In the earliest telephone practice, all lines were grounded ones. The wires were of iron, supported by poles and insulated from them by glass, earthenware, or rubber insulators. For certain uses, such lines still represent good practice. For telegraph service, they represent the present standard practice.

Copper is a better conductor than iron, does not rust, and when drawn into wire in such a way as to have a sufficient tensile strength to support itself is the best available conductor for telephone lines. Only one metal surpasses it in any quality for the purpose: silver is a better conductor by 1 or 2 per cent. Copper is better than silver in strength and price.

In the open country, telephone lines consist of bare wires of copper, of iron, of steel, or of copper-covered steel supported on insulators borne by poles. If the wires on the poles be many, cross-arms carry four to ten wires each and the insulators are mounted on pins in the cross-arms. If the wires on the poles be few, the insulators are mounted on brackets nailed to the poles. Wires so carried are called open wires.

In towns and cities where many wires are to be carried along the same route, the wires are reduced in size, insulated by a covering over each, and assembled into a group. Such a bundle of insulated wires is called a cable. It may be drawn into a duct in the earth and be called an underground cable; it may be laid on the bottom of the sea or other water and be called a submarine cable; or it may be suspended on poles and be called an aerial cable. In the most general practice each wire is insulated from all others by a wrapping of paper ribbon, which covering is only adequate when very dry. Cables formed of paper-insulated wires, therefore, are covered by a seamless, continuous lead sheath, no part of the paper insulation of the wires being exposed to the atmosphere during the cable's entire life in service. Telephone cables for certain uses are formed of wires insulated with such materials as soft rubber, gutta-percha, and cotton or jute saturated with mineral compounds. When insulated with rubber or gutta-percha, no continuous lead sheath is essential for insulation, as those materials, if continuous upon the wire, insulate even when the cable is immersed in water. Sheaths and other armors can assist in protecting these insulating materials from mechanical injury, and often are used for that purpose. The uses to which such cables are suitable in telephony are not many, as will be shown.

A wire supported on poles requires that it be large enough to support its own weight. The smaller the wire, the weaker it is, and with poles a given distance apart, the strength of the wire must be above a certain minimum. In regions where freezing occurs, wires in the open air can collect ice in winter and everywhere open wires are subject to wind pressure; for these reasons additional strength is required. Speaking generally, the practical and economical spacing of poles requires that wires, to be strong enough to meet the above conditions, shall have a diameter not less than .08 inch, if of hard-drawn copper, and .064 inch, if of iron or steel. The honor of developing ways of drawing copper wire with sufficient tensile strength for open-air uses belongs to Mr. Thomas B. Doolittle of Massachusetts.

Lines whose lengths are limited to a few miles do not require a conductivity as great as that of copper wire of .08-inch diameter. A wire of that size weighs approximately 100 pounds per mile. Less than 100 pounds of copper per mile of wire will not give strength enough for use on poles; but as little as 10 pounds per mile of wire gives the necessary conductivity for the lines of the thousands of telephone stations in towns and cities.

Open wires, being exposed to the elements, suffer damage from storms; their insulation is injured by contact with trees; they may make contact with electric power circuits, perhaps injuring apparatus, themselves, and persons; they endanger life and property by the possibility of falling; they and their cross-arm supports are less sightly than a more compact arrangement.

Grouping small wires of telephone lines into cables has, therefore, the advantage of allowing less copper to be used, of reducing the space required, of improving appearance, and of increasing safety. On the other hand, this same grouping introduces negative advantages as well as the foregoing positive ones. It is not possible to talk as far or as well over a line in an ordinary cable as over a line of two open wires. Long-distance telephone circuits, therefore, have not yet been placed in cables for lengths greater than 200 or 300 miles, and special treatment of cable circuits is required to talk through them for even 100 miles. One may talk 2,000 miles over open wires. The reasons for the superiority of the open wires have to do with position rather than material. Obviously it is possible to insulate and bury any wire which can be carried in the air. The differences in the properties of lines whose wires are differently situated with reference to each other and surrounding things are interesting and important.

A telephone line composed of two conductors always possesses four principal properties in some amount: (1) conductivity of the conductors; (2) electrostatic capacity between the conductors; (3) inductance of the circuit; (4) insulation of each conductor from other things.

Conductivity of Conductors. The conductivity of a wire depends upon its material, its cross-section, its length, and its temperature. Conductivity of a copper wire, for example, increases in direct ratio to its weight, in inverse ratio to its length, and its conductivity falls as the temperature rises. Resistance is the reciprocal of conductivity and the properties, conductivity and resistance, are more often expressed in terms of resistance. The unit of the latter is the ohm; of the former the mho. A conductor having a resistance of 100 ohms has a conductivity of .01 mho. The exact correlative terms are resistance and conductance, resistivity and conductivity. The use of the terms as in the foregoing is in accordance with colloquial practice.

Current in a circuit having resistance only, varies inversely as the resistance. Electromotive force being a cause, and resistance a state, current is the result. The formula of this relation, Ohm's law, is

C = E/R

C being the current which results from E, the electromotive force, acting upon R, the resistance. The units are: of current, the ampere; of electromotive force, the volt; of resistance, the ohm.

As the conductivity or resistance of a line is the property of controlling importance in telegraphy, a similar relation was expected in early telephony. As the current in the telephone line varies rapidly, certain other properties of the line assume an importance they do not have in telegraphy in any such degree.

The importance that these properties assume is, that if they did not act and the resistance of the conductors alone limited speech, transmission would be possible direct from Europe to America over a pair of wires weighing 200 pounds per mile of wire, which is less than half the weight of the wire of the best long-distance land lines now in service. The distance from Europe to America is about twice as great as the present commercial radius by land lines of 435-pound wire. In other words, good speech is possible through a mere resistance twenty times greater than the resistance of the longest actual open-wire line it is possible to talk through. The talking ratio between a mere resistance and the resistance of a regular telephone cable is still greater.

Electrostatic Capacity. It is the possession of electrostatic capacity which enables the condenser, of which the Leyden jar is a good example, to be useful in a telephone line. The simplest form of a condenser is illustrated in Fig. 28, in which two conducting surfaces are separated by an insulating material. The larger the surfaces, the closer they are together; and the higher the specific inductive capacity of the insulator, the greater the capacity of the device. An insulator used in this relation to two conducting surfaces is called the dielectric.



Two conventional signs are used to illustrate condensers, the upper one of Fig. 29 growing out of the original condenser of two metal plates, the lower one suggesting the thought of interleaved conductors of tin foil, as for many years was the practice in condenser construction.

With relation to this property, a telephone line is just as truly a condenser as is any other arrangement of conductors and insulators. Assume such a line to be open at the distant end and its wires to be well insulated from each other and the earth. Telegraphy through such a line by ordinary means would be impossible. All that the battery or other source could do would be to cause current to flow into the line for an infinitesimal time, raising the wires to its potential, after which no current would flow. But, by virtue of electrostatic capacity, the condition is much as shown in Fig. 30. The condensers which that figure shows bridged across the line from wire to wire are intended merely to fix in the mind that there is a path for the transfer of electrical energy from wire to wire.



A simple test will enable two of the results of a short-circuiting capacity to be appreciated. Conceive a very short line of two wires to connect two local battery telephones. Such a line possesses negligible resistance, inductance, and shunt capacity. Its insulation is practically infinite. Let condensers be bridged across the line, one by one, while conversation goes on. The listening observer will notice that the sounds reaching his ear steadily grow less loud as the capacity across the line increases. The speaking observer will notice that the sounds he hears through the receiver in series with the line steadily grow louder as the capacity across the line increases. Fig. 31 illustrates the test.

The speaker's observation in this test shows that increasing the capacity across the line increased the amount of current entering it. The hearer's observation in this test shows that increasing the capacity across the line decreased the amount of energy turned into sound at his receiver.



The unit of electrostatic capacity is the farad. As this unit is inconveniently large, for practical applications the unit microfarad—millionth of a farad—is employed. If quantities are known in microfarads and are to be used in calculations in which the values of the capacity require to be farads, care should be taken to introduce the proper corrective factor.

The electrostatic capacity between the conductors of a telephone line depends upon their surface area, their length, their position, and the nature of the materials separating them from each other and from other things. For instance, in an open wire line of two wires, the electrostatic capacity depends upon the diameter of the wires, upon the length of the line, upon their distance apart, upon their distance above the earth, and upon the specific inductive capacity of the air. Air being so common an insulating medium, it is taken as a convenient material whose specific inductive capacity may be used as a basis of reference. Therefore, the specific inductive capacity of air is taken as unity. All solid matter has higher specific inductive capacity than air.

The electrostatic capacity of two open wires .165 inch diameter, 1 ft. apart, and 30 ft. above the earth, is of the order of .009 microfarads per mile. This quantity would be higher if the wires were closer together; or nearer the earth; or if they were surrounded by a gas other than the air or hydrogen; or if the wires were insulated not by a gas but by any solid covering. As another example, a line composed of two wires of a diameter of .036 inch, if wrapped with paper and twisted into a pair as a part of a telephone-cable, has a mutual electrostatic capacity of approximately .08 microfarads per mile, this quantity being greater if the cable be more tightly compressed.

The use of paper as an insulator for wires in telephone cables is due to its low specific inductive capacity. This is because the insulation of the wires is so largely dry air. Rubber and similar insulating materials give capacities as great as twice that of dry paper.

The condenser or other capacity acts as an effective barrier to the steady flow of direct currents. Applying a fixed potential causes a mere rush of current to charge its surface to a definite degree, dependent upon the particular conditions. The condenser does not act as such a barrier to alternating currents, for it is possible to talk through a condenser by means of the alternating voice currents of telephony, or to pass through it alternating currents of much lower frequency. A condenser is used in series with a polarized ringer for the purpose of letting through alternating current for ringing the bell, and of preventing the flow of direct current.

The degree to which the condenser allows alternating currents to pass while stopping direct currents, depends on the capacity of the condenser and on the frequencies of alternating current. The larger the condenser capacity or the higher the frequency of the alternations, the greater will be the current passing through the circuit. The degree to which the current is opposed by the capacity is the reactance of that capacity for that frequency. The formula is

Capacity reactance = 1 /C[omega]

wherein C is the capacity in farads and [omega] is 2[pi]n, or twice 3.1416 times the frequency.

All the foregoing leads to the generalization that the higher the frequency, the less the opposition of a capacity to an alternating current. If the frequency be zero, the reactance is infinite, i.e., the circuit is open to direct current. If the frequency be infinite, the reactance is zero, i.e., the circuit is as if the condenser were replaced by a solid conductor of no resistance. Compare this statement with the correlative generalization which follows the next thought upon inductance.

Inductance of the Circuit. Inductance is the property of a circuit by which change of current in it tends to produce in itself and other conductors an electromotive force other than that which causes the current. Its unit is the henry. The inductance of a circuit is one henry when a change of one ampere per second produces an electromotive force of one volt. Induction between circuits occurs because the circuits possess inductance; it is called mutual induction. Induction within a circuit occurs because the circuit possesses inductance; it is called self-induction. Lenz' law says: In all cases of electromagnetic induction, the induced currents have such a direction that their reaction tends to stop the motion which produced them.



All conductors possess inductance, but straight wires used in lines have negligible inductance in most actual cases. All wires which are wound into coils, such as electromagnets, possess inductance in a greatly increased degree. A wire wound into a spiral, as indicated in Fig. 32, possesses much greater inductance than when drawn out straight. If iron be inserted into the spiral, as shown in Fig. 33, the inductance is still further increased. It is for the purpose of eliminating inductance that resistance coils are wound with double wires, so that current passing through such coils turns in one direction half the way and in the other direction the other half.

A simple test will enable the results of a series inductance in a line to be appreciated. Conceive a very short line of two wires to connect two local battery telephones. Such a line possesses negligible resistance, inductance, and shunt capacity. Its insulation is practically infinite. Let inductive coils such as electromagnets be inserted serially in the wires of the line one by one, while conversation goes on. The listening observer will notice that the sounds reaching his ear steadily grow faint as the inductance in the line increases and the speaking observer will notice the same thing through the receiver in series with the line.

Both observations in this test show that the amount of current entering and emerging from the line decreased as the inductance increased. Compare this with the test with bridged capacity and the loading of lines described later herein, observing the curious beneficial result when both hurtful properties are present in a line. The test is illustrated in Fig. 34.

The degree in which any current is opposed by inductance is termed the reactance of that inductance. Its formula is

Inductive reactance = L[omega]

wherein L is the inductance in henrys and [omega] is 2[pi]n, or twice 3.1416 times the frequency. To distinguish the two kinds of reactance, that due to the capacity is called capacity reactance and that due to inductance is called inductive reactance.

All the foregoing leads to the generalization that the higher the frequency, the greater the opposition of an inductance to an alternating current. If the frequency be zero, the reactance is zero, i.e., the circuit conducts direct current as mere resistance. If the frequency be infinite, the reactance is infinite, i.e., the circuit is "open" to the alternating current and that current cannot pass through it. Compare this with the correlative generalization following the preceding thought upon capacity.



Capacity and inductance depend only on states of matter. Their reactances depend on states of matter and actions of energy.

In circuits having both resistance and capacity or resistance and inductance, both properties affect the passage of current. The joint reaction is expressed in ohms and is called impedance. Its value is the square root of the sum of the squares of the resistance and reactance, or, Z being impedance,

————————————- / 1 Z = / R^{2} + ———————— / C^{2}[omega]^{2}

and

————————————— Z = / R^{2} + L^{2}[omega]^{2} /

the symbols meaning as before.

In words, these formulas mean that, knowing the frequency of the current and the capacity of a condenser, or the frequency of the current and the inductance of a circuit (a line or piece of apparatus), and in either case the resistance of the circuit, one may learn the impedance by calculation.

Insulation of Conductors. The fourth property of telephone lines, insulation of the conductors, usually is expressed in ohms as an insulation resistance. In practice, this property needs to be intrinsically high, and usually is measured by millions of ohms resistance from the wire of a line to its mate or to the earth. It is a convenience to employ a large unit. A million ohms, therefore, is called a megohm. In telephone cables, an insulation resistance of 500 megohms per mile at 60 deg. Fahrenheit is the usual specification. So high an insulation resistance in a paper-insulated conductor is only attained by applying the lead sheath to the cable when its core is made practically anhydrous and kept so during the splicing and terminating of the cable.

Insulation resistance varies inversely as the length of the conductor. If a piece of cable 528 feet long has an insulation resistance of 6,750 megohms, a mile (ten times as much) of such cable, will have an insulation resistance of 675 megohms, or one-tenth as great.

Inductance vs. Capacity. The mutual capacity of a telephone line is greater as its wires are closer together. The self-induction of a telephone line is smaller as its wires are closer together. The electromotive force induced by the capacity of a line leads the impressed electromotive force by 90 degrees. The inductive electromotive force lags 90 degrees behind the impressed electromotive force. And so, in general, the natures of these two properties are opposite. In a cable, the wires are so close together that their induction is negligible, while their capacity is so great as to limit commercial transmission through a cable having .06 microfarads per mile capacity and 94 ohms loop resistance per mile, to a distance of about 30 miles. In the case of open wires spaced 12 inches apart, the limit of commercial transmission is greater, not only because the wires are larger, but because the capacity is lower and the inductance higher.

Table I shows-the practical limiting conversation distance over uniform lines with present standard telephone apparatus.

TABLE I

Limiting Transmission Distances

+ -+ + SIZE AND GAUGE OF WIRE LIMITING DISTANCE + -+ + No. 8 B. W. G. copper 900 miles 10 B. W. G. copper 700 miles 10 B. & S. copper 400 miles 12 N. B. S. copper 400 miles 12 B. & S. copper 240 miles 14 N. B. S. copper 240 miles 8 B. W. G. iron 135 miles 10 B. W. G. iron 120 miles 12 B. W. G. iron 90 miles 16 B. & S. cable, copper 40 miles 19 B. & S. cable, copper 30 miles 22 B. & S. cable, copper 20 miles + -+ +

In 1893, Oliver Heaviside proposed that the inductance of telephone lines be increased above the amount natural for the inter-axial spacing, with a view to counteracting the hurtful effects of the capacity. His meaning was that the increased inductance—a harmful quality in a circuit not having also a harmfully great capacity—would act oppositely to the capacity, and if properly chosen and applied, should decrease or eliminate distortion by making the line's effect on fundamentals and harmonics more nearly uniform, and as well should reduce the attenuation by neutralizing the action of the capacity in dissipating energy.

There are two ways in which inductance might be introduced into a telephone line. As the capacity whose effects are to be neutralized is distributed uniformly throughout the line, the counteracting inductance must also be distributed throughout the line. Mere increase of distance between two wires of the line very happily acts both to increase the inductance and to lower the capacity; unhappily for practical results, the increase of separation to bring the qualities into useful neutralizing relation is beyond practical limits. The wires would need to be so far above the earth and so far apart as to make the arrangement commercially impossible.

Practical results have been secured in increasing the distributed inductance by wrapping fine iron wire over each conductor of the line. Such a treatment increases the inductance and improves transmission.

The most marked success has come as a result of the studies of Professor Michael Idvorsky Pupin. He inserts inductances in series with the wires of the line, so adapting them to the constants of the circuit that attenuation and distortion are diminished in a gratifying degree. This method of counteracting the effects of a distributed capacity by the insertion of localized inductance requires not only that the requisite total amount of inductance be known, but that the proper subdivision and spacing of the local portions of that inductance be known. Professor Pupin's method is described in a paper entitled "Wave Transmission Over Non-uniform Cables and Long-Distance Air Lines," read by him at a meeting of the American Institute of Electrical Engineers in Philadelphia, May 19, 1900.

NOTE. United States Letters Patent were issued to Professor Pupin on June 19, 1900, upon his practical method of reducing attenuation of electrical waves. A paper upon "Propagation of Long Electric Waves" was read by Professor Pupin before the American Institute of Electrical Engineers on March 22, 1899, and appears in Vol. 15 of the Transactions of that society. The student will find these documents useful in his studies on the subject. He is referred also to "Electrical Papers" and "Electromagnetic Theory" of Oliver Heaviside.

Professor Pupin likens the transmission of electric waves over long-distance circuits to the transmission of mechanical waves over a string. Conceive an ordinary light string to be fixed at one end and shaken by the hand at the other; waves will pass over the string from the shaken to the fixed end. Certain reflections will occur from the fixed end. The amount of energy which can be sent in this case from the shaken to the fixed point is small, but if the string be loaded by attaching bullets to it, uniformly throughout its length, it now may transmit much more energy to the fixed end.



The addition of inductance to a telephone line is analogous to the addition of bullets to the string, so that a telephone line is said to be loaded when inductances are inserted in it, and the inductances themselves are known as loading coils.

Fig. 35 shows the general relation of Pupin loading coils to the capacity of the line. The condensers of the figure are merely conventionals to represent the condenser which the line itself forms. The inductances of the figure are the actual loading coils.



The loading of open wires is not as successful in practice as is that of cables. The fundamental reason lies in the fact that two of the properties of open wires—insulation and capacity—vary with atmospheric change. The inserted inductance remaining constant, its benefits may become detriments when the other two "constants" change.

The loading of cable circuits is not subject to these defects. Such loading improves transmission; saves copper; permits the use of longer underground cables than are usable when not loaded; lowers maintenance costs by placing interurban cables underground; and permits submarine telephone cables to join places not otherwise able to speak with each other.

Underground long-distance lines now join or are joining Boston and New York, Philadelphia and New York, Milwaukee and Chicago. England and France are connected by a loaded submarine cable. There is no theoretical reason why Europe and America should not speak to each other.

The student wishing to determine for himself what are the effects of the properties of lines upon open or cable circuits will find most of the subject in the following equation. It tells the value of a in terms of the four properties, a being the attenuation constant of the line.

That is, the larger a is, the more the voice current is reduced in passing over the line. The equation is

———————————————————————————————————- / ———————————————————————- a= /1/2 /(R^{2}L^{2}[omega]^{2})(S^{2}C^{2}[omega]^{2} + 1/2(RS-LC[omega]^{2} / /

The quantities are

R = Resistance in ohms L = Inductance in henrys C = Mutual (shunt) capacity in farads [omega] = 2[pi]n = 6.2832 times the frequency S = Shunt leakage in mhos

The quantity S is a measure of the combined direct-current conductance (reciprocal of insulation resistance) and the apparent conductance due to dielectric hysteresis.

NOTE. An excellent paper, assisting such study, and of immediate practical value as helping the understanding of cables and their reasons, is that of Mr. Frank B. Jewett, presented at the Thousand Islands Convention of the American Institute of Electrical Engineers, July 1, 1909.

Chapter 43 treats cables in further detail. They form a most important part of telephone wire-plant practice, and their uses are becoming wider and more valuable.

Possible Ways of Improving Transmission. Practical ways of improving telephone transmission are of two kinds: to improve the lines and to improve the apparatus. The foregoing shows what are the qualities of lines and the ways they require to be treated. Apparatus treatment, in the present state of the art, is addressed largely to the reduction of losses. Theoretical considerations seem to show, however, that great advance in apparatus effectiveness still is possible. More powerful transmitters—and more faithful ones—more sensitive and accurate receivers, and more efficient translating devices surely are possible. Discovery may need to intervene, to enable invention to restimulate.

In both telegraphy and telephony, the longer the line the weaker the current which is received at the distant end. In both telegraphy and telephony, there is a length of line with a given kind and size of wire and method of construction over which it is just possible to send intelligible speech or intelligible signals. A repeater, in telegraphy, is a device in the form of a relay which is adapted to receive these highly attenuated signal impulses and to re-transmit them with fresh power over a new length of line. An arrangement of two such relays makes it possible to telegraph both ways over a pair of lines united by such a repeater. It is practically possible to join up several such links of lines to repeating devices and, if need be, even submarine cables can be joined to land lines within practical limits. If it were necessary, it probably would be possible to telegraph around the world in this way.

If it were possible to imitate the telegraph repeater in telephony, attenuated voice currents might be caused to actuate it so as to send on those voice currents with renewed power over a length of line, section by section. Such a device has been sought for many years, and it once was quoted in the public press that a reward of one million dollars had been offered by Charles J. Glidden for a successful device of that kind. The records of the patent offices of the world show what effort has been made in that direction and many more devices have been invented than have been patented in all the countries together.

Like some other problems in telephony, this one seems simpler at first sight than it proves to be after more exhaustive study. It is possible for any amateur to produce at once a repeating device which will relay telephone circuits in one direction. It is required, however, that in practice the voice currents be relayed in both directions, and further, that the relay actually augment the energy which passes through it; that is, that it will send on a more powerful current than it receives. Most of the devices so far invented fail in one or the other of these particulars. Several ways have been shown of assembling repeating devices which will talk both ways, but not many assembling repeating devices have been shown that will talk both ways and augment in both directions.



Practical repeaters have been produced, however, and at least one type is in daily successful use. It is not conclusively shown even of it that it augments in the same degree all of the voice waves which reach it, or even that it augments some of them at all. Its action, however, is distinctly an improvement in commercial practice. It is the invention of Mr. Herbert E. Shreeve and is shown in Fig. 39. Primarily it consists of a telephone receiver, of a particular type devised by Gundlach, associated with a granular carbon transmitter button. It is further associated with an arrangement of induction coils or repeating coils, the object of these being to accomplish the two-way action, that is, of speaking in both directions and of preventing reactive interference between the receiving and transmitting elements. The battery 1 energizes the field of the receiving element; the received line current varies that field; the resulting motion varies the resistance of the carbon button and transforms current from battery 2 into a new alternating line current.

By reactive interference is meant action whereby the transmitter element, in emitting a wave, affects its own controlling receiver element, thus setting up an action similar to that which occurs when the receiver of a telephone is held close to its transmitter and humming or singing ensues. No repeater is successful unless it is free from this reactive interference.



Enough has been accomplished by practical tests of the Shreeve device and others like it to show that the search for a method of relaying telephone voice currents is not looking for a pot of gold at the end of the rainbow. The most remarkable truth established by the success of repeaters of the Shreeve type is that a device embodying so large inertia of moving parts can succeed at all. If this mean anything, it is that a device in which inertia is absolutely eliminated might do very much better. Many of the methods already proposed by inventors attack the problem in this way and one of the most recent and most promising ways is that of Mr. J.B. Taylor, the circuit of whose telephone-relay patent is shown in Fig. 37. In it, 1 is an electromagnet energized by voice currents; its varying field varies an arc between the electrodes 2-2 and 3 in a vacuum tube. These fluctuations are transformed into line currents by the coil 4.



CHAPTER V

TRANSMITTERS

Variable Resistance. As already pointed out in Chapter II, the variable-resistance method of producing current waves, corresponding to sound waves for telephonic transmission, is the one that lends itself most readily to practical purposes. Practically all telephone transmitters of today employ this variable-resistance principle. The reason for the adoption of this method instead of the other possible ones is that the devices acting on this principle are capable, with great simplicity of construction, of producing much more powerful results than the others. Their simplicity is such as to make them capable of being manufactured at low cost and of being used successfully by unskilled persons.

Materials. Of all the materials available for the variable-resistance element in telephone transmitters, carbon is by far the most suitable, and its use is well nigh universal. Sometimes one of the rarer metals, such as platinum or gold, is to be found in commercial transmitters as part of the resistance-varying device, but, even when this is so, it is always used in combination with carbon in some form or other. Most of the transmitters in use, however, depend solely upon carbon as the conductive material of the variable-resistance element.

Arrangement of Electrodes. Following the principles pointed out by Hughes, the transmitters of today always employ as their variable-resistance elements one or more loose contacts between one or more pairs of electrodes, which electrodes, as just stated, are usually of carbon. Always the arrangement is such that the sound waves will vary the intimacy of contact between the electrodes and, therefore, the resistance of the path through the electrodes.

A multitude of arrangements have been proposed and tried. Sometimes a single pair of electrodes has been employed having a single point of loose contact between them. These may be termed single-contact transmitters. Sometimes the variable-resistance element has included a greater number of electrodes arranged in multiple, or in series, or in series-multiple, and these have been termed multiple-electrode transmitters, signifying a plurality of electrodes. A later development, an outgrowth of the multiple-electrode transmitter, makes use of a pair of principal electrodes, between which is included a mass of finely divided carbon in the form of granules or small spheres or pellets. These, regardless of the exact form of the carbon particles, are called granular-carbon transmitters.



Single Electrode. Blake. The most notable example of the single-contact transmitter is the once familiar Blake instrument. At one time this formed a part of the standard equipment of almost every telephone in the United States, and it was also largely used abroad. Probably no transmitter has ever exceeded it in clearness of articulation, but it was decidedly deficient in power in comparison with the modern transmitters. In this instrument, which is shown in Fig. 38, the variable-resistance contact was that between a carbon and a platinum electrode. The diaphragm 1 was of sheet iron mounted, as usual in later transmitters, in a soft rubber gasket 2. The whole diaphragm was mounted in a cast-iron ring 3, supported on the inside of the box containing the entire instrument. The front electrode 4 was mounted on a light spring 5, the upper end of which was supported by a movable bar or lever 6, flexibly supported on a spring 7 secured to the casting which supported the diaphragm. The tension of this spring 5 was such as to cause the platinum point to press lightly away from the center of the diaphragm. The rear electrode was of carbon in the form of a small block 9, secured in a heavy brass button 10. The entire rear electrode structure was supported on a heavier spring 11 carried on the same lever as the spring 5. The tension of this latter spring was such as to press against the front electrode and, by its greater strength, press this against the center of the diaphragm. The adjustment of the instrument was secured by means of the screw 12, carried in a lug extending rearwardly from the diaphragm supporting casting, this screw, by its position, determining the strength with which the rear electrode pressed against the front electrode and that against the diaphragm. This instrument was ordinarily mounted in a wooden box together with the induction coil, which is shown in the upper portion of the figure.

The Blake transmitter has passed almost entirely out of use in this country, being superseded by the various forms of granular instruments, which, while much more powerful, are not perhaps capable of producing quite such clear and distinct articulation.

The great trouble with the single-contact transmitters, such as the Blake, was that it was impossible to pass enough current through the single point of contact to secure the desired power of transmission without overheating the contact. If too much current is sent through such transmitters, an undue amount of heat is generated at the point of contact and a vibration is set up which causes a peculiar humming or squealing sound which interferes with the transmission of other sounds.

Multiple Electrode. To remedy this difficulty the so-called multiple-electrode transmitter was brought out. This took a very great number of forms, of which the one shown in Fig. 39 is typical. The diaphragm shown at 1, in this particular form, was made of thin pine wood. On the rear side of this, suspended from a rod 3 carried in a bracket 4, were a number of carbon rods or pendants 5, loosely resting against a rod 2, carried on a bracket 6 also mounted on the rear of the diaphragm. The pivotal rod 3 and the rod 2, against which the pendants rested, were sometimes, like the pendant rods, made of carbon and sometimes of metal, such as brass. When the diaphragm vibrated, the intimacy of contact between the pendant rod 5 and the rod 2 was altered, and thus the resistance of the path through all of the pendant rods in multiple was changed.



A multitude of forms of such transmitters came into use in the early eighties, and while they in some measure remedied the difficulty encountered with the Blake transmitter, i.e., of not being able to carry a sufficiently large current, they were all subject to the effects of extreme sensitiveness, and would rattle or break when called upon to transmit sounds of more than ordinary loudness. Furthermore, the presence of such large masses of material, which it was necessary to throw into vibration by the sound waves, was distinctly against this form of transmitter. The inertia of the moving parts was so great that clearness of articulation was interfered with.

Granular Carbon. The idea of employing a mass of granular carbon, supported between two electrodes, one of which vibrated with the sound waves and the other was stationary, was proposed by Henry Hunnings in the early eighties. While this idea forms the basis of all modern telephone transmitters, yet it did not prevent the almost universal adoption of the single-contact form of instrument during the next decade.

Western Electric Solid-Back Transmitter. In the early nineties, however, the granular-carbon transmitter came into its own with the advent and wide adoption of the transmitter designed by Anthony C. White, known as the White, or solid-back, transmitter. This has for many years been the standard instrument of the Bell companies operating throughout the United States, and has found large use abroad. A horizontal cross-section of this instrument is shown in Fig. 40, and a rear view of the working parts in Fig. 41. The working parts are all mounted on the front casting 1. This is supported in a cup 2, in turn supported on the lug 3, which is pivoted on the transmitter arm or other support. The front and rear electrodes of this instrument are formed of thin carbon disks shown in solid black. The rear electrode, the larger one of these disks, is securely attached by solder to the face of a brass disk having a rearwardly projecting screw-threaded shank, which serves to hold it and the rear electrode in place in the bottom of a heavy brass cup 4. The front electrode is mounted on the rear face of a stud. Clamped against the head of this stud, by a screw-threaded clamping ring 7, is a mica washer, or disk 6. The center portion of this mica washer is therefore rigid with respect to the front electrode and partakes of its movements. The outer edge of this mica washer is similarly clamped against the front edge of the cup 4, a screw-threaded ring 9 serving to hold the edge of the mica rigidly against the front of the cup. The outer edge of this washer is, therefore, rigid with respect to the rear electrode, which is fixed. Whatever relative movement there is between the two electrodes must, therefore, be permitted by the flexing of the mica washer. This mica washer not only serves to maintain the electrodes in their normal relative positions, but also serves to close the chamber which contains the electrodes, and, therefore, to prevent the granular carbon, with which the space between the electrodes is filled, from falling out.



The cup 4, containing the electrode chamber, is rigidly fastened with respect to the body of the transmitter by a rearwardly projecting shank held in a bridge piece 8 which is secured at its ends to the front block. The needed rigidity of the rear electrode is thus obtained and this is probably the reason for calling the instrument the solid-back. The front electrode, on the other hand, is fastened to the center of the diaphragm by means of a shank on the stud, which passes through a hole in the diaphragm and is clamped thereto by two small nuts. Against the rear face of the diaphragm of this transmitter there rest two damping springs. These are not shown in Fig. 40 but are in Fig. 41. They are secured at one end to the rear flange of the front casting 1, and bear with their other or free ends against the rear face of the diaphragm. The damping springs are prevented from coming into actual contact with the diaphragm by small insulating pads. The purpose of the damping springs is to reduce the sensitiveness of the diaphragm to extraneous sounds. As a result, the White transmitter does not pick up all of the sounds in its vicinity as readily as do the more sensitive transmitters, and thus the transmission is not interfered with by extraneous noises. On the other hand, the provision of these heavy damping springs makes it necessary that this transmitter shall be spoken into directly by the user.



The action of this transmitter is as follows: Sound waves are concentrated against the center of the diaphragm by the mouth-piece, which is of the familiar form. These waves impinge against the diaphragm, causing it to vibrate, and this, in turn, produces similar vibrations in the front electrode. The vibrations of the front electrode are permitted by the elasticity of the mica washer 6. The rear electrode is, however, held stationary within the heavy chambered block 4 and which in turn is held immovable by its rigid mounting. As a result, the front electrode approaches and recedes from the rear electrode, thus compressing and decompressing the mass of granular carbon between them. As a result, the intimacy of contact between the electrode plates and the granules and also between the granules themselves is altered, and the resistance of the path from one electrode to the other through the mass of granules is varied.

New Western Electric Transmitter. The White transmitter was the prototype of a large number of others embodying the same features of having the rear electrode mounted in a stationary cup or chamber and the front electrode movable with the diaphragm, a washer of mica or other flexible insulating material serving to close the front of the electrode chamber and at the same time to permit the necessary vibration of the front electrode with the diaphragm.



One of these transmitters, embodying these same features but with modified details, is shown in Fig. 42, this being the new transmitter manufactured by the Western Electric Company. In this the bridge of the original White transmitter is dispensed with, the electrode chamber being supported by a pressed metal cup 1, which supports the chamber as a whole. The electrode cup, instead of being made of a solid block as in the White instrument, is composed of two portions, a cylindrical or tubular portion 2 and a back 3. The cylindrical portion is externally screw-threaded so as to engage an internal screw thread in a flanged opening in the center of the cup 1. By this means the electrode chamber is held in place in the cup 1, and by the same means the mica washer 4 is clamped between the flange in this opening and the tubular portion 2 of the electrode chamber. The front electrode is carried, as in the White transmitter, on the mica washer and is rigidly attached to the center of the diaphragm so as to partake of the movement thereof. It will be seen, therefore, that this is essentially a White transmitter, but with a modified mounting for the electrode chamber.

A feature in this transmitter that is not found in the White transmitter is that both the front and the rear electrodes, in fact, the entire working portions of the transmitter, are insulated from the exposed metal parts of the instrument. This is accomplished by insulating the diaphragm and the supporting cup 1 from the transmitter front. The terminal 5 on the cup 1 forms the electrical connection for the rear electrode, while the terminal 6, which is mounted on but insulated from the cup 1 and is connected with the front electrode by a thin flexible connecting strip, forms the electrical connection for the front electrode.

Kellogg Transmitter. The transmitter of the Kellogg Switchboard and Supply Company, originally developed by Mr. W.W. Dean and modified by his successors in the Kellogg Company, is shown in Fig. 43. In this, the electrode chamber, instead of being mounted in a stationary and rigid position, as in the case of the White instrument, is mounted on, and, in fact, forms a part of the diaphragm. The electrode which is associated with the mica washer instead of moving with the diaphragm, as in the White instrument, is rigidly connected to a bridge so as to be as free as possible from all vibrations.

Referring to Fig. 43, which is a horizontal cross-section of the instrument, 1 indicates the diaphragm. This is of aluminum and it has in its center a forwardly deflected portion forming a chamber for the electrodes. The front electrode 2 of carbon is backed by a disk of brass and rigidly secured in the front of this chamber, as clearly indicated. The rear electrode 3, also of carbon, is backed by a disk of brass, and is clamped against the central portion of a mica disk by means of the enlarged head of stud 6. A nut 7, engaging the end of a screw-threaded shank from the back of the rear electrode, serves to bind these two parts together securely, clamping the mica washer between them. The outer edge of the mica washer is clamped to the main diaphragm 1 by an aluminum ring and rivets, as clearly indicated. It is seen, therefore, that the diaphragm itself contains the electrode chamber as an integral part thereof. The entire structure of the diaphragm, the front and back electrodes, and the granular carbon within are permanently assembled in the factory and cannot be dissociated without destroying some of the parts. The rear electrode is held rigidly in place by the bridge 5 and the stud 6, this stud passing through a block 9 mounted on the bridge but insulated from it. The stud 6 is clamped in the block 9 by means of the set screw 8, so as to hold the rear electrode in proper position after this position has been determined.



In this transmitter, as in the transmitter shown in Fig. 42, all of the working parts are insulated from the exposed metal casing. The diaphragm is insulated from the front of the instrument by means of a washer 4 of impregnated cloth, as indicated. The rear electrode is insulated from the other portions of the instrument by means of the mica washer and by means of the insulation between the block 9 and the bridge 5. The terminal for the rear electrode is mounted on the block 9, while the terminal for the front electrode, shown at 10, is mounted on, but insulated from, the bridge. This terminal 10 is connected with the diaphragm and therefore with the front electrode by means of a thin, flexible metallic connection. This transmitter is provided with damping springs similar to those of the White instrument.

It is claimed by advocates of this type of instrument that, in addition to the ordinary action due to the compression and decompression of the granular carbon between the electrodes, there exists another action due to the agitation of the granules as the chamber is caused to vibrate by the sound waves. In other words, in addition to the ordinary action, which may be termed the piston action between the electrodes, it is claimed that the general shaking-up effect of the granules when the chamber vibrates produces an added effect. Certain it is, however, that transmitters of this general type are very efficient and have proven their capability of giving satisfactory service through long periods of time.

Another interesting feature of this instrument as it is now manufactured is the use of a transmitter front that is struck up from sheet metal rather than the employment of a casting as has ordinarily been the practice. The formation of the supporting lug for the transmitter from the sheet metal which forms the rear casing or shell of the instrument is also an interesting feature.

Automatic Electric Company Transmitter. The transmitter of the Automatic Electric Company, of Chicago, shown in Fig. 44, is of the same general type as the one just discussed, in that the electrode chamber is mounted on and vibrates with the diaphragm instead of being rigidly supported on the bridge as in the case of the White or solid-back type of instrument. In this instrument the transmitter front 1 is struck up from sheet metal and contains a rearwardly projecting flange, carrying an internal screw thread. A heavy inner cup 2, together with the diaphragm 3, form an enclosure containing the electrode chamber. The diaphragm is, in this case, permanently secured at its edge to the periphery of the inner cup 2 by a band of metal 4 so formed as to embrace the edges of both the cup and the diaphragm and permanently lock them together. This inner chamber is held in place in the transmitter front 1 by means of a lock ring 5 externally screw-threaded to engage the internal screw-thread on the flange on the front. The electrode chamber proper is made in the form of a cup, rigidly secured to the diaphragm so as to move therewith, as clearly indicated. The rear electrode is mounted on a screw-threaded stud carried in a block which is fitted to a close central opening in the cup 2.

This transmitter does not make use of a mica washer or diaphragm, but employs a felt washer which surrounds the shank of the rear electrode and serves to close and seal the carbon containing cup. By this means the granular carbon is retained in the chamber and the necessary flexibility or freedom of motion is permitted between the front and the rear electrodes. As in the Kellogg and the later Bell instruments, the entire working parts of this transmitter are insulated from the metal containing case, the inner chamber, formed by the cup 2 and the diaphragm 3, being insulated from the transmitter front and its locking ring by means of insulating washers, as shown.

Fig. 44. Automatic Electric Company Transmitter

Monarch Transmitter. The transmitter of the Monarch Telephone Manufacturing Company, shown in Fig. 45, differs from both the stationary-cup and the vibrating-cup types, although it has the characteristics of both. It might be said that it differs from each of these two types of transmitters in that it has the characteristics of both.

This transmitter, it will be seen, has two flexible mica washers between the electrodes and the walls of the electrode cup. The front and the back electrodes are attached to the diaphragm and the bridge, respectively, by a method similar to that employed in the solid-back transmitters, while the carbon chamber itself is free to vibrate with the diaphragm as is characteristic of the Kellogg transmitter.



An aluminum diaphragm is employed, the circumferential edge of which is forwardly deflected to form a seat. The edge of the diaphragm rests against and is separated from the brass front by means of a one-piece gasket of specially treated linen. This forms an insulator which is not affected by heat or moisture. As in the transmitters previously described, the electrodes are firmly soldered to brass disks which have solid studs extending from their centers. In the case of both the front and the rear electrodes, a mica disk is placed over the supporting stud and held in place by a brass hub which has a base of the same size as the electrode. The carbon-chamber wall consists of a brass ring to which are fastened the mica disks of the front and the back electrodes by means of brass collars clamped over the edge of the mica and around the rim of the brass ring forming the chamber.



Electrodes. The electrode plates of nearly all modern transmitters are of specially treated carbon. These are first copper-plated and soldered to their brass supporting disks. After this they are turned and ground so as to be truly circular in form and to present absolutely flat faces toward each other. These faces are then highly polished and the utmost effort is made to keep them absolutely clean. Great pains are taken to remove from the pores of the carbon, as well as from the surface, all of the acids or other chemicals that may have entered them during the process of electroplating them or of soldering them to the brass supporting disk. That the two electrodes, when mounted in a transmitter, should be parallel with each other, is an item of great importance as will be pointed out later.

In a few cases, as previously stated, gold or platinum has been substituted for the carbon electrodes in transmitters. These are capable of giving good results when used in connection with the proper form of granular carbon, but, on the whole, the tendency has been to abandon all forms of electrode material except carbon, and its use is now well nigh universal.

Preparation of Carbon. The granular carbon is prepared from carefully selected anthracite coal, which is specially treated by roasting or "re-carbonizing" and is then crushed to approximately the proper fineness. The crushed carbon is then screened with extreme care to eliminate all dust and to retain only granules of uniform size.

Packing. In the earlier forms of granular-carbon transmitters a great deal of trouble was experienced due to the so-called packing of the instrument. This, as the term indicates, was a trouble due to the tendency of the carbon granules to settle into a compact mass and thus not respond to the variable pressure. This was sometimes due to the presence of moisture in the electrode chamber; sometimes to the employment of granules of varying sizes, so that they would finally arrange themselves under the vibration of the diaphragm into a fairly compact mass; or sometimes, and more frequently, to the granules in some way wedging the two electrodes apart and holding them at a greater distance from each other than their normal distance. The trouble due to moisture has been entirely eliminated by so sealing the granule chambers as to prevent the entrance of moisture. The trouble due to the lack of uniformity in size of the granules has been entirely eliminated by making them all of one size and by making them of sufficient hardness so that they would not break up into granules of smaller size. The trouble due to the settling of the granules and wedging the electrodes apart has been practically eliminated in well-designed instruments, by great mechanical nicety in manufacture.

Almost any transmitter may be packed by drawing the diaphragm forward so as to widely separate the electrodes. This allows the granules to settle to a lower level than they normally occupy and when the diaphragm is released and attempts to resume its normal position it is prevented from doing so by the mass of granules between. Transmitters of the early types could be packed by placing the lips against the mouthpiece and drawing in the breath. The slots now provided at the base of standard mouthpieces effectually prevent this.

In general it may be said that the packing difficulty has been almost entirely eliminated, not by the employment of remedial devices, such as those often proposed for stirring up the carbon, but by preventing the trouble by the design and manufacture of the instruments in such forms that they will not be subject to the evil.

Carrying Capacity. Obviously, the power of a transmitter is dependent on the amount of current that it may carry, as well as on the amount of variation that it may make in the resistance of the path through it. Granular carbon transmitters are capable of carrying much heavier current than the old Blake or other single or multiple electrode types. If forced to carry too much current, however, the same frying or sizzling sound is noticeable as in the earlier types. This is due to the heating of the electrodes and to small arcs that occur between the electrodes and the granules.

One way to increase the current-carrying capacity of a transmitter is to increase the area of its electrodes, but a limit is soon reached in this direction owing to the increased inertia of the moving electrode, which necessarily comes with its larger size.

The carrying capacity of transmitters may also be increased by providing special means for carrying away the heat generated in the variable-resistance medium. Several schemes have been proposed for this. One is to employ unusually heavy metal for the electrode chamber, and this practice is best exemplified in the White solid-back instrument. It has also been proposed by others to water-jacket the electrode chamber, and also to keep it cool by placing it in close proximity to the relatively cool joints of a thermopile. Neither of these two latter schemes seems to be warranted in ordinary commercial practice.

Sensitiveness. In all the transmitters so far discussed damping springs of one form or another have been employed to reduce the sensitiveness of the instrument. For ordinary commercial use too great a degree of sensitiveness is a fault, as has already been pointed out. There are, however, certain adaptations of the telephone transmitter which make a maximum degree of sensitiveness desirable. One of these adaptations is found in the telephone equipments for assisting partially deaf people to hear. In these the transmitter is carried on some portion of the body of the deaf person, the receiver is strapped or otherwise held at his ear, and a battery for furnishing the current is carried in his pocket. It is not feasible, for this sort of use, that the sound which this transmitter is to reproduce shall always occur immediately in front of the transmitter. It more often occurs at a distance of several feet. For this reason the transmitter is made as sensitive as possible, and yet is so constructed that it will not be caused to produce too loud or unduly harsh sounds in response to a loud sound taking place immediately in front of it. Another adaptation of such highly sensitive transmitters is found in the special intercommunicating telephone systems for use between the various departments or desks in business offices. In these it is desirable that the transmitter shall be able to respond adequately to sounds occurring anywhere in a small-sized room, for instance.

Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted for such use. This has been termed by its makers the acousticon transmitter. Like all the transmitters previously discussed, this is of the variable-resistance type, but it differs from them all in that it has no damping springs; in that carbon balls are substituted for carbon granules; and in that the diaphragm itself serves as the front electrode.

This transmitter consists of a cup 1, into which is set a cylindrical block 2, in one face of which are a number of hemispherical recesses. The diaphragm 3 is made of thin carbon and is so placed in the transmitter as to cover the openings of the recesses in the carbon block, and lie close enough to the carbon block, without engaging it, to prevent the carbon particles from falling out. The diaphragm thus serves as the front electrode and the carbon block as the rear electrode. The recesses in the carbon block are about two-thirds filled with small carbon balls, which are about the size of fine sand. The front piece 4 of the transmitter is of sheet metal and serves to hold the diaphragm in place. To admit the sound waves it is provided with a circular opening opposite to and about the size of the rear electrode block. On this front piece are mounted the two terminals of the transmitter, connected respectively to the two electrodes, terminal 5 being insulated from the front piece and connected by a thin metal strip with the diaphragm, while terminal 6 is mounted directly on the front piece and connected through the cup 1 with the carbon block 2, or back electrode of the transmitter.



When this transmitter is used in connection with outfits for the deaf, it is placed in a hard rubber containing case, consisting of a hollow cylindrical piece 7, which has fastened to it a cover 8. This cover has a circular row of openings or holes near its outer edge, as shown at 9, through which the sound waves may pass to the chamber within, and thence find their way through the round hole in the center of the front plate 4 to the diaphragm 3. It is probable also that the front face of the cover 8 of the outer case vibrates, and in this way also causes sound waves to impinge against the diaphragm. This arrangement provides a large receiving surface for the sound waves, but, owing to the fact that the openings in the containing case are not opposite the opening in the transmitter proper, the sound waves do not impinge directly against the diaphragm. This peculiar arrangement is probably the result of an endeavor to prevent the transmitter from being too strongly actuated by violent sounds close to it. Instruments of this kind are very sensitive and under proper conditions are readily responsive to words spoken in an ordinary tone ten feet away.



Switchboard Transmitter. Another special adaptation of the telephone transmitter is that for use of telephone operators at central-office switchboards. The requirements in this case are such that the operator must always be able to speak into the transmitter while seated before the switchboard, and yet allow both of her hands to be free for use. This was formerly accomplished by suspending an ordinary granular-carbon transmitter in front of the operator, but a later development has resulted in the adoption of the so-called breast transmitter, shown in Fig. 47. This is merely an ordinary granular-carbon transmitter mounted on a plate which is strapped on the breast of the operator, the transmitter being provided with a long curved mouthpiece which projects in such a manner as to lie just in front of the operator's lips. This device has the advantage of automatically following the operator in her movements. The breast transmitter shown in Fig. 47, is that of the Dean Electric Company.



Conventional Diagram. There are several common ways of illustrating transmitters in diagrams of circuits in which they are employed. The three most common ways are shown in Fig. 48. The one at the left is supposed to be a side view of an ordinary instrument, the one in the center a front view, and the one at the right to be merely a suggestive arrangement of the diaphragm and the rear electrode. The one at the right is best and perhaps most common; the center one is the poorest and least used.



CHAPTER VI

RECEIVERS

The telephone receiver is the device which translates the energy of the voice currents into the energy of corresponding sound waves. All telephone receivers today are of the electromagnetic type, the voice currents causing a varying magnetic pull on an armature or diaphragm, which in turn produces the sound waves corresponding to the undulations of the voice currents.

Early Receivers. The early forms of telephone receivers were of the single-pole type; that is, the type wherein but one pole of the electromagnet was presented to the diaphragm. The single-pole receiver that formed the companion piece to the old Blake transmitter and that was the standard of the Bell companies for many years, is shown in Fig. 49. While this has almost completely passed out of use, it may be profitably studied in order that a comparison may be made between certain features of its construction and those of the later forms of receivers.

The coil of this receiver was wound on a round iron core 2, flattened at one end to afford means for attaching the permanent magnet. The permanent magnet was of laminated construction, consisting of four hard steel bars 1, extending nearly the entire length of the receiver shell. These steel bars were all magnetized separately and placed with like poles together so as to form a single bar magnet. They were laid together in pairs so as to include between the pairs the flattened end of the pole piece 2 at one end and the flattened portion of the tail piece 3 at the other end. This whole magnet structure, including the core, the tail piece, and the permanently magnetized steel bars, was clamped together by screws as shown. The containing shell was of hard rubber consisting of three pieces, the barrel 4, the ear-piece 5, and the tail cap 6. The barrel and the ear piece engaged each other by means of a screw thread and served to clamp the diaphragm between them. The compound bar magnet was held in place within the shell by means of a screw 7 passing through the hard rubber tail cap 6 and into the tail block 3 of the magnet. External binding posts mounted on the tail cap, as shown, were connected by heavy leading-in wires to the terminals of the electromagnet.

A casual consideration of the magnetic circuit of this instrument will show that it was inefficient, since the return path for the lines of force set up by the bar magnet was necessarily through a very long air path. Notwithstanding this, these receivers were capable of giving excellent articulation and were of marvelous delicacy of action. A very grave fault was that the magnet was supported in the shell at the end farthest removed from the diaphragm. As a result it was difficult to maintain a permanent adjustment between the pole piece and the diaphragm. One reason for this was that hard rubber and steel contract and expand under changes of temperature at very different rates, and therefore the distance between the pole piece and the diaphragm changed with changes of temperature. Another grave defect, brought about by this tying together of the permanent magnet and the shell which supported the diaphragm at the end farthest from the diaphragm, was that any mechanical shocks were thus given a good chance to alter the adjustment.



Modern Receivers. Receivers of today differ from this old single-pole receiver in two radical respects. In the first place, the modern receiver is of the bi-polar type, consisting essentially of a horseshoe magnet presenting both of its poles to the diaphragm. In the second place, the modern practice is to either support all of the working parts of the receiver, i.e., the magnet, the coils, and the diaphragm, by an inner metallic frame entirely independent of the shell; or, if the shell is used as a part of the structure, to rigidly fasten the several parts close to the diaphragm rather than at the end farthest removed from the diaphragm.

Western Electric Receiver. The standard bi-polar receiver of the Western Electric Company, in use by practically all of the Bell operating companies throughout this country and in large use abroad, is shown in Fig. 50. In this the shell is of three pieces, consisting of the barrel 1, the ear cap 2, and the tail cap 3. The tail cap and the barrel are permanently fastened together to form substantially a single piece. Two permanently magnetized bar magnets 4-4 are employed, these being clamped together at their upper ends, as shown, so as to include the soft iron block 5 between them. The north pole of one of these magnets is clamped to the south pole of the other, so that in reality a horseshoe magnet is formed. At their lower ends, these two permanent magnets are clamped against the soft iron pole pieces 6-6, a threaded block 7 also being clamped rigidly between these pole pieces at this point. On the ends of the pole pieces the bobbins are wound. The whole magnet structure is secured within the shell 1 by means of a screw thread on the block 7 which engages a corresponding internal screw thread in the shell 1. As a result of this construction the whole magnet structure is bound rigidly to the shell structure at a point close to the diaphragm, comparatively speaking, and as a result of this close coupling, the relation between the diaphragm and the pole piece is very much more rigid and substantial than in the case where the magnet structure and the shell were secured together at the end farthest removed from the diaphragm.



Although this receiver shown in Fig. 50 is the standard in use by the Bell companies throughout this country, its numbers running well into the millions, it cannot be said to be a strictly modern receiver, because of at least one rather antiquated feature. The binding posts, by which the circuit conductors are led to the coils of this instrument, are mounted on the outside of the receiver shell, as indicated, and are thus subject to danger of mechanical injury and they are also exposed to the touch of the user, so that he may, in case of the wires being charged to an abnormal potential, receive a shock. Probably a more serious feature than either one of these is that the terminals of the flexible cords which attach to these binding posts are attached outside of the receiver shell, and are therefore exposed to the wear and tear of use, rather than being protected as they should be within the shell. Notwithstanding this undesirable feature, this receiver is a very efficient one and is excellently constructed.



Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with internal or concealed binding posts. This particular receiver is typical of a large number of similar kinds and is manufactured by the Kellogg Switchboard and Supply Company. Two straight permanently magnetized bar magnets 1-1 are clamped together at their opposite ends so as to form a horseshoe magnet. At the end opposite the diaphragm these bars clamp between them a cylindrical piece of iron 2, so as to complete the magnetic circuit at the end. At the end nearest the diaphragm they clamp between them the ends of the soft iron pole pieces 3-3, and also a block of composite metal 4 having a large circular flange 4' which serves as a means for supporting the magnet structure within the shell. The screws by means of which the disk 4' is clamped to the shouldered seat in the shell do not enter the shell directly, but rather enter screw-threaded brass blocks which are moulded into the structure of the shell. It is seen from this construction that the diaphragm and the pole pieces and the magnet structure itself are all rigidly secured together through the medium of the shell at a point as close as possible to the diaphragm.

Between the magnets 1-1 there is clamped an insulating block 5, to which are fastened the terminal plates 6, one on each side of the receiver. These terminal plates are thoroughly insulated from the magnets themselves and from all other metallic parts by means of sheets of fiber, as indicated by the heavy black lines. On these plates 6 are carried the binding posts for the receiver cord terminals. A long tongue extends from each of the plates 6 through a hole in the disk 4', into the coil chamber of the receiver, at which point the terminal of the magnet winding is secured to it. This tongue is insulated from the disk 4', where it passes through it, by means of insulating bushing, as shown. The other terminal of the magnet coils is brought out to the other plate 6 by means of a similar tongue on the other side.

In order that the receiver terminals proper may not be subjected to any strain in case the receiver is dropped and its weight caught on the receiver cord, a strain loop is formed as a continuation of the braided covering of the receiver cord, and this is tied to the permanent magnet structure, as shown. By making this strain loop short, it is obvious that whatever pull the cord receives will not be taken by the cord conductors leading to the binding posts or by the binding posts or the cord terminals themselves.

A number of other manufacturers have gone even a step further than this in securing permanency of adjustment between the receiver diaphragm and pole pieces. They have done this by not depending at all on the hard rubber shell as a part of the structure, but by enclosing the magnet coil in a cup of metal upon which the diaphragm is mounted, so that the permanency of relation between the diaphragm and the pole pieces is dependent only upon the metallic structure and not at all upon the less durable shell.

Direct-Current Receiver. Until about the middle of the year 1909, it was the universal practice to employ permanent magnets for giving the initial polarization to the magnet cores of telephone receivers. This is still done, and necessarily so, in receivers employed in connection with magneto telephones. In common-battery systems, however, where the direct transmitter current is fed from the central office to the local stations, it has been found that this current which must flow at any rate through the line may be made to serve the additional purpose of energizing the receiver magnets so as to give them the necessary initial polarity. A type of receiver has come into wide use as a result, which is commonly called the direct-current receiver, deriving its name from the fact that it employs the direct current that is flowing in the common-battery line to magnetize the receiver cores. The Automatic Electric Company, of Chicago, was probably the first company to adopt this form of receiver as its standard type. Their receiver is shown in cross-section in Fig. 52, and a photograph of the same instrument partially disassembled is given in Fig. 53. The most noticeable thing about the construction of this receiver is the absence of permanent magnets. The entire working parts are contained within the brass cup 1, which serves not only as a container for the magnet, but also as a seat for the diaphragm. This receiver is therefore illustrative of the type mentioned above, wherein the relation between the diaphragm and the pole pieces is not dependent upon any connection through the shell.



The coil of this instrument consists of a single cylindrical spool 2, mounted on a cylindrical core. This bobbin lies within a soft iron-punching 3, the form of which is most clearly shown in Fig. 53, and this punching affords a return path to the diaphragm for the lines of force set up in the magnet core. Obviously a magnetizing current passing through the winding of the coil will cause the end of the core toward the diaphragm to be polarized, say positively, while the end of the enclosing shell will be polarized in the other polarity, negatively. Both poles of the magnet are therefore presented to the diaphragm and the only air gap in the magnetic circuit is that between the diaphragm and these poles. The magnetic circuit is therefore one of great efficiency, since it consists almost entirely of iron, the only air gap being that across which the attraction of the diaphragm is to take place.

The action of this receiver will be understood when it is stated that in common-battery practice, as will be shown in later chapters, a steady current flows over the line for energizing the transmitter. On this current is superposed the incoming voice currents from a distant station. The steady current flowing in the line will, in the case of this receiver, pass through the magnet winding and establish a normal magnetic field in the same way as if a permanent magnet were employed. The superposed incoming voice currents will then be able to vary this magnetic field in exactly the same way as in the ordinary receiver.

An astonishing feature of this recent development of the so-called direct-current receiver is that it did not come into use until after about twenty years of common-battery practice. There is nothing new in the principles involved, as all of them were already understood and some of them were employed by Bell in his original telephone; in fact, the idea had been advanced time and again, and thrown aside as not being worth consideration. This is an illustration of a frequent occurrence in the development of almost any rapidly growing art. Ideas that are discarded as worthless in the early stages of the art are finally picked up and made use of. The reason for this is that in some cases the ideas come in advance of the art, or they are proposed before the art is ready to use them. In other cases the idea as originally proposed lacked some small but essential detail, or, as is more often the case, the experimenter in the early days did not have sufficient skill or knowledge to make it fit the requirements as he saw them.

Monarch Receiver. The receiver of the Automatic Electric Company just discussed employs but a single electromagnet by which the initial magnetization of the cores and also the variable magnetization necessary for speech reproduction is secured. The problem of the direct-current receiver has been attacked in another way by Ernest E. Yaxley, of the Monarch Telephone Manufacturing Company, with the result shown in Fig. 54. The construction in this case is not unlike that of an ordinary permanent-magnet receiver, except that in the place of the permanent magnets two soft iron cores 1-1 are employed. On these are wound two long bobbins of insulated wire so that the direct current flowing over the telephone line will pass through these and magnetize the cores to the same degree and for the same purpose as in the case of permanent magnets. These soft iron magnet cores 1-1 continue to a point near the coil chamber, where they join the two soft iron pole pieces 2-2, upon which the ordinary voice-current coils are wound. The two long coils 4-4, which may be termed the direct-current coils, are of somewhat lower resistance than the two voice-current coils 3-3. They are, however, by virtue of their greater number of turns and the greater amount of iron that is included in their cores, of much higher impedance than the voice-current coils 3-3. These two sets of coils 4-4 and 3-3 are connected in multiple. As a result of their lower ohmic resistance the coils 4-4 will take a greater amount of the steady current which comes over the line, and therefore the greater proportion of the steady current will be employed in magnetizing the bar magnets. On account of their higher impedance to alternating currents, however, nearly all of the voice currents which are superposed on the steady currents, flowing in the line will pass through the voice-current coils 3-3, and, being near the diaphragm, these currents will so vary the steady magnetism in the cores 2-2 as to produce the necessary vibration of the diaphragm.



This receiver, like the one of the Automatic Electric Company, does not rely on the shell in any respect to maintain the permanency of relation between the pole pieces and the diaphragm. The cup 5, which is of pressed brass, contains the voice-current coils and also acts as a seat for the diaphragm. The entire working parts of this receiver may be removed by merely unscrewing the ear piece from the hard rubber shell, thus permitting the whole works to be withdrawn in an obvious manner.

Dean Receiver. Of such decided novelty as to be almost revolutionary in character is the receiver recently put on the market by the Dean Electric Company and shown in Fig. 55. This receiver is of the direct-current type and employs but a single cylindrical bobbin of wire. The core of this bobbin and the return path for the magnetic lines of force set up in it are composed of soft iron punchings of substantially E shape. These punchings are laid together so as to form a laminated soft-iron field, the limbs of which are about square in cross-section. The coil is wound on the center portion of this E as a core, the core being, as stated, approximately square in cross-section. The general form of magnetic circuit in this instrument is therefore similar to that of the Automatic Electric Company's receiver, shown in Figs. 52 and 53, but the core is laminated instead of being solid as in that instrument.



The most unusual feature of this Dean receiver is that the use of hard rubber or composition does not enter into the formation of the shell, but instead a shell composed entirely of steel stampings has been substituted therefor. The main portion of this shell is the barrel 1. Great skill has evidently been exercised in the forming of this by the cold-drawn process, it presenting neither seams nor welds. The ear piece 2 is also formed of steel of about the same gauge as the barrel 1. Instead of screw-threading the steel parts, so that they would directly engage each other, the ingenious device has been employed of swaging a brass ring 3 in the barrel portion and a similar brass ring 4 in the ear cap portion, these two being slotted and keyed, as shown at 8, so as to prevent their turning in their respective seats. The ring 3 is provided with an external screw thread and the ring 4 with an internal screw thread, so that the receiver cap is screwed on to the barrel in the same way as in the ordinary rubber shell. By the employment of these brass screw-threaded rings, the rusting together of the parts so that they could not be separated when required—a difficulty heretofore encountered in steel construction of similar parts—has been remedied.



The entire working parts of this receiver are contained within the cup 5, the edge of which is flanged outwardly to afford a seat for the diaphragm. The diaphragm is locked in place on the shell by a screw-threaded ring 6, as is clearly indicated. A ring 7 of insulating material is seated within the enlarged portion of the barrel 1, and against this the flange of the cup 5 rests and is held in place by the cap 2 when it is screwed home. The working parts of this receiver partially disassembled are shown in Fig. 56, which gives a clear idea of some of the features not clearly illustrated in Fig. 55.

It cannot be denied that one of the principal items of maintenance of subscribers' station equipment has been due to the breakage of receiver shells. The users frequently allow their receiver to fall and strike heavily against the wall or floor, thus not only subjecting the cords to great strain, but sometimes cracking or entirely breaking the receiver shell. The innovation thus proposed by the Dean Company of making the entire receiver shell of steel is of great interest. The shell, as will be seen, is entirely insulated from the circuit of the receiver so that no contact exists by which a user could receive a shock. The shell is enameled inside and out with a heavy black insulating enamel baked on, and said to be of great durability. How this enamel will wear remains to be seen. The insulation of the interior portions of the receiver is further guarded by providing a lining of fiber within the shell at all points where it seems possible that a cross could occur between some of the working parts and the metal of the shell. This type of receiver has not been on the market long enough to draw definite conclusions, based on experience in use, as to what its permanent performance will be.

Thus far in this chapter only those receivers which are commonly called hand receivers have been discussed. These are the receivers that are ordinarily employed by the general public.



Operator's Receiver. At the central office in telephone exchanges the operators are provided with receivers in order that they may communicate with the subscribers or with other operators. In order that they may have both of their hands free to set up and take down the connections and to perform all of the switching operations required, a special form of receiver is employed for this purpose, which is worn as a part of a head-gear and is commonly termed a head receiver. These are necessarily of very light construction, in order not to be burdensome to the operators, and obviously they must be efficient. They are ordinarily held in place at the ear by a metallic head band fitting over the head of the operator.



Such a receiver is shown in cross-section in Fig. 57, and completely assembled with its head band in Fig. 58. Referring to Fig. 57 the shell 1 of the receiver is of aluminum and the magnets are formed of steel rings 2, cross-magnetized so as to present a north pole on one side of the ring and a south pole on the other. The two L-shaped pole pieces 3 are secured by screws to the poles of these ring magnets, and these pole pieces carry the magnet coils, as is clearly indicated. These poles are presented to a soft iron diaphragm in exactly the same way as in the larger hand receivers, the diaphragm being clamped in place by a hard rubber ear piece, as shown. The head bands are frequently of steel covered with leather. They have assumed numerous forms, but the general form shown in Fig. 58 is the one commonly adopted.



Conventional Symbols. The usual diagrammatic symbols for hand and head receivers are shown in Fig. 59. They are self-explanatory. The symbol at the left in this figure, showing the general outline of the receiver, is the one most commonly used where any sort of a receiver is to be indicated in a circuit diagram, but where it becomes desirable to indicate in the diagram the actual connections with the coil or coils of the receiver, the symbol shown at the right is to be preferred, and obviously it may be modified as to number of windings and form of core as desired.



CHAPTER VII

PRIMARY CELLS

Galvani, an Italian physician, discovered, in 1786, that a current of electricity could be produced by chemical action. In 1800, Volta, a physicist, also an Italian, threw further light on Galvani's discovery and produced what we know as the voltaic, or galvanic, cell. In honor of these two discoverers we have the words volt, galvanic, and the various words and terms derived therefrom.

Simple Voltaic Cell. A very simple voltaic cell may be made by placing two plates, one of copper and one of zinc, in a glass vessel partly filled with dilute sulphuric acid, as shown in Fig. 60. When the two plates are not connected by a wire or other conductor, experiment shows that the copper plate bears a positive charge with respect to the zinc plate, and the zinc plate bears a negative charge with respect to the copper. When the two plates are connected by a wire, a current flows from the copper to the zinc plate through the metallic path of the wire, just as is to be expected when any conductor of relatively high electrical potential is joined to one of relatively low electrical potential. Ordinarily, when one charged body is connected to another of different potential, the resulting current is of but momentary duration, due to the redistribution of the charges and consequent equalization of potential. In the case of the simple cell, however, the current is continuous, showing that some action is maintaining the charges on the two plates and therefore maintaining the difference of potential between them. The energy of this current is derived from the chemical action of the acid on the zinc. The cell is in reality a sort of a zinc-burning furnace.

In the action of the cell, when the two plates are joined by a wire, it may be noticed that the zinc plate is consumed and that bubbles of hydrogen gas are formed on the surface of the copper plate.

Theory. Just why or how chemical action in a voltaic cell results in the production of a negative charge on the consumed plate is not known. Modern theory has it that when an acid is diluted in water the molecules of the acid are split up or dissociated into two oppositely charged atoms, or groups of atoms, one bearing a positive charge and the other a negative charge of electricity. Such charged atoms or groups of atoms are called ions. This separation of the molecules of a chemical compound into positively and negatively charged ions is called dissociation.

Thus, in the simple cell under consideration the sulphuric acid, by dissociation, splits up into hydrogen ions bearing positive charges, and SO_{4} ions bearing negative charges. The solution as a whole is neutral in potential, having an equal number of equal and opposite charges.



It is known that when a metal is being dissolved by an acid, each atom of the metal which is torn off by the solution leaves the metal as a positively charged ion. The carrying away of positive charges from a hitherto neutral body leaves that body with a negative charge. Hence the zinc, or consumed plate, becomes negatively charged.

In the chemical attack of the sulphuric acid on the zinc, the positive hydrogen ions are liberated, due to the affinity of the negative SO_{4} ions for the positive zinc ions, this resulting in the formation of zinc sulphate in the solution. Now the solution itself becomes positively charged, due to the positive charges leaving the zinc plate with the zinc ions, and the free positively charged hydrogen ions liberated in the solution as just described are repelled to the copper plate, carrying their positive charges thereto. Hence the copper plate, or the _unconsumed_ plate, becomes positively charged and also coated with hydrogen bubbles.

The plates or electrodes of a voltaic cell need not consist of zinc and copper, nor need the fluid, called the electrolyte, be of sulphuric acid; any two dissimilar elements immersed in an electrolyte that attacks one of them more readily than the other will form a voltaic cell. In every such cell it will be found that one of the plates is consumed, and that on the other plate some element is deposited, this element being sometimes a gas and sometimes a solid. The plate which is consumed is always the negative plate, and the one on which the element is deposited is always the positive, the current through the connecting wire always being, therefore, from the unconsumed to the consumed plate. Thus, in the simple copper-zinc cell just considered, the zinc is consumed, the element hydrogen is deposited on the copper, and the current flow through the external circuit is from the copper to the zinc.

The positive charges, leaving the zinc, or consumed, plate, and passing through the electrolyte to the copper, or unconsumed, plate, constitute in effect a current of electricity flowing within the electrolyte. The current within the cell passes, therefore, from the zinc plate to the copper plate. The zinc is, therefore, said to be positive with respect to the copper.

Difference of Potential. The amount of electromotive force, that is generated between two dissimilar elements immersed in an electrolyte is different for different pairs of elements and for different electrolytes. For a given electrolyte each element bears a certain relation to another; i.e., they are either electro-positive or electro-negative relative to each other. In the following list a group of elements are arranged with respect to the potentials which they assume with respect to each other with dilute sulphuric acid as the electrolyte. The most electro-positive elements are at the top and the most electro-negative at the bottom.

+Sodium Lead Copper Magnesium Iron Silver Zinc Nickel Gold Cadmium Bismuth Platinum Tin Antimony -Graphite (Carbon)

Any two elements selected from this list and immersed in dilute sulphuric acid will form a voltaic cell, the amount of difference of potential, or electromotive force, depending on the distance apart in this series of the two elements chosen. The current within the cell will always flow from the one nearest the top of the list to the one nearest the bottom, i.e., from the most electro-positive to the most electro-negative; and, therefore, the current in the wire joining the two plates will flow from the one lowest down in the list to the one highest up.

From this series it is easy to see why zinc and copper, and also zinc and carbon, are often chosen as elements of voltaic cells. They are widely separated in the series and comparatively cheap.

This series may not be taken as correct for all electrolytes, for different electrolytes alter somewhat the order of the elements in the series. Thus, if two plates, one of iron and the other of copper, are immersed in dilute sulphuric acid, a current is set up which proceeds through the liquid from the iron to the copper; but, if the plates after being carefully washed are placed in a solution of potassium sulphide, a current is produced in the opposite direction. The copper is now the positive element.

Table II shows the electrical deportment of the principal metals in three different liquids. It is arranged like the preceding one, each metal being electro-positive to any one lower in the list.

TABLE II

Behavior of Metals in Different Electrolytes

- CAUSTIC POTASH HYDROCHLORIC ACID POTASSIUM SULPHIDE - + Zinc + Zinc + Zinc Tin Cadmium Copper Cadmium Tin Cadmium Antimony Lead Tin Lead Iron Silver Bismuth Copper Antimony Iron Bismuth Lead Copper Nickel Bismuth Nickel Silver Nickel - Silver - Antimony - Iron -

It is important to remember that in all cells, no matter what elements or what electrolyte are used, the electrode which is consumed is the one that becomes negatively charged and its terminal, therefore, becomes the negative terminal or pole, while the electrode which is not consumed is the one that becomes positively charged, and its terminal is, therefore, the positive terminal or pole of the cell. However, because the current in the electrolyte flows from the consumed plate to the unconsumed plate, the consumed plate is called the positive plate and the unconsumed, the negative. This is likely to become confusing, but if one remembers that the active plate is the positive plate, because it sends forth positive ions in the electrolyte, and, therefore, itself becomes negatively charged, one will have the proper basis always to determine the direction of the current flow, which is the important thing.

Polarization. If the simple cell already described have its terminals connected by a wire for some time, it will be found that the current rapidly weakens until it ceases to be manifest. This weakening results from two causes: first, the hydrogen gas which is liberated in the action of the cell is deposited in a layer on the copper plate, thereby covering the plate and reducing the area of contact with the liquid. This increases the internal resistance of the cell, since hydrogen is a non-conductor. Second, the plate so covered becomes in effect a hydrogen electrode, and hydrogen stands high as an electro-positive element. There is, therefore, actual reduction in the electromotive force of the cell, as well as an increase in internal resistance. This phenomenon is known as polarization, and in commercial cells means must be taken to prevent such action as far as possible.

The means by which polarization of cells is prevented or reduced in practice may be divided into three general classes:

First—mechanical means. If the hydrogen bubbles be simply brushed away from the surface of the electrode the resistance and the counter polarity which they cause will be diminished. The same result may be secured if air be blown into the solution through a tube, or if the liquid be kept agitated. If the surface of the electrode be roughened or covered with points, the bubbles collect more freely at the points and are more quickly carried away to the surface of the liquid. These means are, however, hardly practical except in cells for laboratory use.

Second—chemical means. If a highly oxidizing substance be added to the electrolyte, it will destroy the hydrogen bubbles by combining with them while they are in a nascent state, and this will prevent the increase in internal resistance and the opposing electromotive force. Such substances are bichromate of potash, nitric acid, and chlorine, and are largely used.

Third—electro-chemical means. Double cells, arranged to separate the elements and liquids by means of porous partitions or by gravity, may be so arranged that solid copper is liberated instead of hydrogen at a point where the current leaves the liquid, thereby entirely obviating polarization. This method also is largely used.