This hook switch is accessible in an easy manner and yet not subject to the tampering of idle or mischievous persons. By taking out the screw 7 the entire hook switch may be lifted out of the tube forming the standard, the cords leading to the various binding posts being slid along through the tube. By this means the connections to the hook switch, as well as the contact of the switch itself, are readily inspected or repaired by those whose duty it is to perform such operations.

Kellogg. In Fig. 87 is shown a sectional view of the desk-stand hook switch of the Kellogg Switchboard and Supply Company. In this it will be seen that instead of placing the switch-hook springs within the standard or tube, as in the case of the Western Electric Company, they are mounted in the base where they are readily accessible by merely taking off the base plate from the bottom of the stand. The hook lever operates on the long spring of the group of switch springs by means of a toggle joint in an obvious manner. This switch spring itself serves by its own strength to raise the hook lever when released from the weight of the receiver.



In this switch, the hook lever, and in fact the entire exposed metal portions of the instrument, are insulated from all of the contact springs and, therefore, there is little liability of shocks on the part of the person using the instrument.

Conventional Symbols. The hook switch plays a very important part in the operation of telephone circuits; for this reason readily understood conventional symbols, by which they may be conveniently represented in drawings of circuits, are desirable. In Fig. 88 are shown several symbols such as would apply to almost any circuit, regardless of the actual mechanical details of the particular hook switch which happened to be employed. Thus diagram A in Fig. 88 shows a hook switch having a single make contact and this diagram might be used to refer to the hook switch of the Dean Electric Company shown in Fig. 85, in which only a single contact is made when the receiver is removed, and none is made when it is on the hook. Similarly, diagram B might be used to represent the hook switch of the Kellogg Company, shown in Fig. 83, the arrangement being for two make and two break contacts. Likewise diagram C might be used to represent the hook switch of the Western Electric Company, shown in Fig. 84, which, as before stated, has two make contacts only. Diagram D shows another modification in which contacts made by the hook switch, when the receiver is removed, control two separate circuits. Assuming that the solid black portion represents insulation, it is obvious that the contacts are divided into two groups, one insulated from the other.



CHAPTER X

ELECTROMAGNETS AND INDUCTIVE COILS

Electromagnet. The physical thing which we call an electromagnet, consisting of a coil or helix of wire, the turns of which are insulated from each other, and within which is usually included an iron core, is by far the most useful of all the so-called translating devices employed in telephony. In performing the ordinary functions of an electromagnet it translates the energy of an electrical current into the energy of mechanical motion. An almost equally important function is the converse of this, that is, the translation of the energy of mechanical motion into that of an electrical current. In addition to these primary functions which underlie the art of telephony, the electromagnetic coil or helix serves a wide field of usefulness in cases where no mechanical motion is involved. As impedance coils, they serve to exert important influences on the flow of currents in circuits, and as induction coils, they serve to translate the energy of a current flowing in one circuit into the energy of a current flowing in another circuit, the translation usually, but not always, being accompanied by a change in voltage.

When a current flows through the convolutions of an ordinary helix, the helix will exhibit the properties of a magnet even though the substance forming the core of the helix is of non-magnetic material, such as air, or wood, or brass. If, however, a mass of iron, such as a rod or a bundle of soft iron wires, for instance, is substituted as a core, the magnetic properties will be enormously increased. The reason for this is, that a given magnetizing force will set up in iron a vastly greater number of lines of magnetic force than in air or in any other non-magnetic material.

Magnetizing Force. The magnetizing force of a given helix is that force which tends to drive magnetic lines of force through the magnetic circuit interlinked with the helix. It is called magnetomotive force and is analogous to electromotive force, that is, the force which tends to drive an electric current through a circuit.

The magnetizing force of a given helix depends on the product of the current strength and the number of turns of wire in the helix. Thus, when the current strength is measured in amperes, this magnetizing force is expressed as ampere-turns, being the product of the number of amperes flowing by the number of turns. The magnetizing force exerted by a given current, therefore, is independent of anything except the number of turns, and the material within the core or the shape of the core has no effect upon it.

Magnetic Flux. The total magnetization resulting from a magnetizing force is called the magnetic flux, and is analogous to current. The intensity of a magnetic flux is expressed by the number of magnetic lines of force in a square centimeter or square inch.

While the magnetomotive force or magnetizing force of a given helix is independent of the material of the core, the flux which it sets up is largely dependent on the material and shape of the core—not only upon this but on the material that lies in the return path for the flux outside of the core. We may say, therefore, that the amount of flux set up by a given current in a given coil or helix is dependent on the material in the magnetic path or magnetic circuit, and on the shape and length of that circuit. If the magnetic circuit be of air or brass or wood or any other non-magnetic material, the amount of flux set up by a given magnetizing force will be relatively small, while it will be very much greater if the magnetic circuit be composed in part or wholly of iron or steel, which are highly magnetic substances.

Permeability. The quality of material, which permits of a given magnetizing force setting up a greater or less number of lines of force within it, is called its permeability. More accurately, the permeability is the ratio existing between the amount of magnetization and the magnetizing force which produces such magnetization.

The permeability of a substance is usually represented by the Greek letter mu (pronounced mu). The intensity of the magnetizing force is commonly symbolized by H, and since the permeability of air is always taken as unity, we may express the intensity of magnetizing force by the number of lines of force per square centimeter which it sets up in air.

Now, if the space on which the given magnetizing force H were acting were filled with iron instead of air, then, owing to the greater permeability of iron, there would be set up a very much greater number of lines of force per square centimeter, and this number of lines of force per square centimeter in the iron is the measure of the magnetization produced and is commonly expressed by the letter B.

From this we have

mu = B/H

Thus, when we say that the permeability of a given specimen of wrought iron under given conditions is 2,000, we mean that 2,000 times as many lines of force would be induced in a unit cross-section of this sample as would be induced by the same magnetizing force in a corresponding unit cross-section of air. Evidently for air B = H, hence mu becomes unity.

The permeability of air is always a constant. This means that whether the magnetic density of the lines of force through the air be great or small the number of lines will always be proportional to the magnetizing force. Unfortunately for easy calculations in electromagnetic work, however, this is not true of the permeability of iron. For small magnetic densities the permeability is very great, but for large densities, that is, under conditions where the number of lines of force existing in the iron is great, the permeability becomes smaller, and an increase in the magnetizing force does not produce a corresponding increase in the total flux through the iron.

Magnetization Curves. This quality of iron is best shown by the curves of Fig. 89, which illustrate the degree of magnetization set up in various kinds of iron by different magnetizing forces. In these curves the ordinates represent the total magnetization B, while the abscissas represent the magnetizing force H. It is seen from an inspection of these curves that as the magnetizing force H increases, the intensity of flux also increases, but at a gradually lessening rate, indicating a reduction in permeability at the higher densities. These curves are also instructive as showing the great differences that exist between the permeability of the different kinds of iron; and also as showing how, when the magnetizing force becomes very great, the iron approaches what is called saturation, that is, a point at which the further increase in magnetizing force will result in no further magnetization of the core.

From the data of the curves of Fig. 89, which are commonly called magnetization curves, it is easy to determine other data from which so-called permeability curves may be plotted. In permeability curves the total magnetization of the given pieces of iron are plotted as abscissas, while the corresponding permeabilities are plotted as ordinates.



Direction of Lines of Force. The lines of force set up within the core of a helix always have a certain direction. This direction always depends upon the direction of the flow of current around the core. An easy way to remember the direction is to consider the helix as grasped in the right hand with the fingers partially encircling it and the thumb pointing along its axis. Then, if the current through the convolutions of the helix be in the direction in which the fingers of the hand are pointed around the helix, the magnetic lines of force will proceed through the core of the helix along the direction in which the thumb is pointed.

In the case of a simple bar electromagnet, such as is shown in Fig. 90, the lines of force emerging from one end of the bar must pass back through the air to the other end of the bar, as indicated by dotted lines and arrows. The path followed by the magnetic lines of force is called the magnetic circuit, and, therefore, the magnetic circuit of the magnet shown in Fig. 90 is composed partly of iron and partly of air. From what has been said concerning the relative permeability of air and of iron, it will be obvious that the presence of such a long air path in the magnetic circuit will greatly reduce the number of lines of force that a given magnetizing force can set up. The presence of an air gap in a magnetic circuit has much the same effect on the total flow of lines of force as the presence of a piece of bad conductor in a circuit composed otherwise of good conductor, in the case of the flow of electric current.

Reluctance. As the property which opposes the flow of electric current in an electrical circuit is called resistance, so the property which opposes the flow of magnetic lines of force in a magnetic circuit is called reluctance. In the case of the electric circuit, the resistance is the reciprocal of the conductivity; in the case of the magnetic circuit, the reluctance is the reciprocal of the permeability. As in the case of an electrical circuit, the amount of flow of current is equal to the electromotive force divided by the resistance; so in a magnetic circuit, the magnetic flux is equal to the magnetizing force or magnetomotive force divided by the reluctance.



Types of Low-Reluctance Circuits. As the pull of an electromagnet upon its armature depends on the total number of lines of force passing from the core to the armature—that is, on the total flux—and as the total flux depends for a given magnetizing force on the reluctance of the magnetic circuit, it is obvious that the design of the electromagnetic circuit is of great importance in influencing the action of the magnet. Obviously, anything that will reduce the amount of air or other non-magnetic material that is in the magnetic circuit will tend to reduce the reluctance, and, therefore, to increase the total magnetization resulting from a given magnetizing force.

Horseshoe Form. One of the easiest and most common ways of reducing reluctance in a circuit is to bend the ordinary bar electromagnet into horseshoe form. In order to make clear the direction of current flow, attention is called to Fig. 91. This is intended to represent a simple bar of iron with a winding of one direction throughout its length. The gap in the middle of the bar, which divides the winding into two parts, is intended merely to mark the fact that the winding need not cover the whole length of the bar and still will be able to magnetize the bar when the current passes through it. In Fig. 92 a similar bar is shown with similar winding upon it, but bent into U-form, exactly as if it had been grasped in the hand and bent without further change. The magnetic polarity of the two ends of the bar remain the same as before for the same direction of current, and it is obvious that the portion of the magnetic circuit which extends through air has been very greatly shortened by the bending. As a result, the magnetic reluctance of the circuit has been greatly decreased and the strength of the magnet correspondingly increased.



If the armature of the electromagnet shown in Fig. 92 is long enough to extend entirely across the air gap from the south to the north pole, then the air gap in the magnetic circuit is still further shortened, and is now represented only by the small gap between the ends of the armature and the ends of the core. Such a magnet, with an armature closely approaching the poles, is called a closed-circuit magnet, since the only gap in the iron of the magnetic circuit is that across which the magnet pulls in attracting its armature.

In Fig. 93 is shown the electrical and magnetic counterpart of Fig. 92. The fact that the magnetic circuit is not a single iron bar but is made up of two cores and one backpiece rigidly secured together, has no bearing upon the principle, but only shows that a modification of construction is possible. In the construction of Fig. 93 the armature 1 is shown as being pulled directly against the two cores 2 and 3, these two cores being joined by a yoke 4, which, like the armature and the core, is of magnetic material. The path of the lines of force is indicated by dotted lines. This is a very important form of electromagnet and is largely used in telephony.

Iron-Clad Form. Another way of forming a closed-circuit magnet that is widely used in telephony is to enclose the helix or winding in a shell of magnetic material which joins the core at one end. This construction results in what is known as the tubular or iron-clad electromagnet, which is shown in section and in end view in Fig. 94. In this the core 1 is a straight bar of iron and it lies centrally within a cylindrical shell 2, also of iron. The bar is usually held in place within the shell by a screw, as shown. The lines of force set up in the core by the current flowing through the coil, pass to the center of the bottom of the iron shell and thence return through the metal of the shell, through the air gap between the edges of the shell and the armature, and then concentrate at the center of the armature and pass back to the end of the core. This is a highly efficient form of closed-circuit magnet, since the magnetic circuit is of low reluctance.



Such forms of magnets are frequently used where it is necessary to mount a large number of them closely together and where it is desired that the current flowing in one magnet shall produce no inductive effect in the coils of the adjacent magnets. The reason why mutual induction between adjacent magnets is obviated in the case of the iron-clad or tubular magnet is that practically all stray field is eliminated, since the return path for the magnetic lines is so completely provided for by the presence of the iron shell.

Special Horseshoe Form. In Fig. 95 is shown a type of relay commonly employed in telephone circuits. The purpose of illustrating it in this chapter is not to discuss relays, but rather to show an adaptation of an electromagnet wherein low reluctance of the magnetic circuit is secured by providing a return leg for the magnetic lines developed in the core, thus forming in effect a horseshoe magnet with a winding on one of its limbs only. To the end of the core 1 there is secured an L-shaped piece of soft iron 2. This extends upwardly and then forwardly throughout the entire length of the magnet core. An L-shaped armature 3 rests on the front edge of the piece 2 so that a slight rocking motion will be permitted on the "knife-edge" bearing thus afforded. It is seen from the dotted lines that the magnetic circuit is almost a closed one. The only gap is that between the lower end of the armature 3 and the front end of the core. When the coil is energized, this gap is closed by the attraction of the armature. As a result, the rearwardly projecting end of the armature 3 is raised and this raises the spring 4 and causes it to break the normally existing contact with the spring 5 and to establish another contact with the spring 6. Thus the energy developed within the coil of the magnet is made to move certain parts which in turn operate the switching devices to produce changes in electrical circuits. These relays and other adaptations of the electromagnet will be discussed more fully later on.



There are almost numberless forms of electromagnets, but we have illustrated here examples of the principal types employed in telephony, and the modifications of these types will be readily understood in view of the general principles laid down.

Direction of Armature Motion. It may be said in general that the armature of an electromagnet always moves or tends to move, when the coil is energized, in such a way as to reduce the reluctance of the magnetic circuit through the coil. Thus, in all of the forms of electromagnets discussed, the armature, when attracted, moves in such a direction as to shorten the air gap and to introduce the iron of the armature as much as possible into the path of the magnetic lines, thus reducing the reluctance. In the case of a solenoid type of electromagnet, or the coil and plunger type, which is a better name than solenoid, the coil, when energized, acts in effect to suck the iron core or plunger within itself so as to include more and more of the iron within the most densely occupied portion of the magnetic circuit.



Differential Electromagnet. Frequently in telephony, the electromagnets are provided with more than one winding. One purpose of the double-wound electromagnet is to produce the so-called differential action between the two windings, i.e., making one of the windings develop magnetization in the opposite direction from that of the other, so that the two will neutralize each other, or at least exert different and opposite influences. The principle of the differential electromagnet may be illustrated in connection with Fig. 96. Here two wires 1 and 2 are shown wrapped in the same direction about an iron core, the ends of the wire being joined together at 3. Obviously, if one of these windings only is employed and a current sent through it, as by connecting the terminals of a battery with the points 4 and 3, for instance, the core will be magnetized as in an ordinary magnet. Likewise, the core will be energized if a current be sent from 5 to 3. Assuming that the two windings are of equal resistance and number of turns, the effects so produced, when either the coil 1 or the coil 2 is energized, will be equal. If the battery be connected between the terminals 4 and 5 with the positive pole, say, at 5, then the current will proceed through the winding 2 and tend to generate magnetism in the core in the direction of the arrow. After traversing the winding 2, however, it will then begin to traverse the other winding 1 and will pass around the core in the opposite direction throughout the length of that winding. This will tend to set up magnetism in the core in the opposite direction to that indicated by the arrow. Since the two currents are equal and also the number of turns in each winding, it is obvious that the two magnetizing influences will be exactly equal and opposite and no magnetic effect will be produced. Such a winding, as is shown in Fig. 96, where the two wires are laid on side by side, is called a parallel differential winding.

Another way of winding magnets differentially is to put one winding on one end of the core and the other winding on the other end of the core and connect these so as to cause the currents through them to flow around the core in opposite directions. Such a construction is shown in Fig. 97 and is called a tandem differential winding. The tandem arrangement, while often good enough for practical purposes, cannot result in the complete neutralization of magnetic effect. This is true because of the leakage of some of the lines of force from intermediate points in the length of the core through the air, resulting in some of the lines passing through more of the turns of one coil than of the other. Complete neutralization can only be attained by first twisting the two wires together with a uniform lay and then winding them simultaneously on the core.



Mechanical Details. We will now consider the actual mechanical construction of the electromagnet. This is a very important feature of telephone work, because, not only must the proper electrical and magnetic effects be produced, but also the whole structure of the magnet must be such that it will not easily get out of order and not be affected by moisture, heat, careless handling, or other adverse conditions.

The most usual form of magnet construction employed in telephony is shown in Fig. 98. On the core, which is of soft Norway iron, usually cylindrical in form, are forced two washers of either fiber or hard rubber. Fiber is ordinarily to be preferred because it is tougher and less liable to breakage. Around the core, between the two heads, are then wrapped several layers of paper or specially prepared cloth in order that the wire forming the winding may be thoroughly insulated from the core. One end of the wire is then passed through a hole in one of the spool heads or washers, near the core, and the wire is then wound on in layers. Sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers. When the last layer is wound on, the end of the wire is passed out through a hole in the head, thus leaving both ends projecting.



Magnet Wire. The wire used in winding magnets is, of course, an important part of the electromagnet. It is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pass around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next, as would be the case if the turns were not insulated. For this purpose the wire is usually covered with a coating of some insulating material. There are, however, methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way, as will be pointed out.

Insulated wire for the purpose of winding magnet coils is termed magnet wire. Copper is the material almost universally employed for the conductor. Its high conductivity, great ductility, and low cost are the factors which make it superior to all other metals. However, in special cases, where exceedingly high conductivity is required with a limited winding space, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding space, either iron or German silver or some other high-resistance alloy is used.

Wire Gauges. Wire for electrical purposes is drawn to a number of different standard gauges. Each of the so-called wire gauges consists of a series of graded sizes of wire, ranging from approximately one-half an inch in diameter down to about the fineness of a lady's hair. In certain branches of telephone work, such as line construction, the existence of the several wire gauges or standards is very likely to lead to confusion. Fortunately, however, so far as magnet wire is concerned, the so-called Brown and Sharpe, or American, wire gauge is almost universally employed in this country. The abbreviations for this gauge are B.&S. or A.W.G.

TABLE III

Copper Wire Table

Giving weights, lengths, and resistances of wire @ 68 deg. F., of Matthiessen's Standard Conductivity.

+ -+ + + -+ + -+ RESISTANCE LENGTH WEIGHT A.W.G. DIAMETER AREA + -+ + -+ B.&S. MILS CIRCULAR OHMS PER OHMS PER FEET PER FEET PER POUNDS PER POUNDS PER MILS POUND FOOT POUND OHM FOOT OHM + -+ + + -+ -+ + -+ + + 0000 460. 211,600. 0.00007639 0.0000489 1.561 20,440. 0.6405 13,090. 000 409.6 167,800. 0.0001215 0.0000617 1.969 16,210. 0.5080 8,232. 00 364.8 133,100. 0.0001931 0.0000778 2.482 12,850. 0.4028 5,177. 0 324.9 105,500. 0.0003071 0.0000981 3.130 10,190. 0.3195 3,256. + -+ + + -+ -+ + -+ + + 1 289.3 83,690. 0.0004883 0.0001237 3.947 8,083. 0.2533 2,048. 2 257.6 66,370. 0.0007765 0.0001560 4.977 6,410. 0.2009 1,288. 3 229.4 52,630. 0.001235 0.0001967 6.276 5,084. 0.1593 810.0 4 204.3 41,740. 0.001963 0.0002480 7.914 4,031. 0.1264 509.4 5 181.9 33,100. 0.003122 0.0003128 9.980 3,197. 0.1002 320.4 6 162.0 26,250. 0.004963 0.0003944 12.58 2,535. 0.07946 201.5 7 144.3 20,820. 0.007892 0.0004973 15.87 2,011. 0.06302 126.7 8 128.5 16,510. 0.01255 0.0006271 20.01 1,595. 0.04998 79.69 9 114.4 13,090. 0.01995 0.0007908 25.23 1,265. 0.03963 50.12 10 101.9 10,380. 0.03173 0.0009273 31.82 1,003. 0.03143 31.52 + -+ + + -+ -+ + -+ + + 11 90.74 8,234. 0.05045 0.001257 40.12 795.3 0.02493 19.82 12 80.81 6,530. 0.08022 0.001586 50.59 630.7 0.01977 12.47 13 71.96 5,178. 0.1276 0.001999 63.79 500.1 0.01568 7.840 14 64.08 4,107. 0.2028 0.002521 80.44 396.6 0.01243 4.931 15 57.07 3,257. 0.3225 0.003179 101.4 314.5 0.009858 3.101 16 50.82 2,583. 0.5128 0.004009 127.9 249.4 0.007818 1.950 17 45.26 2,048. 0.8153 0.005055 161.3 197.8 0.006200 1.226 18 40.30 1,624. 1.296 0.006374 203.4 156.9 0.004917 0.7713 19 35.89 1,288. 2.061 0.008038 256.5 124.4 0.003899 0.4851 20 31.96 1,022. 3.278 0.01014 323.4 98.66 0.003092 0.3051 + -+ + + -+ -+ + -+ + + 21 28.46 810.1 5.212 0.01278 407.8 78.24 0.002452 0.1919 22 25.35 642.4 8.287 0.01612 514.2 62.05 0.001945 0.1207 23 22.57 509.5 13.18 0.02032 648.4 49.21 0.001542 0.07589 24 20.10 404.0 20.95 0.02563 817.6 39.02 0.001223 0.04773 25 17.90 320.4 33.32 0.03231 1,031. 30.95 0.0009699 0.03002 26 15.94 254.1 52.97 0.04075 1,300. 24.54 0.0007692 0.1187 27 14.2 201.5 84.23 0.05138 1,639. 19.46 0.0006100 0.01888 28 12.64 159.8 133.9 0.06479 2,067. 15.43 0.0004837 0.007466 29 11.26 126.7 213.0 0.08170 2,607. 12.24 0.0003836 0.004696 30 10.03 100.5 338.6 0.1030 3,287. 9.707 0.0003042 0.002953 + -+ + + -+ -+ + -+ + + 31 8.928 79.70 538.4 0.1299 4,145. 7.698 0.0002413 0.001857 32 7.950 63.21 856.2 0.1638 5,227. 6.105 0.0001913 0.001168 33 7.080 50.13 1,361. 0.2066 6,591. 4.841 0.0001517 0.0007346 34 6.305 39.75 2,165. 0.2605 8,311. 3.839 0.0001203 0.0004620 35 5.615 31.52 3,441. 0.3284 10,480. 3.045 0.00009543 0.0002905 36 5.0 25.0 5,473. 0.4142 13,210. 2.414 0.00007568 0.0001827 37 4.453 19.83 8,702. 0.5222 16,660. 1.915 0.00006001 0.0001149 38 3.965 15.72 13,870. 0.6585 21,010. 1.519 0.00004759 0.00007210 39 3.531 12.47 22,000. 0.8304 26,500. 1.204 0.00003774 0.00004545 40 3.145 9.888 34,980. 1.047 33,410. 0.9550 0.00002993 0.00002858 + -+ + + -+ -+ + -+ + +



In the Brown and Sharpe gauge the sizes, beginning with the largest, are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger than about No. 16 B.&S. gauge are seldom used as magnet wire in telephony, but for the purpose of making the list complete, Table III is given, including all of the sizes of the B.&S. gauge.

In Table III there is given for each gauge number the diameter of the wire in mils (thousandths of an inch); the cross-sectional area in circular mils (a unit area equal to that of a circle having a diameter of one one-thousandth of an inch); the resistance of the wire in various units of length and weight; the length of the wire in terms of resistance and of weight; and the weight of the wire in terms of its length and resistance.

It is to be understood that in Table III the wire referred to is bare wire and is of pure copper. It is not commercially practicable to use absolutely pure copper, and the ordinary magnet wire has a conductivity equal to about 98 per cent of that of pure copper. The figures given in this table are sufficiently accurate for all ordinary practical purposes.

Silk and Cotton Insulation. The insulating material usually employed for covering magnet wire is of silk or cotton. Of these, silk is by far the better material for all ordinary purposes, since it has a much higher insulating property than cotton, and is very much thinner. Cotton, however, is largely employed, particularly in the larger sizes of magnet wire. Both of these materials possess the disadvantage of being hygroscopic, that is, of readily absorbing moisture. This disadvantage is overcome in many cases by saturating the coil after it is wound in some melted insulating compound, such as wax or varnish or asphaltum, which will solidify on cooling. Where the coils are to be so saturated the best practice is to place them in a vacuum chamber and exhaust the air, after which the hot insulating compound is admitted and is thus drawn into the innermost recesses of the winding space.

Silk-insulated wire, as regularly produced, has either one or two layers of silk. This is referred to commercially as single silk wire or as double silk wire. The single silk has a single layer of silk fibers wrapped about it, while the double silk has a double layer, the two layers being put on in reverse direction. The same holds true of cotton insulated wire. Frequently, also, there is a combination of the two, consisting of a single or a double wrapping of silk next to the wire with an outer wrapping of cotton. Where this is done the cotton serves principally as a mechanical protection for the silk, the principal insulating properties residing in the silk.

Enamel. A later development in the insulation of magnet wire has resulted in the so-called enamel wire. In this, instead of coating the wire with some fibrous material such as silk or cotton, the wire is heated and run through a bath of fluid insulating material or liquid enamel, which adheres to the wire in a very thin coating. The wire is then run through baking ovens, so that the enamel is baked on. This process is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succession.

The characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth: It is desirable for the insulation to possess the highest insulating qualities; to have a glossy, flawless surface; to be hard without being brittle; to adhere tenaciously and stand all reasonable handling without cracking or flaking; to have a coefficient of elasticity greater than the wire itself; to withstand high temperatures; to be moisture-proof and inert to corrosive agencies; and not to "dry out" or become brittle over a long period of time.

Space Utilization. The utilization of the winding space in an electromagnet is an important factor in design, since obviously the copper or other conductor is the only part of the winding that is effective in setting up magnetizing force. The space occupied by the insulation is, in this sense, waste space. An ideally perfect winding may be conceived as one in which the space is all occupied by wire; and this would necessarily involve the conception of wire of square cross-section and insulation of infinite thinness. In such a winding there would be no waste of space and a maximum amount of metal employed as a conductor. Of course, such a condition is not possible to attain and in practice some insulating material must be introduced between the layers of wire and between the adjacent convolutions of wire. The ratio of the space occupied by the conductor to the total space occupied by the winding, that is, by the conductor and the insulation, is called the coefficient of space utilization of the coil. For the ideal coil just conceived the coefficient of space utilization would be 1. Ordinarily the coefficient of space utilization is greater for coarse wire than for fine wire, since obviously the ratio of the diameter of the wire to the thickness of the insulation increases as the size of the wire grows larger.

The chief advantage of enamel insulation for magnet wire is its thinness, and the high coefficient of space utilization which may be secured by its use. In good enamel wire the insulation will average about one-quarter the thickness of the standard single silk insulation, and the dielectric strength is equal or greater. Where economy of winding space is desirable the advantages of this may readily be seen. For instance, in a given coil wound with No. 36 single silk wire about one-half of the winding space is taken up with the insulation, whereas when the same coil is wound with No. 36 enameled wire only about one-fifth of the winding space is taken up by the insulation. Thus the coefficient of space utilization is increased from .50 to .80. The practical result of this is that, in the case of any given winding space where No. 36 wire is used, about 60 per cent more turns can be put on with enameled wire than with single silk insulation, and of course this ratio greatly increases when the comparison is made with double silk insulation or with cotton insulation. Again, where it is desired to reduce the winding space and keep the same number of turns, an equal number of turns may be had with a corresponding reduction of winding space where enameled wire is used in place of silk or cotton.

In the matter of heat-resisting properties the enameled wire possesses a great advantage over silk and cotton. Cotton or silk insulation will char at about 260 deg. Fahrenheit, while good enameled wire will stand 400 deg. to 500 deg. Fahrenheit without deterioration of the insulation. It is in the matter of liability to injury in rough or careless handling, or in winding coils having irregular shapes, that enamel wire is decidedly inferior to silk or cotton-covered wire. It is likely to be damaged if it is allowed to strike against the sharp corners of the magnet spool during winding, or run over the edge of a hard surface while it is being fed on to the spool. Coils having other than round cores, or having sharp corners on their spool heads, should not ordinarily be wound with enamel wire.

The dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickness. This is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire. If wire insulated with enamel is given a single wrapping of silk or of cotton, the insulating and dielectric properties of the enamel is secured, while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel.

Winding Methods. In winding a coil, the spool, after being properly prepared, is placed upon a spindle which may be made to revolve rapidly. Ordinarily the wire is guided on by hand; sometimes, however, machinery is used, the wire being run over a tool which moves to and fro along the length of the spool, just fast enough to lay the wire on at the proper rate. The movement of this tool is much the same as that of the tool in a screw cutting lathe.

Unless high voltages are to be encountered, it is ordinarily not necessary to separate the layers of wire with paper, in the case of silk-or cotton-insulated magnet wire; although where especially high insulation resistance is needed this is often done. It is necessary to separate the successive layers of a magnet that is wound with enamel wire, by sheets of paper or thin oiled cloth.



In Fig. 99 is shown a method, that has been used with some success, of winding magnets with bare wire. In this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire. Each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper. This is essentially a machine-wound coil, and machines for winding it have been so perfected that several coils are wound simultaneously, the paper being fed in automatically at the end of each layer.

Another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil, due to the slight sinking into the layer of paper on which it is wound. In this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire, so that a small distance will be left between each pair of adjacent turns.

Upon the completion of the winding of a coil, regardless of what method is used, it is customary to place a layer of bookbinders' cloth over the coil so as to afford a certain mechanical protection for the insulated wire.

Winding Terminals. The matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes. Where the winding is of fine wire, it is always well to reinforce its ends by a short piece of larger wire. Where this is done the larger wire is given several turns around the body of the coil, so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire. Great care is necessary in the bringing out of the inner terminal—i.e., the terminal which connects with the inner layer—that the terminal wire shall not come in contact with any of the subsequent layers that are wound on.



Where economy of space is necessary, a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head. This, in the case of a fiber spool head, may be done by driving heavy metal terminals into the fiber. The connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder. A coil having such terminals is shown in its finished condition in Fig. 100.

Winding Data. The two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding and the resistance of the wound wire. The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing. While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter. When the coil has been completed it is a simple matter to measure its resistance. But it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool, and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound.

TABLE IV

Winding Data for Insulated Wires—Silk and Cotton Covering

A.W.G. B & S 20 21 22 23 24 25 - DIAMETER Mils 31.961 28.462 25.347 22.571 20.100 17.900 - AREA Circular Mils 1021.20 810.10 642.70 509.45 404.01 320.40 - DIAMETER OVER INSULATION SINGLE COTTON 37.861 34.362 31.247 28.471 26.000 23.800 DOUBLE COTTON 42.161 38.662 35.547 32.771 30.300 28.100 SINGLE SILK 34.261 30.762 27.647 24,871 22.401 20.200 DOUBLE SILK 36.161 32.662 29.547 26.771 24.300 22.100 - TURNS PER LINEAR INCH SINGLE COTTON 25.7 28.3 31.0 34.4 36.9 38.0 DOUBLE 22.5 24.5 26.7 28.97 31.35 33.92 COTTON SINGLE SILK 27.70 30.97 34.39 38.19 42.37 47.02 DOUBLE SILK 26.22 29.07 32.11 35.53 39.14 42.94 - TURNS PER SQUARE INCH SINGLE COTTON 660.5 800.9 961.0 1183.0 1321.6 1444.0 DOUBLE COTTON 506.3 600.2 712.9 839.2 982.8 1150.8 SINGLE SILK 767.3 959.1 1182.7 1458.5 1795.2 2210.9 DOUBLE SILK 687.5 845.0 1031.0 1262.4 1532.0 1843.8 - OHMS PER CUBIC INCH SINGLE COTTON .646 .981 1.502 2.359 3.528 5.831 DOUBLE COTTON .533 .795 1.188 1.772 2.595 3.802 SINGLE SILK .801 1.261 1.956 3.049 4.739 7.489 -

A.W.G. B & S 26 27 28 29 30 31 - DIAMETER Mils 15.940 14.195 12.641 11.257 10.025 8.928 - AREA Circular Mils 254.01 201.50 159.79 126.72 100.50 79.71 - DIAMETER OVER INSULATION SINGLE COTTON 21.840 20.095 18.541 17.157 15.925 14.828 DOUBLE COTTON 26.140 24.395 22.841 21.457 20.225 19.128 SINGLE SILK 18.240 16.495 14.941 13.557 12.325 11.228 DOUBLE SILK 20.140 18.395 16.841 15.457 14.225 13.128 - TURNS PER LINEAR INCH SINGLE COTTON 42.0 48.0 53.0 56.5 59.66 64.125 DOUBLE COTTON 36.29 38.95 41.61 44.27 46.93 49.78 SINGLE SILK 52.06 57.67 63.36 70.11 77.14 84.64 DOUBLE SILK 46.81 51.59 56.43 61.56 66.79 72.39 - TURNS PER SQUARE INCH SINGLE COTTON 1764.0 2304.0 2809.9 3192.3 3359.2 4112.2 DOUBLE COTTON 1317.0 1517.2 1731.0 1959.9 2202.5 2478.0 SINGLE SILK 2710.3 3326.0 4014.5 4915.5 5950.2 7164.0 DOUBLE SILK 2191.2 2661.6 3184.5 3789.8 4461.0 5240.0 - OHMS PER CUBIC INCH SINGLE COTTON 6.941 10.814 17.617 25.500 34.800 48.5 DOUBLE COTTON 5.552 8.078 11.54 16.47 23.43 32.83 SINGLE SILK 9.031 13.92 26.86 41.29 62.98 95.70 -

A.W.G. B & S 32 33 34 35 36 37 DIAMETER Mils 7.950 7.080 6.304 5.614 5.000 4.453 AREA Circular Mils 63.20 50.13 39.74 31.52 25.00 19.83 DIAMETER OVER INSULATION SINGLE COTTON 13.850 12.980 12.204 11.514 10.900 10.353 DOUBLE COTTON 18.150 17.280 16.504 15.814 15.200 14.653 SINGLE SILK 10.250 9.380 8.504 7.914 7.300 6.753 DOUBLE SILK 12.150 11.280 10.504 9.814 9.200 8.653 TURNS PER LINEAR INCH SINGLE COTTON 68.600 73.050 77.900 82.600 87.100 91.870 DOUBLE COTTON 52.34 55.10 57.57 60.04 62.51 64.70 SINGLE SILK 92.72 101.65 112.11 119.7 130.15 140.6 DOUBLE SILK 78.19 84.17 90.44 96.90 103.55 110.20 TURNS PER SQUARE INCH SINGLE 4692.5 5333.5 6068.5 6773.3 7586.5 8440.0 COTTON DOUBLE COTTON 2739.5 3036.1 3314.2 3605.0 3907.5 4186.1 SINGLE SILK 8597.5 10332.0 12570.0 14327.0 16940.0 19770.0 DOUBLE SILK 6114.0 7085.0 8179.5 9389.5 10772.0 12145.0 - OHMS PER CUBIC INCH SINGLE COTTON 73.8 104.5 151.4 202.0 298.8 418.0 DOUBLE COTTON 46.19 64.30 70.58 125.9 166.3 225.6 SINGLE SILK 144.70 217.8 342.1 489.0 721.1 1062.0 -

A.W.G. B & S 38 39 40 DIAMETER Mils 3.965 3.531 3.144 AREA Circular Mils 15.72 12.47 9.89 DIAMETER OVER INSULATION SINGLE COTTON 9.865 9.431 9.044 DOUBLE COTTON 14.165 13.731 13.344 SINGLE SILK 6.265 5.831 5.344 DOUBLE SILK 8.165 7.731 7.344 TURNS PER LINEAR INCH SINGLE COTTON 95.000 100.700 106.000 DOUBLE COTTON 66.80 68.80 71.20 SINGLE SILK 151.05 163.04 177.65 DOUBLE SILK 116.85 122.55 129.20 TURNS PER SQUARE INCH SINGLE COTTON 9025.0 10140.5 11236.0 DOUBLE 4462.2 4733.6 5069.8 COTTON SINGLE SILK 22820.0 26700.0 31559.0 DOUBLE SILK 13655.0 15018.0 16692.0 OHMS PER CUBIC INCH SINGLE COTTON 567.0 811.0 1113.0 DOUBLE 305.5 409.8 545.5 COTTON SINGLE SILK 1557.0 2266.0 3400.0 -

If the length and the depth of the winding space of the coil as well as the diameter of the core are known, it is not difficult to determine how much bare copper wire of a given size may be wound on it, but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk. Yet something may be done, and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation. As a result of facts collected from a large number of actually wound coils, the number of turns per linear inch and per square inch of B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these, supplemented by a tabulation of the number of ohms per cubic inch of winding space for wires of three different kinds of insulation, are given in Table IV.

Bearing in mind that the calculations of Table IV are all based upon the "diameter over insulation," which it states at the outset for each of four different kinds of covering, it is evident what is meant by "turns per linear inch." The columns referring to "turns per square inch" mean the number of turns, the ends of which would be exposed in one square inch if the wound coil were cut in a plane passing through the axis of the core. Knowing the distance between the head, and the depth to which the coil is to be wound, it is easy to select a size of wire which will give the required number of turns in the provided space. It is to be noted that the depth of winding space is one-half of the difference between the core diameter and the complete diameter of the wound coil. The resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding space and multiplying this by the ohms per cubic inch of the selected wire; that is, one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding space, and this in turn by .7854. This result, times the ohms per cubic inch, as given in the table, gives the resistance of the winding.

There is a considerable variation in the method of applying silk insulation to the finer wires, and it is in the finer sizes that the errors, if any, pile up most rapidly. Yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils. It should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires. This table has been compared with many examples and has been used in calculating windings in advance, and is found to be as close an approximation as is afforded by any of the formulas on the subject, and with the further advantage that it is not so cumbersome to apply.

Winding Calculations. In experimental work, involving the winding of coils, it is frequently necessary to try one winding to determine its effect in a given circuit arrangement, and from the knowledge so gained to substitute another just fitted to the conditions. It is in such a substitution that the table is of most value. Assume a case in which are required a spool and core of a given size with a winding of, say No. 25 single silk-covered wire, of a resistance of 50 ohms. Assume also that the circuit regulations required that this spool should be rewound so as to have a resistance of, say 1,000 ohms. What size single silk-covered wire shall be used? Manifestly, the winding space remains the same, or nearly so. The resistance is to be increased from 50 to 1,000 ohms, or twenty times its first value. Therefore, the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in No. 25, the known first size. This amount would be twenty times 7.489, which is 149.8, but there is no size giving this exact resistance. No. 32, however, is very nearly of that resistance and if wound to exactly the same depth would give about 970 ohms. A few turns more would provide the additional thirty ohms.

Similarly, in a coil known to possess a certain number of turns, the table will give the size to be selected for rewinding to a greater or smaller number of turns. In this case, as in the case of substituting a winding of different resistance, it is unnecessary to measure and calculate upon the dimensions of the spool and core. Assume a spool wound with No. 30 double silk-covered wire, which requires to be wound with a size to double the number of turns. The exact size to do this would have 8922. turns per square inch and would be between No. 34 and No. 35. A choice of these two wires may be made, using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire.

Impedance Coils. In telephony electromagnets frequently serve, as already stated, to perform other functions than the producing of motion by attracting or releasing their armatures. They are required to act as impedance coils to present a barrier to the passage of alternating or other rapidly fluctuating currents, and at the same time to allow the comparatively free passage of steady currents. Where it is desired that an electromagnet coil shall possess high impedance, it is usual to employ a laminated instead of a solid core. This is done by building up a core of suitable size by laying together thin sheets of soft iron, or by forming a bundle of soft iron wires. The use of laminated cores is for the purpose of preventing eddy currents, which, if allowed to flow, would not only be wasteful of energy but would also tend to defeat the desired high impedance. Sometimes in iron-clad impedance coils, the iron shell is slotted longitudinally to break up the flow of eddy currents in the shell.

Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed impedance, or retardation, or choke coils, since they are employed to impede or to retard or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each.

Number of Turns. It should be remembered that an impedance coil obstructs the passage of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force. Other things being equal, the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil, and this latter factor depends also on the reluctance of the magnetic circuit. Therefore, to secure high impedance we need many turns or low reluctance, or both. Often, owing to requirements for direct-current carrying capacity and limitations of space, a very large number of turns is not permissible, in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance, usually a completely closed circuit.

Kind of Iron. An important factor in the design of impedance coils is the grade of iron used in the magnetic circuit. Obviously, it should be of the highest permeability and, furthermore, there should be ample cross-section of core to prevent even an approach to saturation. The iron should, if possible, be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects.

Types. Open-Circuit:—Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding.



Closed-Circuit:—A type of retardation coil which is largely used in systems of simultaneous telegraphy and telephony, known as composite systems, is shown in Fig. 103. In the construction of this coil the core is made of a bundle of fine iron wires first bent into U-shape, and then after the coils are in place, the free ends of the core are brought together to form a closed magnetic circuit. The coils have a large number of turns of rather coarse wire. The conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity, and fortunately the added requirement, that it shall be placed in a very small space, does not exist.

Toroidal:—Another type of retardation coil, called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire, is shown in diagram in Fig. 104. The two windings of this coil may be connected in series to form in effect a single winding, or it may be used as a "split-winding" coil, the two windings being in series but having some other element, such as a battery, connected between them in the circuit. Evidently such a coil, however connected, is well adapted for high impedance, on account of the low reluctance of its core.



This coil is usually mounted on a base-board, the coil being enclosed in a protecting iron case, as shown in Fig. 105. The terminal wires of both windings of each coil are brought out to terminal punchings on one end of the base-board to facilitate the making of the necessary circuit connections.



The usual diagrammatic symbol for an impedance coil is shown in Fig. 106. This is the same as for an ordinary bar magnet, except that the parallel lines through the core may be taken as indicating that the core is laminated, thus conveying the idea of high impedance. The symbol of Fig. 104 is a good one for the toroidal type of impedance coil.



Induction Coil. An induction coil consists of two or more windings of wire interlinked by a common magnetic circuit. In an induction coil having two windings, any change in the strength of the current flowing in one of the windings, called the primary, will cause corresponding changes in the magnetic flux threading the magnetic circuit, and, therefore, changes in flux through the other winding, called the secondary. This, by the laws of electromagnetic induction, will produce corresponding electromotive forces in the secondary winding and, therefore, corresponding currents in that winding if its circuit be closed.

Current and Voltage Ratios. In a well-designed induction coil the energy in the secondary, i.e., the induced current, is for all practical purposes equal to that of the primary current, yet the values of the voltage and the amperage of the induced current may vary widely from the values of the voltage and the amperage of the primary current. With simple periodic currents, such as the commercial alternating lighting currents, the ratio between the voltage in the primary and that in the secondary will be equal to the ratio of the number of turns in the primary to the number of turns in the secondary. Since the energy in the two circuits will be practically the same, it follows that the ratio between the current in the primary and that in the secondary will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary. In telephony, where the currents are not simple periodic currents, and where the variations in current strength take place at different rates, such a law as that just stated does not hold for all cases; but it may be stated in general that the induced currents will be of higher voltage and smaller current strength than those of the primary in all coils where the secondary winding has a greater number of turns than the primary, and vice versa.

Functions. The function of the induction coil in telephony is, therefore, mainly one of transformation, that is, either of stepping up the voltage of a current, or in other cases stepping it down. The induction coil, however, does serve another purpose in cases where no change in voltage and current strength is desired, that is, it serves as a means for electrically separating two circuits so far as any conductive relation exists, and yet of allowing the free transmission by induction from one of these circuits to the other. This is a function that in telephony is scarcely of less importance than the purely transforming function.

Design. Induction coils, as employed in telephony, may be divided into two general types: first, those having an open magnetic circuit; and, second, those having a closed magnetic circuit. In the design of either type it is important that the core should be thoroughly laminated, and this is done usually by forming it of a bundle of soft Swedish or Norway iron wire about .02 of an inch in diameter. The diameter and the length of the coil, and the relation between the number of turns in the primary and in the secondary, and the mechanical construction of the coil, are all matters which are subject to very wide variation in practice. While the proper relationship of these factors is of great importance, yet they may not be readily determined except by actual experiment with various coils, owing to the extreme complexity of the action which takes place in them and to the difficulty of obtaining fundamental data as to the existing facts. It may be stated, therefore, that the design of induction coils is nearly always carried out by "cut-and-try" methods, bringing to bear, of course, such scientific and practical knowledge as the experimenter may possess.



Use and Advantage. The use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters. Such induction coils are nearly always of the open magnetic circuit type, consisting of a long, straight core comprised of a bundle of small annealed iron wires, on which is wound a primary of comparatively coarse wire and having a small number of turns, and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns. A view of such a coil mounted on a base is shown in Fig. 107, and a sectional view of a similar coil is shown in Fig. 108. The method of bringing out the winding terminals is clearly indicated in this figure, the terminal wires 2 and 4 being those of the primary winding and 1 and 3 those of the secondary winding. It is customary to bring out these wires and attach them by solder to suitable terminal clips. In the case of the coil shown in Fig. 108 these clips are mounted on the wooden heads of the coil, while in the design shown in Fig. 107 they are mounted on the base, as is clearly indicated.

Repeating Coil. The so-called repeating coil used in telephony is really nothing but an induction coil. It is used in a variety of ways and usually has for its purpose the inductive association of two circuits that are conductively separated. Usually the repeating coil has a one to one ratio of turns, that is, there are the same number of turns in the primary as in the secondary. However, this is not always the case, since sometimes they are made to have an unequal number of turns, in which case they are called step-up or step-down repeating coils, according to whether the primary has a smaller or a greater number of turns than the secondary. Repeating coils are almost universally of the closed magnetic circuit type.

Ringing and Talking Considerations. Since repeating coils often serve to connect two telephones, it follows that it is sometimes necessary to ring through them as well as talk through them. By this is meant that it is necessary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents. Ringing currents ordinarily have a frequency ranging from about 16 to 75 cycles per second, while voice currents have frequencies ranging from a few hundred up to perhaps ten thousand per second. Ordinarily, therefore, the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and vice versa. If the comparatively heavy ringing currents alone were to be considered, the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit. On the other hand, for carrying voice currents alone it is usually made with a small amount of iron and with small windings, in order to prevent waste of energy in the core, and to give a high degree of responsiveness with the least amount of distortion of wave form, so that the voice currents will retain as far as possible their original characteristics. When, therefore, a coil is required to carry both ringing and talking currents, a compromise must be effected.

Types. The form of repeating coil largely used for both ringing and talking through is shown in Fig. 109. This coil comprises a soft iron core made up of a bundle of wires about .02 inch in diameter, the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core. The windings of this particular coil are four in number, and contain about 2,400 turns each, and have a resistance of about 60 ohms. In this coil, when connected for local battery work, the windings are connected in pairs in series, thus forming effectively two windings having about 120 ohms resistance each. The whole coil is enclosed in a protecting case of iron. The terminals are brought out to suitable clips on the wooden base, as shown. An external perspective view of this coil is shown in Fig. 110. By bringing out each terminal of each winding, eight in all, as shown in this figure, great latitude of connection is provided for, since the windings may be connected in circuit in any desirable way, either by connecting them together in pairs to form virtually a primary and a secondary, or, as is frequently the case, to split the primary and the secondary, connecting a battery between each pair of windings.



Fig. 111 illustrates in section a commercial type of coil designed for talking through only. This coil is provided with four windings of 1,357 turns each, and when used for local battery work the coils are connected in pairs in series, thus giving a resistance of about 190 ohms in each half of the repeating coil. The core of this coil consists of a bundle of soft iron wires, and the shell which forms the return path for the magnetic lines is of very soft sheet iron. This shell is drawn into cup shape and its open end is closed, after the coil is inserted, by the insertion of a soft iron head, as indicated. As in the case of the coil shown in Figs. 109 and 110, eight terminals are brought out on this coil, thus providing the necessary flexibility of connection.



Still another type of repeating coil is illustrated in diagram in Fig. 112, and in view in Fig. 113. This coil, like the impedance coil shown in Fig. 104, comprises a core made up of a bundle of soft iron wires wound into the form of a ring. It is usually provided with two primary windings placed opposite each other upon the core, and with two secondary windings, one over each primary. In practice these two primary windings are connected in one circuit and the two secondaries in another. This is the standard repeating coil now used by the Bell companies in their common-battery cord circuits.



Conventional Symbols. The ordinary symbol for the induction coil used in local battery work is shown in Fig. 114. This consists merely of a pair of parallel zig-zag lines. The primary winding is usually indicated by a heavy line having a fewer number of zig-zags, and the secondary by a finer line having a greater number of zig-zags. In this way the fact that the primary is of large wire and of comparatively few turns is indicated. This diagrammatic symbol may be modified to suit almost any conditions, and where a tertiary as well as a secondary winding is provided it may be shown by merely adding another zig-zag line.



The repeating coil is indicated symbolically in the two diagrams of Fig. 115. Where there is no necessity for indicating the internal connections of the coil, the symbol shown in the left of this figure is usually employed. Where, however, the coil consists of four windings rather than two and the method of connecting them is to be indicated, the symbol at the right hand is employed. In Fig. 116 another way of indicating a four-winding repeating coil or induction coil is shown. Sometimes such windings may be combined by connection to form merely a primary and a secondary winding, and in other cases the four windings all act separately, in which case one may be considered the primary and the others, respectively, the secondary, tertiary, and quaternary.



Where the toroidal type of repeating coil is employed, the diagram of Fig. 112, already referred to, is a good symbolic representation.



CHAPTER XI

NON-INDUCTIVE RESISTANCE DEVICES

It is often desired to introduce simple ohmic resistance into telephone circuits, in order to limit the current flow, or to create specific differences of potential at given points in the circuit.

Temperature Coefficient. The design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed. The resistance of conductors is subject to change by changes in temperature. While nearly all metals show an increase, carbon shows a decrease in its resistance when heated.

The temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree.

TABLE V

Temperature Coefficients

+ -+ -+ PURE METALS TEMPERATURE COEFFICIENTS + -+ + + CENTIGRADE FAHRENHEIT + -+ + + Silver (annealed) 0.00400 0.00222 Copper (annealed) 0.00428 0.00242 Gold (99.9%) 0.00377 0.00210 Aluminum (99%) 0.00423 0.00235 Zinc 0.00406 0.00226 Platinum (annealed) 0.00247 0.00137 Iron 0.00625 0.00347 Nickel 0.0062 0.00345 Tin 0.00440 0.00245 Lead 0.00411 0.00228 Antimony 0.00389 0.00216 Mercury 0.00072 0.00044 Bismuth 0.00354 0.00197 + -+ + +

Positive and Negative Coefficients. Those conductors, in which a rise in temperature produces an increase in resistance, are said to have positive temperature coefficients, while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients.

The temperature coefficients of pure metals are always positive and for some of the more familiar metals, have values, according to Foster, as in Table V.

Iron, it will be noticed, has the highest temperature coefficient of all. Carbon, on the other hand, has a large negative coefficient, as proved by the fact that the filament of an ordinary incandescent lamp has nearly twice the resistance when cold as when heated to full candle-power.

Certain alloys have been produced which have very low temperature coefficients, and these are of value in producing resistance units which have practically the same resistance for all ordinary temperatures. Some of these alloys also have very high resistance as compared with copper and are of value in enabling one to obtain a high resistance in small space.

One of the most valuable resistance wires is of an alloy known as German silver. The so-called eighteen per cent alloy has approximately 18.3 times the resistance of copper and a temperature coefficient of .00016 per degree Fahrenheit. The thirty per cent alloy has approximately 28 times the resistance of copper and a temperature coefficient of .00024 per degree Fahrenheit.

For facilitating the design of resistance coils of German silver wire, Tables VI and VII are given, containing information as to length, resistance, and weight of the eighteen per cent and the thirty per cent alloys, respectively, for all sizes of wire smaller than No. 20 B. & S. gauge.

Special resistance alloys may be obtained having temperature coefficients as low as .000003 per degree Fahrenheit. Other alloys of nickel and steel are adapted for use where the wire must carry heavy currents and be raised to comparatively high temperatures thereby; for such use non-corrosive properties are specially to be desired. Such wire may be obtained having a resistance of about fifty times that of copper.

TABLE VI

18 Per Cent German Silver Wire

- - - No. B. & S. DIAMETER WEIGHT LENGTH RESISTANCE GAUGE INCHES POUNDS PER FOOT FEET PER POUND OHMS PER FOOT - - - 21 .02846 .002389 418.6 .2333 22 .02535 .001894 527.9 .2941 23 .02257 .001502 665.8 .3710 24 .02010 .001191 839.5 .4678 25 .01790 .0009449 1058. .5899 26 .01594 .0007493 1335. .7438 27 .01419 .0005943 1683. .9386 28 .01264 .0004711 2123. 1.183 29 .01126 .0003735 2677. 1.491 30 .01003 .0002962 3376. 1.879 31 .008928 .0002350 4255. 2.371 32 .007950 .0001864 5366. 2.990 33 .007080 .0001478 6766. 3.771 34 .006304 .0001172 8532. 4.756 35 .005614 .00009295 10758. 5.997 36 .005000 .00007369 13569. 7.560 37 .004453 .00005845 17108. 9.532 38 .003965 .00004636 21569. 12.02 39 .003531 .00003675 27209. 15.16 40 .003145 .00002917 34282. 19.11 - - -

Inductive Neutrality. Where the resistance unit is required to be strictly non-inductive, and is to be in the form of a coil, special designs must be employed to give the desired inductive neutrality.

Provisions Against Heating. In cases where a considerable amount of heat is to be generated in the resistance, due to the necessity of carrying large currents, special precautions must be taken as to the heat-resisting properties of the structure, and also as to the provision of sufficient radiating surface or its equivalent to provide for the dissipation of the heat generated.

Types. Mica Card Unit. One of the most common resistance coils used in practice is shown in Fig. 117. This comprises a coil of fine, bare German silver wire wound on a card of mica, the windings being so spaced that the loops are not in contact with each other. The winding is protected by two cards of mica and the whole is bound in place by metal strips, to which the ends of the winding are attached. Binding posts are provided on the extended portions of the terminals to assist in mounting the resistance on a supporting frame, and the posts terminate in soldering terminals by which the resistance is connected into the circuit.

TABLE VII

30 Per Cent German Silver Wire

- - - No. B. & S. DIAMETER WEIGHT LENGTH RESISTANCE GAUGE INCHES POUNDS PER FOOT FEET PER POUND OHMS PER FOOT - - - 21 .02846 .002405 415.8 .3581 22 .02535 .001907 524.4 .4513 23 .02257 .001512 661.3 .5693 24 .02010 .001199 833.9 .7178 25 .01790 .0009513 1051. .9051 26 .01594 .0007544 1326. 1.141 27 .01419 .0005983 1671. 1.440 28 .01264 .0004743 2108. 1.815 29 .01126 .0003761 2659. 2.287 30 .01003 .0002982 3353. 2.883 31 .008928 .0002366 4227. 3.638 32 .007950 .0001876 5330. 4.588 33 .007080 .0001488 6721. 5.786 34 .006304 .0001180 8475. 7.297 35 .005614 .00009358 10686. 9.201 36 .005000 .00007419 13478. 11.60 37 .004453 .00005885 16994. 14.63 38 .003965 .00004668 21424. 18.45 39 .003531 .00003700 27026. 23.26 40 .003145 .00002937 34053. 29.32 - - -

Differentially-Wound Unit. Another type of resistance coil is that in which the winding is placed upon an insulating core of heat-resisting material and wound so as to overcome inductive effects. In order to accomplish this, the wire to be bound on the core is doubled back on itself at its middle portion to form two strands, and these are wound simultaneously on the core, thus forming two spirals of equal number of turns. The current in traversing the entire coil must flow through one spiral in one direction with relation to the core, and in the opposite direction in the other spiral, thereby nullifying the inductive effects of one spiral by those of the other. This is called a non-inductive winding and is in reality an example of differential winding.

Lamp Filament. An excellent type of non-inductive resistance is the ordinary carbon-filament incandescent lamp. This is used largely in the circuits of batteries, generators, and other sources of supply to prevent overload in case of short circuits on the line. These are cheap, durable, have large current-carrying capacities, and are not likely to set things afire when overheated. An additional advantage incident to their use for this purpose is that an overload on a circuit in which they are placed is visibly indicated by the glowing of the lamp.



Obviously, the carbon-filament incandescent lamp, when used as a resistance, has, on account of the negative temperature coefficient of carbon, the property of presenting the highest resistance to the circuit when carrying no current, and of presenting a lower and lower resistance as the current and consequent heating increases. For some conditions of practice this is not to be desired, and the opposite characteristic of presenting low resistance to small currents and comparatively high resistance to large currents would best meet the conditions of practice.

Iron-Wire Ballast. Claude D. Enochs took advantage of the very high positive temperature coefficient of iron to produce a resistance device having these characteristics. His arrangement possesses the compactness of the carbon-filament lamp and is shown in Fig. 118. The resistance element proper is an iron wire, wound on a central stem of glass, and this is included in an exhausted bulb so as to avoid oxidation. Such a resistance is comparatively low when cold, but when traversed by currents sufficient to heat it considerably will offer a very large increase of resistance to oppose the further increase of current. In a sense, it is a self-adjusting resistance, tending towards the equalization of the flow of current in the circuit in which it is placed.



CHAPTER XII

CONDENSERS

Charge. A conducting body insulated from all other bodies will receive and hold a certain amount of electricity (a charge), if subjected to an electrical potential. Thus, referring to Fig. 119, if a metal plate, insulated from other bodies, be connected with, say, the positive pole of a battery, the negative pole of which is grounded, a current will flow into the plate until the plate is raised to the same potential as that of the battery pole to which it is connected. The amount of electricity that will flow into the plate will depend, other things being equal, on the potential of the source from which it is charged; in fact, it is proportional to the potential of the source from which it is charged. This amount of electricity is a measure of the capacity of the plate, just as the amount of water that a bath-tub will hold is a measure of the capacity of the bath-tub.

Capacity. Instead of measuring the amount of electricity by the quart or pound, as in the case of material things, the unit of electrical quantity is the coulomb. The unit of capacity of an insulated conductor is the farad, and a given insulated conductor is said to have unit capacity, that is, the capacity of one farad, when it will receive a charge of one coulomb of electricity at a potential of one volt.

Referring to Fig. 119, the potential of the negative terminal of the battery may be said to be zero, since it is connected to the earth. If the battery shown be supposed to have exactly one volt potential, then the plate would be said to have the capacity of one farad if one coulomb of electricity flowed from the battery to the plate before the plate was raised to the same potential as that of the positive pole, that is, to a potential of one volt above the potential of the earth; it being assumed that the plate was also at zero potential before the connection was made. Another conception of this quantity may be had by remembering that a coulomb is such a quantity of current as will result from one ampere flowing one second.

The capacity of a conductor depends, among other things, on its area. If the plate of Fig. 119 should be made twice as large in area, other things remaining the same, it would have twice the capacity. But there are other factors governing the capacity of a conductor. Consider the diagram of Fig. 120, which is supposed to represent two such plates as are shown in Fig. 119, placed opposite each other and connected respectively with the positive and the negative poles of the battery. When the connection between the plates and the battery is made, the two plates become charged to a difference of potential equal to the electromotive force of the battery. In order to obtain these charges, assume that the plates were each at zero potential before the connection was made; then current flows from the battery into the plates until they each assume the potential of the corresponding battery terminal. If the two plates be brought closer together, it will be found that more current will now flow into each of them, although the difference of potential between the two plates must obviously remain the same, since each of them is still connected to the battery.



Theory. Due to the proximity of the plates, the positive electricity on plate A is drawn by the negative charge on plate B towards plate B, and likewise the negative electricity on plate B is drawn to the side towards plate A by the positive charge on that plate. These two charges so drawn towards each other will, so to speak, bind each other, and they are referred to as bound charges. The charge on the right-hand side of plate A and on the left-hand side of plate B will, however, be free charges, since there is nothing to attract them, and these are, therefore, neutralized by a further flow of electricity from the battery to the plate.



Obviously, the closer together the plates are the stronger will be the attractive influence of the two charges on each other. From this it follows that in the case of plate A, when the two plates are being moved closer together, more positive electricity will flow into plate A to neutralize the increasing free negative charges on the right-hand side of the plate. As the plates are moved closer together still, a new distribution of charges will take place, resulting in more positive electricity flowing into plate A and more negative electricity flowing into plate B. The closer proximity of the plates, therefore, increases the capacity of the plates for holding charges, due to the increased inductive action across the dielectric separating the plates.

Condenser Defined. A condenser is a device consisting of two adjacent plates of conducting material, separated by an insulating material, called a dielectric. The purpose is to increase by the proximity of the plates, each to the other, the amount of electricity which each plate will receive and hold when subjected to a given potential.

Dielectric. We have already seen that the capacity of a condenser depends upon the area of its plates, and also upon their distance apart. There is still another factor on which the capacity of a condenser depends, i.e., on the character of the insulating medium separating its plates. The inductive action which takes place between a charged conductor and other conductors nearby it, as between plate A and plate B of Fig. 120, is called electrostatic induction, and it plays an important part in telephony. It is found that the ability of a given charged conductor to induce charges on other neighboring conductors varies largely with the insulating medium or dielectric that separates them. This quality of a dielectric, by which it enables inductive action to take place between two separated conductors, is called inductive capacity. Usually this quality of dielectrics is measured in terms of the same quality in dry air, this being taken as unity. When so expressed, it is termed specific inductive capacity. To be more accurate the specific inductive capacity of a dielectric is the ratio between the capacity of a condenser having that substance as a dielectric, to the capacity of the same condenser using dry air at zero degrees Centigrade and at a pressure of 14.7 pounds per square inch as the dielectric. To illustrate, if two condensers having plates of equal size and equal distance apart are constructed, one using air as the dielectric and the other using hard crown glass as the dielectric, the one using glass will have a capacity of 6.96 times that of the one using air. From this we say that crown glass has a specific inductive capacity of 6.96.

Various authorities differ rather widely as to the specific inductive capacity of many common substances. The values given in Table VIII have been chosen from the Smithsonian Physical Tables.

TABLE VIII

Specific Inductive Capacities

+ -+ + DIELECTRIC REFERRED TO AIR AS 1 + -+ + Vacuum .99941 Hydrogen .99967 Carbonic Acid 1.00036 Dry Paper 1.25 to 1.75 Paraffin 1.95 to 2.32 Ebonite 1.9 to 3.48 Sulphur 2.24 to 3.90 Shellac 2.95 to 3.73 Gutta-percha 3.3 to 4.9 Plate Glass 3.31 to 7.5 Porcelain 4.38 Mica 4.6 to 8.0 Glass Light Flint 6.61 Glass Hard Crown 6.96 Selenium 10.2 + -+ +

This data is interesting as showing the wide divergence in specific inductive capacities of various materials, and also showing the wide divergence in different observations of the same material. Undoubtedly, this latter is due mainly to the fact that various materials differ largely in themselves, as in the case of paraffin, for instance, which exhibits widely different specific inductive capacities according to the difference in rapidity with which it is cooled in changing from a liquid to a solid state.

We see then that the capacity of a condenser varies as the area of its plates, as the specific inductive capacity of the dielectric employed, and also inversely as the distance between the plates.

Obviously, therefore, in making a condenser of large capacity, it is important to have as large an area of the plate as possible; to have them as close together as possible; to have the dielectric a good insulating medium so that there will be practically no leakage between the plates; and to have the dielectric of as high a specific inductive capacity as economy and suitability of material in other respects will permit.

Dielectric Materials. Mica. Of all dielectrics mica is the most suitable for condensers, since it has very high insulation resistance and also high specific inductive capacity, and furthermore may be obtained in very thin sheets. High-grade condensers, such as are used for measurements and standardization purposes, usually have mica for the dielectric.



Dry Paper. The demands of telephonic practice are, however, such as to require condensers of very cheap construction with large capacity in a small space. For this purpose thin bond paper, saturated with paraffin, has been found to be the best dielectric. The conductors in condensers are almost always of tinfoil, this being an ideal material on account of its cheapness and its thinness. Before telephony made such urgent demands for a cheap compact condenser, the customary way of making them was to lay up alternate sheets of dielectric material, either of oiled paper or mica and tinfoil, the sheets of tinfoil being cut somewhat smaller than the sheets of dielectric material in order that the proper insulation might be secured at the edges. After a sufficient number of such plates were built up the alternate sheets of tinfoil were connected together to form one composite plate of the condenser, while the other sheets were similarly connected together to form the other plate. Obviously, in this way a very large area of plates could be secured with a minimum degree of separation.



There has been developed for use in telephony, however, and its use has since extended into other arts requiring condensers, what is called the rolled condenser. This is formed by rolling together in a flat roll four sheets of thin bond paper, 1, 2, 3, and 4, and two somewhat narrower strips of tinfoil, 5 and 6, Fig. 121. The strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases. Thin sheet metal terminals 7 and 8 are rolled into the condenser as it is being wound, and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished. After it is rolled, the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture. It is then squeezed in a press and allowed to cool while under pressure. In this way the surplus paraffin is expelled and the plates are brought very close together. It then appears as in Fig. 122. The condenser is now sealed in a metallic case, usually rectangular in form, and presents the appearance shown in Fig. 123.



A later method of condenser making which has not yet been thoroughly proven in practice, but which bids fair to produce good results, varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material. This conducting material is of metallic nature and in reality forms a part of the paper. To form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before.

Sizes. The condensers ordinarily used in telephone practice range in capacity from about 1/4 microfarad to 2 microfarads. When larger capacities than 2 microfarads are desired, they may be obtained by connecting several of the smaller size condensers in multiple. Table IX gives the capacity, shape, and dimensions of a variety of condensers selected from those regularly on the market.

TABLE IX

Condenser Data

+ + -+ -+ DIMENSIONS IN INCHES CAPACITY SHAPE + + -+ Height Width Thickness + + -+ + + -+ 2 m. f. Rectangular 9-1/6 4-3/4 11/16 1 m. f. " 9-1/6 4-3/4 11/16 1 m. f. " 4-3/4 2-3/32 13/16 1/2 m. f. " 2-3/4 1-1/4 3/4 1 m. f. " 4-13/16 2-1/32 25/32 1/2 m. f. " 4-3/4 2-3/32 13/16 3/10 m. f. " 4-3/4 2-3/32 13/16 1 m. f. " 2-3/4 3 l + + -+ + + -+

Conventional Symbols. The conventional symbols usually employed to represent condensers in telephone diagrams are shown in Fig. 124. These all convey the idea of the adjacent conducting plates separated by insulating material.



Functions. Obviously, when placed in a circuit a condenser offers a complete barrier to the flow of direct current, since no conducting path exists between its terminals, the dielectric offering a very high insulation resistance. If, however, the condenser is connected across the terminals of a source of alternating current, this current flows first in one direction and then in the other, the electromotive force in the circuit increasing from zero to a maximum in one direction, and then decreasing back to zero and to a maximum in the other direction, and so on. With a condenser connected so as to be subjected to such alternating electromotive forces, as the electromotive force begins to rise the electromotive force at the condenser terminals will also rise and a current will, therefore, flow into the condenser. When the electromotive force reaches its maximum, the condenser will have received its full charge for that potential, and the current flow into it will cease. When the electromotive force begins to fall, the condenser can no longer retain its charge and a current will, therefore, flow out of it. Apparently, therefore, there is a flow of current through the condenser the same as if it were a conductor.

Means for Assorting Currents. In conclusion, it is obvious that the telephone engineer has within his reach in the various coils—whether non-inductive or inductive, or whether having one or several windings—and in the condenser, a variety of tools by which he may achieve a great many useful ends in his circuit work. Obviously, the condenser affords a means for transmitting voice currents or fluctuating currents, and for excluding steady currents. Likewise the impedance coil affords a means for readily transmitting steady currents but practically excluding voice currents or fluctuating currents. By the use of these very simple devices it is possible to sift out the voice currents from a circuit containing both steady and fluctuating currents, or it is possible in the same manner to sift out the steady currents and to leave the voice currents alone to traverse the circuit.

Great use is made in the design of telephone circuits of the fact that the electromagnets, which accomplish the useful mechanical results in causing the movement of parts, possess the quality of impedance. Thus, the magnets which operate various signaling relays at the central office are often used also as impedance coils in portions of the circuit through which it is desired to have only steady currents pass. If, on the other hand, it is necessary to place a relay magnet, having considerable impedance, directly in a talking circuit, the bad effects of this on the voice currents may be eliminated by shunting this coil with a condenser, or with a comparatively high non-inductive resistance. The voice currents will flow around the high impedance of the relay coil through the condenser or resistance, while the steady currents, which are the ones which must be depended upon to operate the relay, are still forced in whole or in part to pass through the relay coil where they belong.

In a similar way the induction coil affords a means for keeping two circuits completely isolated so far as the direct flow of current between them is concerned, and yet of readily transmitting, by electromagnetic induction, currents from one of these circuits to the other. Here is a means of isolation so far as direct current is concerned, with complete communication for alternating current.



CHAPTER XIII

CURRENT SUPPLY TO TRANSMITTERS

The methods by which current is supplied to the transmitter of a telephone for energizing it, may be classified under two divisions: first, those where the battery or other source of current is located at the station with the transmitter which it supplies; and second, those where the battery or other source of current is located at a distant point from the transmitter, the battery in such cases serving as a common source of current for the supply of transmitters at a number of stations.

The advantages of putting the transmitter and the battery which supplies it with current in a local circuit with the primary of an induction coil, and placing the secondary of the induction coil in the line, have already been pointed out but may be briefly summarized as follows: When the transmitter is placed directly in the line circuit and the line is of considerable length, the current which passes through the transmitter is necessarily rather small unless a battery of high potential is used; and, furthermore, the total change in resistance which the transmitter is capable of producing is but a small proportion of the total resistance of the line, and, therefore, the current changes produced by the transmitter are relatively small. On the other hand, when the transmitter is placed in a local circuit with the battery, this circuit may be of small resistance and the current relatively large, even though supplied by a low-voltage battery; so that the transmitter is capable of producing relatively large changes in a relatively large current.

To draw a comparison between these two general classes of transmitter current supply, a number of cases will be considered in connection with the following figures, in each of which two stations connected by a telephone line are shown. Brief reference to the local battery method of supplying current will be made in order to make this chapter contain, as far as possible, all of the commonly used methods of current supply to transmitters.