An Intermediate Wave Set With Variocoupler Inductance Coils.—By using the coil wound on the rotor of the variocoupler as the tickler the coupling between the detector tube circuits and the aerial wire system increases as the set is tuned for greater wave lengths. This scheme makes the control of the regenerative circuit far more stable than it is where an ordinary loose coupled tuning coil is used.

When the variocoupler is adjusted for receiving very long waves the rotor sets at right angles to the stator and, since when it is in this position there is no mutual induction between them, the tickler coil serves as a loading coil for the detector plate oscillation circuit. Inductance coils for short wave lengths are usually wound in single layers but bank-wound coils, as they are called are necessary to get compactness where long wave lengths are to be received. By winding inductance coils with two or more layers the highest inductance values can be obtained with the least resistance. A wiring diagram of a multipoint inductance coil is shown in Fig. 58. You can buy this intermediate wave set assembled and ready to use or get the parts and connect them up yourself.



The Parts and How to Connect Them Up.—For this regenerative intermediate wave set get: (1) one 12 section triple bank-wound inductance coil, (2) one variometer, and (3) all the other parts shown in the diagram Fig. 58 except the variocoupler. First connect the free end of the condenser in the aerial to one of the terminals of the stator of the variocoupler; then connect the other terminal of the stator with one of the ends of the bank-wound inductance coil and connect the movable contact of this with the ground.

Next connect a wire to the aerial between the variable condenser and the stator and connect this to one post of a .0005 microfarad fixed condenser, then connect the other post of this with the grid of the detector and shunt a 2 megohm grid leak around it. Connect a wire to the ground wire between the bank-wound inductance coil and the ground proper, i.e., the radiator or water pipe, connect the other end of this to the + electrode of the A battery and connect this end also to one of the terminals of the filament. This done connect the other terminal of the filament to one post of the rheostat and the other post of this to the - or negative side of the A battery.

To the + electrode of the A battery connect the - or zinc pole of the B battery and connect the + or carbon pole of the latter with one post of the fixed .001 microfarad condenser. This done connect one terminal of the tickler coil which is on the rotor of the variometer to the plate of the detector and the other terminal of the tickler to the other post of the .001 condenser and around this shunt your headphones. Or if you want to use one or more amplifying tubes connect the circuit of the first one, see Fig. 45, to the posts on either side of the fixed condenser instead of the headphones.

A Long Wave Receiving Set.—The vivid imagination of Jules Verne never conceived anything so fascinating as the reception of messages without wires sent out by stations half way round the world; and in these days of high power cableless stations on the five continents you can listen-in to the messages and hear what is being sent out by the Lyons, Paris and other French stations, by Great Britain, Italy, Germany and even far off Russia and Japan.

A long wave set for receiving these stations must be able to tune to wave lengths up to 20,000 meters. Differing from the way in which the regenerative action of the short wave sets described in the preceding chapter is secured and which depends on a tickler coil and the coupling action of the detector in this long wave set, [Footnote: All of the short wave and intermediate wave receivers described, are connected up according to the wiring diagram used by the A. H. Grebe Company, Richmond Hill, Long Island, N. Y.] this action is obtained by the use of a tickler coil in the plate circuit which is inductively coupled to the grid circuit and this feeds back the necessary amount of current. This is a very good way to connect up the circuits for the reason that: (1) the wiring is simplified, and (2) it gives a single variable adjustment for the entire range of wave lengths the receptor is intended to cover.

The Parts and How to Connect Them Up.—The two chief features as far as the parts are concerned of this long wave length receiving set are (1) the variable condensers, and (2) the tuning inductance coils. The variable condenser used in series with the aerial wire system has 26 plates and is equal to a capacitance of .0008 mfd. which is the normal aerial capacitance. The condenser used in the secondary coil circuit has 14 plates and this is equal to a capacitance of .0004 mfd.

There are a number of inductance coils and these are arranged so that they can be connected in or cut out and combinations are thus formed which give a high efficiency and yet allow them to be compactly mounted. The inductance coils of the aerial wire system and those of the secondary coil circuit are practically alike. For wave lengths up to 2,200 meters bank litz-wound coils are used and these are wound up in 2, 4 and 6 banks in order to give the proper degree of coupling and inductance values.

Where wave lengths of more than 2,200 meters are to be received coto-coils are used as these are the "last word" in inductance coil design, and are especially adapted for medium as well as long wave lengths. [Footnote: Can be had of the Coto Coil Co., Providence, R. I.] These various coils are cut in and out by means of two five-point switches which are provided with auxiliary levers and contactors for dead-ending the right amount of the coils. In cutting in coils for increased wave lengths, that is from 10,000 to 20,000 meters, all of the coils of the aerial are connected in series as well as all of the coils of the secondary circuit. The connections for a long wave receptor are shown in the wiring diagram in Fig. 59.



CHAPTER XIII

HETERODYNE OR BEAT LONG WAVE TELEGRAPH RECEIVING SET

Any of the receiving sets described in the foregoing chapters will respond to either: (1) a wireless telegraph transmitter that uses a spark gap and which sends out periodic electric waves, or to (2) a wireless telephone transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous electric waves. To receive wireless telegraph signals, however, from a transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous waves, either the transmitter or the receptor must be so constructed that the continuous waves will be broken up into groups of audio frequency and this is done in several different ways.

There are four different ways employed at the present time to break up the continuous waves of a wireless telegraph transmitter into groups and these are: (a) the heterodyne, or beat, method, in which waves of different lengths are impressed on the received waves and so produces beats; (b) the tikker, or chopper method, in which the high frequency currents are rapidly broken up; (c) the variable condenser method, in which the movable plates are made to rapidly rotate; (d) the tone wheel, or frequency transformer, as it is often called, and which is really a modified form of and an improvement on the tikker. The heterodyne method will be described in this chapter.

What the Heterodyne or Beat Method Is.—The word heterodyne was coined from the Greek words heteros which means other, or different, and dyne which means power; in other words it means when used in connection with a wireless receptor that another and different high frequency current is used besides the one that is received from the sending station. In music a beat means a regularly recurrent swelling caused by the reinforcement of a sound and this is set up by the interference of sound waves which have slightly different periods of vibration as, for instance, when two tones take place that are not quite in tune with each other. This, then, is the principle of the heterodyne, or beat, receptor.

In the heterodyne, or beat method, separate sustained oscillations, that are just about as strong as those of the incoming waves, are set up in the receiving circuits and their frequency is just a little higher or a little lower than those that are set up by the waves received from the distant transmitter. The result is that these oscillations of different frequencies interfere and reinforce each other when beats are produced, the period of which is slow enough to be heard in the headphones, hence the incoming signals can be heard only when waves from the sending station are being received. A fuller explanation of how this is done will be found in Chapter XV.

The Autodyne or Self-Heterodyne Long-Wave Receiving Set.—This is the simplest type of heterodyne receptor and it will receive periodic waves from spark telegraph transmitters or continuous waves from an arc or vacuum tube telegraph transmitter. In this type of receptor the detector tube itself is made to set up the heterodyne oscillations which interfere with those that are produced by the incoming waves that are a little out of tune with it.

With a long wave autodyne, or self-heterodyne receptor, as this type is called, and a two-step audio-frequency amplifier you can clearly hear many of the cableless stations of Europe and others that send out long waves. For receiving long wave stations, however, you must have a long aerial—a single wire 200 or more feet in length will do—and the higher it is the louder will be the signals. Where it is not possible to put the aerial up a hundred feet or more above the ground, you can use a lower one and still get messages in International Morse fairly strong.

The Parts and Connections of an Autodyne, or Self-Heterodyne, Receiving Set.—For this long wave receiving set you will need: (1) one variocoupler with the primary coil wound on the stator and the secondary coil and tickler coil wound on the rotor, or you can use three honeycomb or other good compact coils of the longest wave you want to receive, a table of which is given in Chapter XII; (2) two .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one .5 to 2 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) one rheostat; (8) one B battery; (9) one potentiometer; (10) one .001 mfd. fixed condenser and (11) one pair of headphones. For the two-step amplifier you must, of course, have besides the above parts the amplifier tubes, variable condensers, batteries rheostats, potentiometers and fixed condensers as explained in Chapter IX. The connections for the autodyne, or self-heterodyne, receiving set are shown in Fig. 60.



The Separate Heterodyne Long Wave Receiving Set.—This is a better long wave receptor than the self heterodyne set described above for receiving wireless telegraph signals sent out by a continuous long wave transmitter. The great advantage of using a separate vacuum tube to generate the heterodyne oscillations is that you can make the frequency of the oscillations just what you want it to be and hence you can make it a little higher or a little lower than the oscillations set up by the received waves.

The Parts and Connections of a Separate Heterodyne Long Wave Receiving Set.—The parts required for this long wave receiving set are: (1) four honeycomb or other good compact inductance coils of the longest wave length that you want to receive; (2) three .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one 1 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) two rheostats; (8) two B batteries, one of which is supplied with taps; (9) one potentiometer; (10) one vacuum tube amplifier, for setting up the heterodyne oscillations; (11) a pair of headphones and (12) all of the parts for a two-step amplifier as detailed in Chapter IX, that is if you are going to use amplifiers. The connections are shown in Fig. 61.



In using either of these heterodyne receivers be sure to carefully adjust the B battery by means of the potentiometer.

[Footnote: The amplifier tube in this case is used as a generator of oscillations.]



CHAPTER XIV

HEADPHONES AND LOUD SPEAKERS

Wireless Headphones.—A telephone receiver for a wireless receiving set is made exactly on the same principle as an ordinary Bell telephone receiver. The only difference between them is that the former is made flat and compact so that a pair of them can be fastened together with a band and worn on the head (when it is called a headset), while the latter is long and cylindrical so that it can be held to the ear. A further difference between them is that the wireless headphone is made as sensitive as possible so that it will respond to very feeble currents, while the ordinary telephone receiver is far from being sensitive and will respond only to comparatively large currents.

How a Bell Telephone Receiver Is Made.—An ordinary telephone receiver consists of three chief parts and these are: (1) a hard-rubber, or composition, shell and cap, (2) a permanent steel bar magnet on one end of which is wound a coil of fine insulated copper wire, and (3) a soft iron disk, or diaphragm, all of which are shown in the cross-section in Fig. 62. The bar magnet is securely fixed inside of the handle so that the outside end comes to within about 1/32 of an inch of the diaphragm when this is laid on top of the shell and the cap is screwed on.



The ends of the coil of wire are connected with two binding posts which are in the end of the shell, but are shown in the picture at the sides for the sake of clearness. This coil usually has a resistance of about 75 ohms and the meaning of the ohmic resistance of a receiver and its bearing on the sensitiveness of it will be explained a little farther along. After the disk, or diaphragm, which is generally made of thin, soft sheet iron that has been tinned or japanned, [Footnote: A disk of photographic tin-type plate is generally used.] is placed over the end of the magnet, the cap, which has a small opening in it, is screwed on and the receiver is ready to use.

How a Wireless Headphone Is Made.—For wireless work a receiver of the watch-case type is used and nearly always two such receivers are connected with a headband. It consists of a permanent bar magnet bent so that it will fit into the shell of the receiver as shown at A in Fig. 63.



The ends of this magnet, which are called poles, are bent up, and hence this type is called a bipolar receiver. The magnets are wound with fine insulated wire as before and the diaphragm is held securely in place over them by screwing on the cap.

About Resistance, Turns of Wire and Sensitivity of Headphones.—If you are a beginner in wireless you will hear those who are experienced speak of a telephone receiver as having a resistance of 75 ohms, 1,000 ohms, 2,000 or 3,000 ohms, as the case may be; from this you will gather that the higher the resistance of the wire on the magnets the more sensitive the receiver is. In a sense this is true, but it is not the resistance of the magnet coils that makes it sensitive, in fact, it cuts down the current, but it is the number of turns of wire on them that determines its sensitiveness; it is easy to see that this is so, for the larger the number of turns the more often will the same current flow round the cores of the magnet and so magnetize them to a greater extent.

But to wind a large number of turns of wire close enough to the cores to be effective the wire must be very small and so, of course, the higher the resistance will be. Now the wire used for winding good receivers is usually No. 40, and this has a diameter of .0031 inch; consequently, when you know the ohmic resistance you get an idea of the number of turns of wire and from this you gather in a general way what the sensitivity of the receiver is.

A receiver that is sensitive enough for wireless work should be wound to not less than 1,000 ohms (this means each ear phone), while those of a better grade are wound to as high as 3,000 ohms for each one. A high-grade headset is shown in Fig. 64. Each phone of a headset should be wound to the same resistance, and these are connected in series as shown. Where two or more headsets are used with one wireless receiving set they must all be of the same resistance and connected in series, that is, the coils of one head set are connected with the coils of the next head set and so on to form a continuous circuit.



The Impedance of Headphones.—When a current is flowing through a circuit the material of which the wire is made not only opposes its passage—this is called its ohmic resistance—but a counter-electromotive force to the current is set up due to the inductive effects of the current on itself and this is called impedance. Where a wire is wound in a coil the impedance of the circuit is increased and where an alternating current is used the impedance grows greater as the frequency gets higher. The impedance of the magnet coils of a receiver is so great for high frequency oscillations that the latter cannot pass through them; in other words, they are choked off.

How the Headphones Work.—As you will see from the cross-sections in Figs. 62 and 63 there is no connection, electrical or mechanical, between the diaphragm and the other parts of the receiver. Now when either feeble oscillations, which have been rectified by a detector, or small currents from a B battery, flow through the magnet coils the permanent steel magnet is energized to a greater extent than when no current is flowing through it. This added magnetic energy makes the magnet attract the diaphragm more than it would do by its own force. If, on the other hand, the current is cut off the pull of the magnet is lessened and as its attraction for the diaphragm is decreased the latter springs back to its original position. When varying currents flow through the coils the diaphragm vibrates accordingly and sends out sound waves.

About Loud Speakers.—The simplest acoustic instrument ever invented is the megaphone, which latter is a Greek word meaning great sound. It is a very primitive device and our Indians made it out of birch-bark before Columbus discovered America. In its simplest form it consists of a cone-shaped horn and as the speaker talks into the small end the concentrated sound waves pass out of the large end in whatever direction it is held.

Now a loud speaker of whatever kind consists of two chief parts and these are: (1) a telephone receiver, and (2) a megaphone, or horn as it is called. A loud speaker when connected with a wireless receiving set makes it possible for a room, or an auditorium, full of people, or an outdoor crowd, to hear what is being sent out by a distant station instead of being limited to a few persons listening-in with headphones. To use a loud speaker you should have a vacuum tube detector receiving set and this must be provided with a one-step amplifier at least.

To get really good results you need a two-step amplifier and then energize the plate of the second vacuum tube amplifier with a 100 volt B battery; or if you have a three-step amplifier then use the high voltage on the plate of the third amplifier tube. Amplifying tubes are made to stand a plate potential of 100 volts and this is the kind you must use. Now it may seem curious, but when the current flows through the coils of the telephone receiver in one direction it gives better results than when it flows through in the other direction; to find out the way the current gives the best results try it out both ways and this you can do by simply reversing the connections.

The Simplest Type of Loud Speaker.—This loud speaker, which is called, the Arkay, [Footnote: Made by the Riley-Klotz Mfg. Co., Newark, N. J.] will work on a one- or two-step amplifier. It consists of a brass horn with a curve in it and in the bottom there is an adapter, or frame, with a set screw in it so that you can fit in one of your headphones and this is all there is to it. The construction is rigid enough to prevent overtones, or distortion of speech or music. It is shown in Fig. 65.



Another Simple Kind of Loud Speaker.—Another loud speaker, see Fig. 66, is known as the Amplitone [Footnote: Made by the American Pattern, Foundry and Machine Co., 82 Church Street, N. Y. C.] and it likewise makes use of the headphones as the sound producer. This device has a cast metal horn which improves the quality of the sound, and all you have to do is to slip the headphones on the inlet tubes of the horn and it is ready for use. The two headphones not only give a longer volume of sound than where a single one is used but there is a certain blended quality which results from one phone smoothing out the imperfections of the other.



A Third Kind of Simple Loud Speaker.—The operation of the Amplitron, [Footnote: Made by the Radio Service Co., 110 W. 40th Street, N. Y.] as this loud speaker is called, is slightly different from others used for the same purpose. The sounds set up by the headphone are conveyed to the apex of an inverted copper cone which is 7 inches long and 10 inches in diameter. Here it is reflected by a parabolic mirror which greatly amplifies the sounds. The amplification takes place without distortion, the sounds remaining as clear and crisp as when projected by the transmitting station. By removing the cap from the receiver the shell is screwed into a receptacle on the end of the loud speaker and the instrument is ready for use. It is pictured in Fig. 67.



A Super Loud Speaker.—This loud speaker, which is known as the Magnavox Telemegafone, was the instrument used by Lt. Herbert E. Metcalf, 3,000 feet in the air, and which startled the City of Washington on April 2, 1919, by repeating President Wilson's Victory Loan Message from an airplane in flight so that it was distinctly heard by 20,000 people below.

This wonderful achievement was accomplished through the installation of the Magnavox and amplifiers in front of the Treasury Building. Every word Lt. Metcalf spoke into his wireless telephone transmitter was caught and swelled in volume by the Telemegafones below and persons blocks away could hear the message plainly. Two kinds of these loud speakers are made and these are: (1) a small loud speaker for the use of operators so that headphones need not be worn, and (2) a large loud speaker for auditorium and out-door audiences.



Either kind may be used with a one- or two-step amplifier or with a cascade of half a dozen amplifiers, according to the degree of loudness desired. The Telemegafone itself is not an amplifier in the true sense inasmuch as it contains no elements which will locally increase the incoming current. It does, however, transform the variable electric currents of the wireless receiving set into sound vibrations in a most wonderful manner.

A telemegafone of either kind is formed of: (1) a telephone receiver of large proportions, (2) a step-down induction coil, and (3) a 6 volt storage battery that energizes a powerful electromagnet which works the diaphragm. An electromagnet is used instead of a permanent magnet and this is energized by a 6-volt storage battery as shown in the wiring diagram at A in Fig. 68. One end of the core of this magnet is fixed to the iron case of the speaker and together these form the equivalent of a horseshoe magnet. A movable coil of wire is supported from the center of the diaphragm the edge of which is rigidly held between the case and the small end of the horn. This coil is placed over the upper end of the magnet and its terminals are connected to the secondary of the induction coil. Now when the coil is energized by the current from the amplifiers it and the core act like a solenoid in that the coil tends to suck the core into it; but since the core is fixed and the coil is movable the core draws the coil down instead. The result is that with every variation of the current that flows through the coil it moves up and down and pulls and pushes the diaphragm down and up with it. The large amplitude of the vibrations of the latter set up powerful sound waves which can be heard several blocks away from the horn. In this way then are the faint incoming signals, speech and music which are received by the amplifying receiving set reproduced and magnified enormously. The Telemegafone is shown complete at B.



CHAPTER XV

OPERATION OF VACUUM TUBE RECEPTORS

From the foregoing chapters you have seen that the vacuum tube can be used either as a detector or an amplifier or as a generator of electric oscillations, as in the case of the heterodyne receiving set. To understand how a vacuum tube acts as a detector and as an amplifier you must first know what electrons are. The way in which the vacuum tube sets up sustained oscillations will be explained in Chapter XVIII in connection with the Operation of Vacuum Tube Transmitters.

What Electrons Are.—Science teaches us that masses of matter are made up of molecules, that each of these is made up of atoms, and each of these, in turn, is made up of a central core of positive particles of electricity surrounded by negative particles of electricity as shown in the schematic diagram, Fig. 69. The little black circles inside the large circle represent positive particles of electricity and the little white circles outside of the large circle represent negative particles of electricity, or electrons as they are called.



It is the number of positive particles of electricity an atom has that determines the kind of an element that is formed when enough atoms of the same kind are joined together to build it up. Thus hydrogen, which is the lightest known element, has one positive particle for its nucleus, while uranium, the heaviest element now known, has 92 positive particles. Now before leaving the atom please note that it is as much smaller than the diagram as the latter is smaller than our solar system.

What Is Meant by Ionization.—A hydrogen atom is not only lighter but it is smaller than the atom of any other element while an electron is more than a thousand times smaller than the atom of which it is a part. Now as long as all of the electrons remain attached to the surface of an atom its positive and negative charges are equalized and it will, therefore, be neither positive nor negative, that is, it will be perfectly neutral. When, however, one or more of its electrons are separated from it, and there are several ways by which this can be done, the atom will show a positive charge and it is then called a positive ion.

In other words a positive ion is an atom that has lost some of its negative electrons while a negative ion is one that has acquired some additional negative electrons. When a number of electrons are being constantly given by the atoms of an element, which let us suppose is a metal, and are being attracted to atoms of another element, which we will say is also a metal, a flow of electrons takes place between the two oppositely charged elements and form a current of negative electricity as represented by the arrows at A in Fig. 70.



When a stream of electrons is flowing between two metal elements, as a filament and a plate in a vacuum tube detector, or an amplifier, they act as carriers for more negative electrons and these are supplied by a battery as we shall presently explain. It has always been customary for us to think of a current of electricity as flowing from the positive pole of a battery to the negative pole of it and hence we have called this the direction of the current. Since the electronic theory has been evolved it has been shown that the electrons, or negative charges of electricity, flow from the negative to the positive pole and that the ionized atoms, which are more positive than negative, flow in the opposite direction as shown at B.

How Electrons are Separated from Atoms.—The next question that arises is how to make a metal throw off some of the electrons of the atoms of which it is formed. There are several ways that this can be done but in any event each atom must be given a good, hard blow. A simple way to do this is to heat a metal to incandescence when the atoms will bombard each other with terrific force and many of the electrons will be knocked off and thrown out into the surrounding space.

But all, or nearly all, of them will return to the atoms from whence they came unless a means of some kind is employed to attract them to the atoms of some other element. This can be done by giving the latter piece of metal a positive charge. If now these two pieces of metal are placed in a bulb from which the air has been exhausted and the first piece of metal is heated to brilliancy while the second piece of metal is kept positively electrified then a stream of electrons will flow between them.

Action of the Two Electrode Vacuum Tube.—Now in a vacuum tube detector a wire filament, like that of an incandescent lamp, is connected with a battery and this forms the hot element from which the electrons are thrown off, and a metal plate with a terminal wire secured to it is connected to the positive or carbon tap of a dry battery; now connect the negative or zinc tap of this with one end of a telephone receiver and the other end of this with the terminals of the filament as shown at A in Fig. 71. If now you heat the filament and hold the phone to your ear you can hear the current from the B battery flowing through the circuit.



Since the electrons are negative charges of electricity they are not only thrown off by the hot wire but they are attracted by the positive charged metal plate and when enough electrons pass, or flow, from the hot wire to the plate they form a conducting path and so complete the circuit which includes the filament, the plate and the B or plate battery, when the current can then flow through it. As the number of electrons that are thrown off by the filament is not great and the voltage of the plate is not high the current that flows between the filament and the plate is always quite small.

How the Two Electrode Tube Acts as a Detector.—As the action of a two electrode tube as a detector [Footnote: The three electrode vacuum tube has entirely taken the place of the two electrode type.] is simpler than that of the three electrode vacuum tube we shall describe it first. The two electrode vacuum tube was first made by Mr. Edison when he was working on the incandescent lamp but that it would serve as a detector of electric waves was discovered by Prof. Fleming, of Oxford University, London. As a matter of fact, it is not really a detector of electric waves, but it acts as: (1) a rectifier of the oscillations that are set up in the receiving circuits, that is, it changes them into pulsating direct currents so that they will flow through and affect a telephone receiver, and (2) it acts as a relay and the feeble received oscillating current controls the larger direct current from the B battery in very much the same way that a telegraph relay does. This latter relay action will be explained when we come to its operation as an amplifier.

We have just learned that when the stream of electrons flow from the hot wire to the cold positive plate in the tube they form a conducting path through which the battery current can flow. Now when the electric oscillations surge through the closed oscillation circuit, which includes the secondary of the tuning coil, the variable condenser, the filament and the plate as shown at B in Fig. 71 the positive part of them passes through the tube easily while the negative part cannot get through, that is, the top, or positive, part of the wave-form remains intact while the lower, or negative, part is cut off as shown in the diagram at C. As the received oscillations are either broken up into wave trains of audio frequency by the telegraph transmitter or are modulated by a telephone transmitter they carry the larger impulses of the direct current from the B battery along with them and these flow through the headphones. This is the reason the vacuum tube amplifies as well as detects.

How the Three Electrode Tube Acts as a Detector.—The vacuum tube as a detector has been made very much more sensitive by the use of a third electrode shown in Fig. 72. In this type of vacuum tube the third electrode, or grid, is placed between the filament and the plate and this controls the number of electrons flowing from the filament to the plate; in passing between these two electrodes they have to go through the holes formed by the grid wires.



If now the grid is charged to a higher negative voltage than the filament the electrons will be stopped by the latter, see A, though some of them will go through to the plate because they travel at a high rate of speed. The higher the negative charge on the grid the smaller will be the number of electrons that will reach the plate and, of course, the smaller will be the amount of current that will flow through the tube and the headphones from the B battery.

On the other hand if the grid is charged positively, see B, then more electrons will strike the plate than when the grid is not used or when it is negatively charged. But when the three electrode tube is used as a detector the oscillations set up in the circuits change the grid alternately from negative to positive as shown at C and hence the voltage of the B battery current that is allowed to flow through the detector from the plate to the filament rises and falls in unison with the voltage of the oscillating currents. The way the positive and negative voltages of the oscillations which are set up by the incoming waves, energize the grid; how the oscillator tube clips off the negative parts of them, and, finally, how these carry the battery current through the tube are shown graphically by the curves at D.

How the Vacuum Tube Acts as an Amplifier.—If you connect up the filament and the plate of a three electrode tube with the batteries and do not connect in the grid, you will find that the electrons which are thrown off by the filament will not get farther than the grid regardless of how high the voltage is that you apply to the plate. This is due to the fact that a large number of electrons which are thrown off by the filament strike the grid and give it a negative charge, and consequently, they cannot get any farther. Since the electrons do not reach the plate the current from the B battery cannot flow between it and the filament.

Now with a properly designed amplifier tube a very small negative voltage on the grid will keep a very large positive voltage on the plate from sending a current through the tube, and oppositely, a very small positive voltage on the grid will let a very large plate current flow through the tube; this being true it follows that any small variation of the voltage from positive to negative on the grid and the other way about will vary a large current flowing from the plate to the filament.

In the Morse telegraph the relay permits the small current that is received from the distant sending station to energize a pair of magnets, and these draw an armature toward them and close a second circuit when a large current from a local battery is available for working the sounder. The amplifier tube is a variable relay in that the feeble currents set up by the incoming waves constantly and proportionately vary a large current that flows through the headphones. This then is the principle on which the amplifying tube works.

The Operation of a Simple Vacuum Tube Receiving Set.—The way a simple vacuum tube detector receiving set works is like this: when the filament is heated to brilliancy it gives off electrons as previously described. Now when the electric waves impinge on the aerial wire they set up oscillations in it and these surge through the primary coil of the loose coupled tuning coil, a diagram of which is shown at B in Fig. 41.

The energy of these oscillations sets up oscillations of the same frequency in the secondary coil and these high frequency currents whose voltage is first positive and then negative, surge in the closed circuit which includes the secondary coil and the variable condenser. At the same time the alternating positive and negative voltage of the oscillating currents is impressed on the grid; at each change from + to - and back again it allows the electrons to strike the plate and then shuts them off; as the electrons form the conducting path between the filament and the plate the larger direct current from the B battery is permitted to flow through the detector tube and the headphones.

Operation of a Regenerative Vacuum Tube Receiving Set.—By feeding back the pulsating direct current from the B battery through the tickler coil it sets up other and stronger oscillations in the secondary of the tuning coil when these act on the detector tube and increase its sensitiveness to a remarkable extent. The regenerative, or feed back, action of the receiving circuits used will be easily understood by referring back to B in Fig. 47.

When the waves set up oscillations in the primary of the tuning coil the energy of them produces like oscillations in the closed circuit which includes the secondary coil and the condenser; the alternating positive and negative voltages of these are impressed on the grid and these, as we have seen before, cause similar variations of the direct current from the B battery which acts on the plate and which flows between the latter and the filament.

This varying direct current, however, is made to flow back through the third, or tickler coil of the tuning coil and sets up in the secondary coil and circuits other and larger oscillating currents and these augment the action of the oscillations produced by the incoming waves. These extra and larger currents which are the result of the feedback then act on the grid and cause still larger variations of the current in the plate voltage and hence of the current of the B battery that flows through the detector and the headphones. At the same time the tube keeps on responding to the feeble electric oscillations set up in the circuits by the incoming waves. This regenerative action of the battery current augments the original oscillations many times and hence produce sounds in the headphones that are many times greater than where the vacuum tube detector alone is used.

Operation of Autodyne and Heterodyne Receiving Sets.—On page 109 [Chapter VII] we discussed and at A in Fig. 36 is shown a picture of two tuning forks mounted on sounding boxes to illustrate the principle of electrical tuning. When a pair of these forks are made to vibrate exactly the same number of times per second there will be a condensation of the air between them and the sound waves that are sent out will be augmented. But if you adjust one of the forks so that it will vibrate 256 times a second and the other fork so that it will vibrate 260 times a second then there will be a phase difference between the two sets of waves and the latter will augment each other 4 times every second and you will hear these rising and falling sounds as beats.

Now electric oscillations set up in two circuits that are coupled together act in exactly the same way as sound waves produced by two tuning forks that are close to each other. Since this is true if you tune one of the closed circuits so that the oscillations in it will have a frequency of a 1,000,000 and tune the other circuit so that the oscillations in it have a frequency of 1,001,000 a second then the oscillations will augment each other 1,000 times every second.

As these rising and falling currents act on the pulsating currents from the B battery which flow through the detector tube and the headphones you will hear them as beats. A graphic representation of the oscillating currents set up by the incoming waves, those produced by the heterodyne oscillator and the beats they form is shown in Fig. 73. To produce these beats a receptor can use: (1) a single vacuum tube for setting up oscillations of both frequencies when it is called an autodyne, or self-heterodyne receptor, or (2) a separate vacuum tube for setting up the oscillations for the second circuit when it is called a heterodyne receptor.



The Autodyne, or Self-Heterodyne Receiving Set.—Where only one vacuum tube is used for producing both frequencies you need only a regenerative, or feed-back receptor; then you can tune the aerial wire system to the incoming waves and tune the closed circuit of the secondary coil so that it will be out of step with the former by 1,000 oscillations per second, more or less, the exact number does not matter in the least. From this you will see that any regenerative set can be used for autodyne, or self-heterodyne, reception.

The Separate Heterodyne Receiving Set.—The better way, however, is to use a separate vacuum tube for setting up the heterodyne oscillations. The latter then act on the oscillations that are produced by the incoming waves and which energize the grid of the detector tube. Note that the vacuum tube used for producing the heterodyne oscillations is a generator of electric oscillations; the latter are impressed on the detector circuits through the variable coupling, the secondary of which is in series with the aerial wire as shown in Fig. 74. The way in which the tube acts as a generator of oscillations will be told in Chapter XVIII.



CHAPTER XVI

CONTINUOUS WAVE TELEGRAPH TRANSMITTING SETS WITH DIRECT CURRENT

In the first part of this book we learned about spark-gap telegraph sets and how the oscillations they set up are damped and the waves they send out are periodic. In this and the next chapter we shall find out how vacuum tube telegraph transmitters are made and how they set up oscillations that are sustained and radiate waves that are continuous.

Sending wireless telegraph messages by continuous waves has many features to recommend it as against sending them by periodic waves and among the most important of these are that the transmitter can be: (1) more sharply tuned, (2) it will send signals farther with the same amount of power, and (3) it is noiseless in operation. The disadvantageous features are that: (1) a battery current is not satisfactory, (2) its circuits are somewhat more complicated, and (3) the oscillator tubes burn out occasionally. There is, however, a growing tendency among amateurs to use continuous wave transmitters and they are certainly more up-to-date and interesting than spark gap sets.

Now there are two practical ways by which continuous waves can be set up for sending either telegraphic signals or telephonic speech and music and these are with: (a) an oscillation arc lamp, and (b) a vacuum tube oscillator. The oscillation arc was the earliest known way of setting up sustained oscillations, and it is now largely used for commercial high power, long distance work. But since the vacuum tube has been developed to a high degree of efficiency and is the scheme that is now in vogue for amateur stations we shall confine our efforts here to explaining the apparatus necessary and how to wire the various parts together to produce several sizes of vacuum tube telegraph transmitters.

Sources of Current for Telegraph Transmitting Sets.—Differing from a spark-gap transmitter you cannot get any appreciable results with a low voltage battery current to start with. For a purely experimental vacuum tube telegraph transmitter you can use enough B batteries to operate it but the current strength of these drops so fact when they are in use, that they are not at all satisfactory for the work.

You can, however, use 110 volt direct current from a lighting circuit as your initial source of power to energize the plate of the vacuum tube oscillator of your experimental transmitter. Where you have a 110 volt direct current lighting service in your home and you want a higher voltage for your plate, you will then have to use a motor-generator set and this costs money. If you have 110 volt alternating current lighting service at hand your troubles are over so far as cost is concerned for you can step it up to any voltage you want with a power transformer. In this chapter will be shown how to use a direct current for your source of initial power and in the next chapter how to use an alternating current for the initial power.

An Experimental Continuous Wave Telegraph Transmitter.—You will remember that in Chapter XV we learned how the heterodyne receiver works and that in the separate heterodyne receiving set the second vacuum tube is used solely to set up oscillations. Now while this extra tube is used as a generator of oscillations these are, of course, very weak and hence a detector tube cannot be used to generate oscillations that are useful for other purposes than heterodyne receptors and measurements.

There is a vacuum tube amplifier [Footnote: This is the radiation UV-201, made by the Radio Corporation of America, Woolworth Bldg., New York City.] made that will stand a plate potential of 100 volts, and this can be used as a generator of oscillations by energizing it with a 110 volt direct current from your lighting service. Or in a pinch you can use five standard B batteries to develop the plate voltage, but these will soon run down. But whatever you do, never use a current from a lighting circuit on a tube of any kind that has a rated plate potential of less than 100 volts.

The Apparatus You Need.—For this experimental continuous wave telegraph transmitter get the following pieces of apparatus: (1) one single coil tuner with three clips; (2) one .002 mfd. fixed condenser; (3) three .001 mfd. condensers; (4) one adjustable grid leak; (5) one hot-wire ammeter; (6) one buzzer; (7) one dry cell; (8) one telegraph key; (9) one 100 volt plate vacuum tube amplifier; (10) one 6 volt storage battery; (11) one rheostat; (12) one oscillation choke coil; (13) one panel cut-out with a single-throw, double-pole switch, and a pair of fuse sockets on it.

The Tuning Coil.—You can either make this tuning coil or buy one. To make it get two disks of wood 3/4-inch thick and 5 inches in diameter and four strips of hard wood, or better, hard rubber or composition strips, such as bakelite, 1/2-inch thick, 1 inch wide and 5-3/4 inches long, and screw them to the disks as shown at A in Fig. 75. Now wrap on this form about 25 turns of No. 8 or 10, Brown and Sharpe gauge, bare copper wire with a space of 1/8-inch between each turn. Get three of the smallest size terminal clips, see B, and clip them on to the different turns, when your tuning coil is ready for use. You can buy a coil of this kind for $4.00 or $5.00.

The Condensers.—For the aerial series condenser get one that has a capacitance of .002 mfd. and that will stand a potential of 3,000 volts. [Footnote: The U C-1014 Faradon condenser made by the Radio Corporation of America will serve the purpose.] It is shown at C. The other three condensers, see D, are also of the fixed type and may have a capacitance of .001 mfd.; [Footnote: List No. 266; fixed receiving condenser, sold by the Manhattan Electrical Supply Co.] the blocking condenser should preferably have a capacitance of 1/2 a mfd. In these condensers the leaves of the sheet metal are embedded in composition. The aerial condenser will cost you $2.00 and the others 75 cents each.



The Aerial Ammeter.—This instrument is also called a hot-wire ammeter because the oscillating currents flowing through a piece of wire heat it according to their current strength and as the wire contracts and expands it moves a needle over a scale. The ammeter is connected in the aerial wire system, either in the aerial side or the ground side—the latter place is usually the most convenient. When you tune the transmitter so that the ammeter shows the largest amount of current surging in the aerial wire system you can consider that the oscillation circuits are in tune. A hot-wire ammeter reading to 2.5 amperes will serve your needs, it costs $6.00 and is shown at E in Fig. 75.



The Buzzer and Dry Cell.—While a heterodyne, or beat, receptor can receive continuous wave telegraph signals an ordinary crystal or vacuum tube detector receiving set cannot receive them unless they are broken up into trains either at the sending station or at the receiving station, and it is considered the better practice to do this at the former rather than at the latter station. For this small transmitter you can use an ordinary buzzer as shown at F. A dry cell or two must be used to energize the buzzer. You can get one for about 75 cents.

The Telegraph Key.—Any kind of a telegraph key will serve to break up the trains of sustained oscillations into dots and dashes. The key shown at G is mounted on a composition base and is the cheapest key made, costing $1.50.

The Vacuum Tube Oscillator.—As explained before you can use any amplifying tube that is made for a plate potential of 100 volts. The current required for heating the filament is about 1 ampere at 6 volts. A porcelain socket should be used for this tube as it is the best insulating material for the purpose. An amplifier tube of this type is shown at H and costs $6.50.

The Storage Battery.—A storage battery is used to heat the filament of the tube, just as it is with a detector tube, and it can be of any make or capacity as long as it will develop 6 volts. The cheapest 6 volt storage battery on the market has a 20 to 40 ampere-hour capacity and sells for $13.00.

The Battery Rheostat.—As with the receptors a rheostat is needed to regulate the current that heats the filament. A rheostat of this kind is shown at I and is listed at $1.25.

The Oscillation Choke Coil.—This coil is connected in between the oscillation circuits and the source of current which feeds the oscillator tube to keep the oscillations set up by the latter from surging back into the service wires where they would break down the insulation. You can make an oscillation choke coil by winding say 100 turns of No. 28 Brown and Sharpe gauge double cotton covered magnet wire on a cardboard cylinder 2 inches in diameter and 2-1/2 inches long.

Transmitter Connectors.—For connecting up the different pieces of apparatus of the transmitter it is a good scheme to use copper braid; this is made of braided copper wire in three sizes and sells for 7,15 and 20 cents a foot respectively. A piece of it is pictured at J.

The Panel Cut-Out.—This is used to connect the cord of the 110-volt lamp socket with the transmitter. It consists of a pair of plug cutouts and a single-throw, double-pole switch mounted on a porcelain base as shown at K. In some localities it is necessary to place these in an iron box to conform to the requirements of the fire underwriters.

Connecting Up the Transmitting Apparatus.—The way the various pieces of apparatus are connected together is shown in the wiring diagram. Fig. 76. Begin by connecting one post of the ammeter with the wire that leads to the aerial and the other post of it to one end of the tuning coil; connect clip 1 to one terminal of the .002 mfd. 3,000 volt aerial condenser and the other post of this with the ground.



Now connect the end of the tuning coil that leads to the ammeter with one end of the .001 mfd. grid condenser and the other end of this with the grid of the vacuum tube. Connect the telegraph key, the buzzer and the dry cell in series and then shunt them around the grid condenser. Next connect the plate of the tube with one end of the .001 mfd. blocking condenser and the other end of this with the clip 2 on the tuning coil.

Connect one end of the filament with the + or positive electrode of the storage battery, the - or negative electrode of this with one post of the rheostat and the other post of the latter with the other end of the filament; then connect clip 3 with the + or positive side of the storage battery. This done connect one end of the choke coil to the conductor that leads to the plate and connect the other end of the choke coil to one of the taps of the switch on the panel cut-out. Connect the + or positive electrode of the storage battery to the other switch tap and between the switch and the choke coil connect the protective condenser across the 110 volt feed wires. Finally connect the lamp cord from the socket to the plug fuse taps when your experimental continuous wave telegraph transmitter is ready to use.

A 100 Mile C. W. Telegraph Transmitter.—Here is a continuous wave telegraph transmitter that will cover distances up to 100 miles that you can rely on. It is built on exactly the same lines as the experimental transmitter just described, but instead of using a 100 volt plate amplifier as a makeshift generator of oscillations it employs a vacuum tube made especially for setting up oscillations and instead of having a low plate voltage it is energized with 350 volts.

The Apparatus You Need.—For this transmitter you require: (1) one oscillation transformer; (2) one hot-wire ammeter; (3) one aerial series condenser; (4) one grid leak resistance; (5) one chopper; (6) one key circuit choke coil; (7) one 5 watt vacuum tube oscillator; (8) one 6 volt storage battery; (9) one battery rheostat; (10) one battery voltmeter; (11) one blocking condenser; (12) one power circuit choke coil, and (13) one motor-generator.

The Oscillation Transformer.—The tuning coil, or oscillation transformer as this one is called, is a conductively coupled tuner—that is, the primary and secondary coils form one continuous coil instead of two separate coils. This tuner is made up of 25 turns of thin copper strip, 3/8 inch wide and with its edges rounded, and this is secured to a wood base as shown at A in Fig. 77. It is fitted with one fixed tap and three clips to each of which a length of copper braid is attached. It has a diameter of 6-1/4 inches, a height of 7-7/8 inches and a length of 9-3/8 inches, and it costs $11.00.



The Aerial Condenser.—This condenser is made up of three fixed condensers of different capacitances, namely .0003, .0004 and .0005 mfd., and these are made to stand a potential of 7500 volts. The condenser is therefore adjustable and, as you will see from the picture B, it has one terminal wire at one end and three terminal wires at the other end so that one, two or three condensers can be used in series with the aerial. A condenser of this kind costs $5.40.

The Aerial Ammeter.—This is the same kind of a hot-wire ammeter already described in connection with the experimental set, but it reads to 5 amperes.

The Grid and Blocking Condensers.—Each of these is a fixed condenser of .002 mfd. capacitance and is rated to stand 3,000 volts. It is made like the aerial condenser but has only two terminals. It costs $2.00.

The Key Circuit Apparatus.—This consists of: (1) the grid leak; (2) the chopper; (3) the choke coil, and (4) the key. The grid leak is connected in the lead from the grid to the aerial to keep the voltage on the grid at the right potential. It has a resistance of 5000 ohms with a mid-tap at 2500 ohms as shown at C. It costs $2.00.

The chopper is simply a rotary interrupter driven by a small motor. It comprises a wheel of insulating material in which 30 or more metal segments are set in an insulating disk as shown at D. A metal contact called a brush is fixed on either side of the wheel. It costs about $7.00 and the motor to drive it is extra. The choke coil is wound up of about 250 turns of No. 30 Brown and Sharpe gauge cotton covered magnet wire on a spool which has a diameter of 2 inches and a length of 3-1/4 inches.

The 5 Watt Oscillator Vacuum Tube.—This tube is made like the amplifier tube described for use with the preceding experimental transmitter, but it is larger, has a more perfect vacuum, and will stand a plate potential of 350 volts while the plate current is .045 ampere. The filament takes a current of a little more than 2 amperes at 7.5 volts. A standard 4-tap base is used with it. The tube costs $8.00 and the porcelain base is $1.00 extra. It is shown at E.

The Storage Battery and Rheostat.—This must be a 5-cell battery so that it will develop 10 volts. A storage battery of any capacity can be used but the lowest priced one costs about $22.00. The rheostat for regulating the battery current is the same as that used in the preceding experimental transmitter.

The Filament Voltmeter.—To get the best results it is necessary that the voltage of the current which heats the filament be kept at the same value all of the time. For this transmitter a direct current voltmeter reading from 0 to 15 volts is used. It is shown at F and costs $7.50. The Oscillation Choke Coil.—This is made exactly like the one described in connection with the experimental transmitter.

The Motor-Generator Set.—Where you have only a 110 or a 220 volt direct current available as a source of power you need a motor-generator to change it to 350 volts, and this is an expensive piece of apparatus. It consists of a single armature core with a motor winding and a generator winding on it and each of these has its own commutator. Where the low voltage current flows into one of the windings it drives its as a motor and this in turn generates the higher voltage current in the other winding. Get a 100 watt 350 volt motor-generator; it is shown at F and costs about $75.00.

The Panel Cut-Out.—This switch and fuse block is the same as that used in the experimental set.

The Protective Condenser.—This is a fixed condenser having a capacitance of 1 mfd. and will stand 750 volts. It costs $2.00.

Connecting Up the Transmitting Apparatus.—From all that has gone before you have seen that each piece of apparatus is fitted with terminal, wires, taps or binding posts. To connect up the parts of this transmitter it is only necessary to make the connections as shown in the wiring diagram Fig. 78.



A 200 Mile C. W. Telegraph Transmitter.—To make a continuous wave telegraph transmitter that will cover distances up to 200 miles all you have to do is to use two 5 watt vacuum tubes in parallel, all of the rest of the apparatus being exactly the same. Connecting the oscillator tubes up in parallel means that the two filaments are connected across the leads of the storage battery, the two grids on the same lead that goes to the aerial and the two plates on the same lead that goes to the positive pole of the generator. Where two or more oscillator tubes are used only one storage battery is needed, but each filament must have its own rheostat. The wiring diagram Fig. 79 shows how the two tubes are connected up in parallel.



A 500 Mile C. W. Telegraph Transmitter.—For sending to distances of over 200 miles and up to 500 miles you can use either: (1) three or four 5 watt oscillator tubes in parallel as described above, or (2) one 50 watt oscillator tube. Much of the apparatus for a 50 watt tube set is exactly the same as that used for the 5 watt sets. Some of the parts, however, must be proportionately larger though the design all the way through remains the same.

The Apparatus and Connections.—The aerial series condenser, the blocking condenser, the grid condenser, the telegraph key, the chopper, the choke coil in the key circuit, the filament voltmeter and the protective condenser in the power circuit are identical with those described for the 5 watt transmitting set.

The 50 Watt Vacuum Tube Oscillator.—This is the size of tube generally used by amateurs for long distance continuous wave telegraphy. A single tube will develop 2 to 3 amperes in your aerial. The filament takes a 10 volt current and a plate potential of 1,000 volts is needed. One of these tubes is shown in Fig. 80 and the cost is $30.00. A tube socket to fit it costs $2.50 extra.



The Aerial Ammeter.—This should read to 5 amperes and the cost is $6.25.

The Grid Leak Resistance.—It has the same resistance, namely 5,000 ohms as the one used with the 5 watt tube transmitter, but it is a little larger. It is listed at $1.65.

The Oscillation Choke Coil.—The choke coil in the power circuit is made of about 260 turns of No. 30 B. & S. cotton covered magnet wire wound on a spool 2-1/4 inches in diameter and 3-1/4 inches long.

The Filament Rheostat.—This is made to take care of a 10 volt current and it costs $10.00.

The Filament Storage Battery.—This must develop 12 volts and one having an output of 40 ampere-hours costs about $25.00.

The Protective Condenser.—This condenser has a capacitance of 1 mfd. and costs $2.00.

The Motor-Generator.—Where you use one 50 watt oscillator tube you will need a motor-generator that develops a plate potential of 1000 volts and has an output of 200 watts. This machine will stand you about $100.00.

The different pieces of apparatus for this set are connected up exactly the same as shown in the wiring diagram in Fig. 78.

A 1000 Mile C. W. Telegraph Transmitter.—All of the parts of this transmitting set are the same as for the 500 mile transmitter just described except the motor generator and while this develops the same plate potential, i.e., 1,000 volts, it must have an output of 500 watts; it will cost you in the neighborhood of $175.00. For this long distance transmitter you use two 50 watt oscillator tubes in parallel and all of the parts are connected together exactly the same as for the 200 mile transmitter shown in the wiring diagram in Fig. 79.



CHAPTER XVII

CONTINUOUS WAVE TELEGRAPH TRANSMITTING SETS WITH ALTERNATING CURRENT

Within the last few years alternating current has largely taken the place of direct current for light, heat and power purposes in and around towns and cities and if you have alternating current service in your home you can install a long distance continuous wave telegraph transmitter with very little trouble and at a comparatively small expense.

A 100 Mile C. W. Telegraph Transmitting Set.—The principal pieces of apparatus for this transmitter are the same as those used for the 100 Mile Continuous Wave Telegraph Transmitting Set described and pictured in the preceding chapter which used direct current, except that an alternating current power transformer is employed instead of the more costly motor-generator.

The Apparatus Required.—The various pieces of apparatus you will need for this transmitting set are: (1) one hot-wire ammeter for the aerial as shown at E in Fig. 75, but which reads to 5 amperes instead of to 2.5 amperes; (2) one tuning coil as shown at A in Fig. 77; (3) one aerial condenser as shown at B in Fig. 77; (4) one grid leak as shown at C in Fig. 77; (5) one telegraph key as shown at G in Fig. 75; (6) one grid condenser, made like the aerial condenser but having only two terminals; (7) one 5 watt oscillator tube as shown at E in Fig. 77; (8) one .002 mfd. 3,000 volt by-pass condenser, made like the aerial and grid condensers; (9) one pair of choke coils for the high voltage secondary circuit; (10) one milli-ammeter; (11) one A. C. power transformer; (12) one rheostat as shown at I in Fig. 75, and (13) one panel cut-out as shown at K in Fig. 75.

The Choke Coils.—Each of these is made by winding about 100 turns of No. 28, Brown and Sharpe gauge, cotton covered magnet wire on a spool 2 inches in diameter and 2-1/2 inches long, when it will have an inductance of about 0.5 millihenry [Footnote: A millihenry is 1/1000th part of a henry.] at 1,000 cycles.

The Milli-ammeter.—This is an alternating current ammeter and reads from 0 to 250 milliamperes; [Footnote: A milliampere is the 1/1000th part of an ampere.] and is used for measuring the secondary current that energizes the plate of the oscillator tube. It looks like the aerial ammeter and costs about $7.50.

The A. C. Power Transformer.—Differing from the motor generator set the power transformer has no moving parts. For this transmitting set you need a transformer that has an input of 325 volts. It is made to work on a 50 to 60 cycle current at 102.5 to 115 volts, which is the range of voltage of the ordinary alternating lighting current. This adjustment for voltage is made by means of taps brought out from the primary coil to a rotary switch.

The high voltage secondary coil which energizes the plate has an output of 175 watts and develops a potential of from 350 to 1,100 volts. The low voltage secondary coil which heats the filament has an output of 175 watts and develops 7.5 volts. This transformer, which is shown in Fig. 81, is large enough to take care of from one to four 5 watt oscillator tubes. It weighs about 15 pounds and sells for $25.00.



Connecting Up the Apparatus.—The wiring diagram Fig. 82 shows clearly how all of the connections are made. It will be observed that a storage battery is not needed as the secondary coil of the transformer supplies the current to heat the filament of the oscillator. The filament voltmeter is connected across the filament secondary coil terminals, while the plate milli-ammeter is connected to the mid-taps of the plate secondary coil and the filament secondary coil.



A 200 to 500 Mile C. W. Telegraph Transmitting Set.—Distances of from 200 to 500 miles can be successfully covered with a telegraph transmitter using two, three or four 5 watt oscillator tubes in parallel. The apparatus needed is identical with that used for the 100 mile transmitter just described. The tubes are connected in parallel as shown in the wiring diagram in Fig. 83.



A 500 to 1,000 Mile C. W. Telegraph Transmitting Set.—With the apparatus described for the above set and a single 50 watt oscillator tube a distance of upwards of 500 miles can be covered, while with two 50 watt oscillator tubes in parallel you can cover a distance of 1,000 miles without difficulty, and nearly 2,000 miles have been covered with this set.

The Apparatus Required.—All of the apparatus for this C. W. telegraph transmitting set is the same as that described for the 100 and 200 mile sets but you will need: (1) one or two 50 watt oscillator tubes with sockets; (2) one key condenser that has a capacitance of 1 mfd., and a rated potential of 1,750 volts; (3) one 0 to 500 milli-ammeter; (4) one aerial ammeter reading to 5 amperes, and (5) an A. C. power transformer for one or two 50 watt tubes.



The Alternating Current Power Transformer.—This power transformer is made exactly like the one described in connection with the preceding 100 mile transmitter and pictured in Fig. 81, but it is considerably larger. Like the smaller one, however, it is made to work with a 50 to 60 cycle current at 102.5 to 115 volts and, hence, can be used with any A. C. lighting current.

It has an input of 750 volts and the high voltage secondary coil which energizes the plate has an output of 450 watts and develops 1,500 to 3,000 volts. The low voltage secondary coil which heats the filament develops 10.5 volts. This transformer will supply current for one or two 50-watt oscillator tubes and it costs about $40.00.

Connecting Up the Apparatus.—Where a single oscillator tube is used the parts are connected as shown in Fig. 82, and where two tubes are connected in parallel the various pieces of apparatus are wired together as shown in Fig. 83. The only difference between the 5 watt tube transmitter and the 50 watt tube transmitter is in the size of the apparatus with one exception; where one or two 50 watt tubes are used a second condenser of large capacitance (1 mfd.) is placed in the grid circuit and the telegraph key is shunted around it as shown in the diagram Fig. 83.



CHAPTER XVIII

WIRELESS TELEPHONE TRANSMITTING SETS WITH DIRECT AND ALTERNATING CURRENTS

In time past the most difficult of all electrical apparatus for the amateur to make, install and work was the wireless telephone. This was because it required a direct current of not less than 500 volts to set up the sustained oscillations and all ordinary direct current for lighting purposes is usually generated at a potential of 110 volts.

Now as you know it is easy to step-up a 110 volt alternating current to any voltage you wish with a power transformer but until within comparatively recent years an alternating current could not be used for the production of sustained oscillations for the very good reason that the state of the art had not advanced that far. In the new order of things these difficulties have all but vanished and while a wireless telephone transmitter still requires a high voltage direct current to operate it this is easily obtained from 110 volt source of alternating current by means of vacuum tube rectifiers.

The pulsating direct currents are then passed through a filtering reactance coil, called a reactor, and one or more condensers, and these smooth them out until they approximate a continuous direct current. The latter is then made to flow through a vacuum tube oscillator when it is converted into high frequency oscillations and these are varied, or modulated, as it is called, by a microphone transmitter such as is used for ordinary wire telephony. The energy of these sustained modulated oscillations is then radiated into space from the aerial in the form of electric waves.

The distance that can be covered with a wireless telephone transmitter is about one-fourth as great as that of a wireless telegraph transmitter having the same input of initial current, but it is long enough to satisfy the most enthusiastic amateur. For instance with a wireless telephone transmitter where an amplifier tube is used to set up the oscillations and which is made for a plate potential of 100 volts, distances up to 10 or 15 miles can be covered.

With a single 5 watt oscillator tube energized by a direct current of 350 volts from either a motor-generator or from a power transformer (after it has been rectified and smoothed out) speech and music can be transmitted to upwards of 25 miles. Where two 5 watt tubes connected in parallel are used wireless telephone messages can be transmitted to distances of 40 or 50 miles. Further, a single 50 watt oscillator tube will send to distances of 50 to 100 miles while two of these tubes in parallel will send from 100 to 200 miles. Finally, where four or five oscillator tubes are connected in parallel proportionately greater distances can be covered.

A Short Distance Wireless Telephone Transmitting Set-With 110 Volt Direct Lighting Current.—For this very simple, short distance wireless telephone transmitting set you need the same apparatus as that described and pictured in the beginning of Chapter XVI for a Short Distance C. W. Telegraph Transmitter, except that you use a microphone transmitter instead of a telegraph key. If you have a 110 volt direct lighting current in your home you can put up this short distance set for very little money and it will be well worth your while to do so.

The Apparatus You Need.—For this set you require: (1) one tuning coil as shown at A and B in Fig. 75; (2) one aerial ammeter as shown at C in Fig. 75; (3) one aerial condenser as shown at C in Fig. 75; (4) one grid, blocking and protective condenser as shown at D in Fig. 75; (5) one grid leak as shown at C in Fig. 77; (6) one vacuum tube amplifier which is used as an oscillator; (7) one 6 volt storage battery; (8) one rheostat as shown at I in Fig. 75; (9) one oscillation choke coil; (10) one panel cut-out as shown at K in Fig. 75 and an ordinary microphone transmitter.

The Microphone Transmitter.—The best kind of a microphone to use with this and other telephone transmitting sets is a Western Electric No. 284-W. [Footnote: Made by the Western Electric Company, Chicago, Ill.] This is known as a solid back transmitter and is the standard commercial type used on all long distance Bell telephone lines. It articulates sharply and distinctly and there are no current variations to distort the wave form of the voice and it will not buzz or sizzle. It is shown in Fig. 84 and costs $2.00. Any other good microphone transmitter can be used if desired.



Connecting Up the Apparatus.—Begin by connecting the leading-in wire with one of the terminals of the microphone transmitter, as shown in the wiring diagram Fig. 85, and the other terminal of this to one end of the tuning coil. Now connect clip 1 of the tuning coil to one of the posts of the hot-wire ammeter, the other post of this to one end of aerial condenser and, finally, the other end of the latter with the water pipe or other ground. The microphone can be connected in the ground wire and the ammeter in the aerial wire and the results will be practically the same.



Next connect one end of the grid condenser to the post of the tuning coil that makes connection with the microphone and the other end to the grid of the tube, and then shunt the grid leak around the condenser. Connect the + or positive electrode of the storage battery with one terminal of the filament of the vacuum tube, the other terminal of the filament with one post of the rheostat and the other post of this with the - or negative electrode of the battery. This done, connect clip 2 of the tuning coil to the + or positive electrode of the battery and bring a lead from it to one of the switch taps of the panel cut-out.

Now connect clip 3 of the tuning coil with one end of the blocking condenser, the other end of this with one terminal of the choke coil and the other terminal of the latter with the other switch tap of the cut-out. Connect the protective condenser across the direct current feed wires between the panel cut-out and the choke coil. Finally connect the ends of a lamp cord to the fuse socket taps of the cut-out, and connect the other ends to a lamp plug and screw it into the lamp socket of the feed wires. Screw in a pair of 5 ampere fuse plugs, close the switch and you are ready to tune the transmitter and talk to your friends.

A 25 to 50 Mile Wireless Telephone Transmitter—With Direct Current Motor Generator.—Where you have to start with 110 or 220 volt direct current and you want to transmit to a distance of 25 miles or more you will have to install a motor-generator. To make this transmitter you will need exactly the same apparatus as that described and pictured for the 100 Mile C. W. Telegraph Transmitting Set in Chapter XVI, except that you must substitute a microphone transmitter and a telephone induction coil, or a microphone transformer, or still better, a magnetic modulator, for the telegraph key and chopper.

The Apparatus You Need.—To reiterate; the pieces of apparatus you need are: (1) one aerial ammeter as shown at E in Fig. 75; (2) one tuning coil as shown at A in Fig. 77; (3) one aerial condenser as shown at B in Fig. 77; (4) one grid leak as shown at C in Fig. 77; (5) one grid, blocking and protective condenser; (6) one 5 watt oscillator tube as shown at E in Fig. 77; (7) one rheostat as shown at I in Fig. 75; (8) one 10 volt (5 cell) storage battery; (9) one choke coil; (10) one panel cut-out as shown at K in Fig. 75, and (11) a motor-generator having an input of 110 or 220 volts and an output of 350 volts.

In addition to the above apparatus you will need: (12) a microphone transmitter as shown in Fig. 84; (13) a battery of four dry cells or a 6 volt storage battery, and either (14) a telephone induction coil as shown in Fig. 86; (15) a microphone transformer as shown in Fig. 87; or a magnetic modulator as shown in Fig. 88. All of these parts have been described, as said above, in Chapter XVI, except the microphone modulators.



The Telephone Induction Coil.—This is a little induction coil that transforms the 6-volt battery current after it has flowed through and been modulated by the microphone transmitter into alternating currents that have a potential of 1,000 volts of more. It consists of a primary coil of No. 20 B. and S. gauge cotton covered magnet wire wound on a core of soft iron wires while around the primary coil is wound a secondary coil of No. 30 magnet wire. Get a standard telephone induction coil that has a resistance of 500 or 750 ohms and this will cost you a couple of dollars.

The Microphone Transformer.—This device is built on exactly the same principle as the telephone induction coil just described but it is more effective because it is designed especially for modulating the oscillations set up by vacuum tube transmitters. As with the telephone induction coil, the microphone transmitter is connected in series with the primary coil and a 6 volt dry or storage battery.

In the better makes of microphone transformer, there is a third winding, called a side tone coil, to which a headphone can be connected so that the operator who is speaking into the microphone can listen-in and so learn if his transmitter is working up to standard.

The Magnetic Modulator.—This is a small closed iron core transformer of peculiar design and having a primary and a secondary coil wound on it. This device is used to control the variations of the oscillating currents that are set up by the oscillator tube. It is made in three sizes and for the transmitter here described you want the smallest size, which has an output of 1/2 to 1-1/2 amperes. It costs about $10.00.

How the Apparatus Is Connected Up.—The different pieces of apparatus are connected together in exactly the same way as the 100 Mile C. W. Telegraph Set in Chapter XVI except that the microphone transmitter and microphone modulator (whichever kind you use) is substituted for the telegraph key and chopper.

Now there are three different ways that the microphone and its modulator can be connected in circuit. Two of the best ways are shown at A and B in Fig. 89. In the first way the secondary terminals of the modulator are shunted around the grid leak in the grid circuit as at A, and in the second the secondary terminals are connected in the aerial as at B. Where an induction coil or a microphone transformer is used they are shunted around a condenser, but this is not necessary with the magnetic modulator. Where a second tube is used as in Fig. 90 then the microphone and its modulator are connected with the grid circuit and clip 3 of the tuning coil.



A 50 to 100 Mile Wireless Telephone Transmitter—With Direct Current Motor Generator.—As the initial source of current available is taken to be a 110 or 220 volt direct current a motor-generator having an output of 350 volts must be used as before. The only difference between this transmitter and the preceding one is that: (1) two 5 watt tubes are used, the first serving as an oscillator and the second as a modulator; (2) an oscillation choke coil is used in the plate circuit; (3) a reactance coil or reactor, is used in the plate circuit; and (4) a reactor is used in the grid circuit.

The Oscillation Choke Coil.—You can make this choke coil by winding about 275 turns of No. 28 B. and S. gauge cotton covered magnet wire on a spool 2 inches in diameter and 4 inches long. Give it a good coat of shellac varnish and let it dry thoroughly.

The Plate and Grid Circuit Reactance Coils.—Where a single tube is used as an oscillator and a second tube is employed as a modulator, a reactor, which is a coil of wire wound on an iron core, is used in the plate circuit to keep the high voltage direct current of the motor-generator the same at all times. Likewise the grid circuit reactor is used to keep the voltage of the grid at a constant value. These reactors are made alike and a picture of one of them is shown in Fig. 91 and each one will cost you $5.75.



Connecting up the Apparatus.—All of the different pieces of apparatus are connected up as shown in Fig. 89. One of the ends of the secondary of the induction coil, or the microphone transformer, or the magnetic modulator is connected to the grid circuit and the other end to clip 3 of the tuning coil.

A 100 to 200 Mile Wireless Telephone Transmitter—With Direct Current Motor Generator.—By using the same connections shown in the wiring diagrams in Fig. 89 and a single 50 watt oscillator tube your transmitter will then have a range of 100 miles or so, while if you connect up the apparatus as shown in Fig. 90 and use two 50 watt tubes you can work up to 200 miles. Much of the apparatus for a 50 watt oscillator set where either one or two tubes are used is of the same size and design as that just described for the 5 watt oscillator sets, but, as in the C. W. telegraph sets, some of the parts must be proportionately larger. The required parts are (1) the 50 watt tube; (2) the grid leak resistance; (3) the filament rheostat; (4) the filament storage battery; and (5) the magnetic modulator. All of these parts, except the latter, are described in detail under the heading of a 500 Mile C. W. Telegraph Transmitting Set in Chapter XVI, and are also pictured in that chapter.

It is not advisable to use an induction coil for the modulator for this set, but use, instead, either a telephone transformer, or better, a magnetic modulator of the second size which has an output of from 1-1/2 to 3-1/2 amperes. The magnetic modulator is described and pictured in this chapter.

A 50 to 100 Mile Wireless Telephone Transmitting Set—With 110 Volt Alternating Current.—If you have a 110 volt [Footnote: Alternating current for lighting purposes ranges from 102.5 volts to 115 volts, so we take the median and call it 110 volts.] alternating current available you can use it for the initial source of energy for your wireless telephone transmitter. The chief difference between a wireless telephone transmitting set that uses an alternating current and one that uses a direct current is that: (1) a power transformer is used for stepping up the voltage instead of a motor-generator, and (2) a vacuum tube rectifier must be used to convert the alternating current into direct current.

The Apparatus You Need.—For this telephone transmitting set you need: (1) one aerial ammeter; (2) one tuning coil; (3) one telephone modulator; (4) one aerial series condenser; (5) one 4 cell dry battery or a 6 volt storage battery; (6) one microphone transmitter; (7) one battery switch; (8) one grid condenser; (9) one grid leak; (10) two 5 watt oscillator tubes with sockets; (11) one blocking condenser; (12) one oscillation choke coil; (13) two filter condensers; (14) one filter reactance coil; (15) an alternating current power transformer, and (16) two 20 watt rectifier vacuum tubes.

All of the above pieces of apparatus are the same as those described for the 100 Mile C. W. Telegraph Transmitter in Chapter XVII, except: (a) the microphone modulator; (b) the microphone transmitter and (c) the dry or storage battery, all of which are described in this chapter; and the new parts which are: (d) the rectifier vacuum tubes; (e) the filter condensers; and (f) the filter reactance coil; further and finally, the power transformer has a third secondary coil on it and it is this that feeds the alternating current to the rectifier tubes, which in turn converts it into a pulsating direct current.

The Vacuum Tube Rectifier.—This rectifier has two electrodes, that is, it has a filament and a plate like the original vacuum tube detector, The smallest size rectifier tube requires a plate potential of 550 volts which is developed by one of the secondary coils of the power transformer. The filament terminal takes a current of 7.5 volts and this is supplied by another secondary coil of the transformer. This rectifier tube delivers a direct current of 20 watts at 350 volts. It looks exactly like the 5 watt oscillator tube which is pictured at E in Fig. 77. The price is $7.50.

The Filter Condensers.—These condensers are used in connection with the reactance coil to smooth out the pulsating direct current after it has passed through the rectifier tube. They have a capacitance of 1 mfd. and will stand 750 volts. These condensers cost about $2.00 each.

The Filter Reactance Coil.—This reactor which is shown in Fig. 92, has about the same appearance as the power transformer but it is somewhat smaller. It consists of a coil of wire wound on a soft iron core and has a large inductance, hence the capacitance of the filter condensers are proportionately smaller than where a small inductance is used which has been the general practice. The size you require for this set has an output of 160 milliamperes and it will supply current for one to four 5 watt oscillator tubes. This size of reactor costs $11.50.



Connecting Up the Apparatus.—The wiring diagram in Fig. 93 shows how the various pieces of apparatus for this telephone transmitter are connected up. You will observe: (1) that the terminals of the power transformer secondary coil which develops 10 volts are connected to the filaments of the oscillator tubes; (2) that the terminals of the other secondary coil which develops 10 volts are connected with the filaments of the rectifier tubes; (3) that the terminals of the third secondary coil which develops 550 volts are connected with the plates of the rectifier tubes; (4) that the pair of filter condensers are connected in parallel and these are connected to the mid-taps of the two filament secondary coils; (5) that the reactance coil and the third filter condenser are connected together in series and these are shunted across the filter condensers, which are in parallel; and, finally, (6) a lead connects the mid-tap of the 550-volt secondary coil of the power transformer with the connection between the reactor and the third filter condenser.



A 100 to 200 Mile Wireless Telephone Transmitting Set—With 110 Volt Alternating Current.—This telephone transmitter is built up of exactly the same pieces of apparatus and connected up in precisely the same way as the one just described and shown in Fig. 93.

Apparatus Required.—The only differences between this and the preceding transmitter are: (1) the magnetic modulator, if you use one, should have an output of 3-1/2 to 5 amperes; (2) you will need two 50 watt oscillator tubes with sockets; (3) two 150 watt rectifier tubes with sockets; (4) an aerial ammeter that reads to 5 amperes; (5) three 1 mfd. filter condensers in parallel; (6) two filter condensers of 1 mfd. capacitance that will stand 1750 volts; and (6) a 300 milliampere filter reactor.

The apparatus is wired up as shown in Fig. 93.



CHAPTER XIX

THE OPERATION OF VACUUM TUBE TRANSMITTERS

The three foregoing chapters explained in detail the design and construction of (1) two kinds of C. W. telegraph transmitters, and (2) two kinds of wireless telephone transmitters, the difference between them being whether they used (A) a direct current, or (B) an alternating current as the initial source of energy. Of course there are other differences between those of like types as, for instance, the apparatus and connections used (a) in the key circuits, and (b) in the microphone circuits. But in all of the transmitters described of whatever type or kind the same fundamental device is used for setting up sustained oscillations and this is the vacuum tube.

The Operation of the Vacuum Tube Oscillator.—The operation of the vacuum tube in producing sustained oscillations depends on (1) the action of the tube as a valve in setting up the oscillations in the first place and (2) the action of the grid in amplifying the oscillations thus set up, both of which we explained in Chapter XIV. In that chapter it was also pointed out that a very small change in the grid potential causes a corresponding and larger change in the amount of current flowing from the plate to the filament; and that if a vacuum tube is used for the production of oscillations the initial source of current must have a high voltage, in fact the higher the plate voltage the more powerful will be the oscillations.

To understand how oscillations are set up by a vacuum tube when a direct current is applied to it, take a look at the simple circuits shown in Fig. 94. Now when you close the switch the voltage from the battery charges the condenser and keeps it charged until you open it again; the instant you do this the condenser discharges through the circuit which includes it and the inductance coil, and the discharge of a condenser is always oscillatory.



Where an oscillator tube is included in the circuits as shown at A and B in Fig. 94, the grid takes the place of the switch and any slight change in the voltage of either the grid or the plate is sufficient to start a train of oscillations going. As these oscillations surge through the tube the positive parts of them flow from the plate to the filament and these carry more of the direct current with them.

To make a tube set up powerful oscillations then, it is only necessary that an oscillation circuit shall be provided which will feed part of the oscillations set up by the tube back to the grid circuit and when this is done the oscillations will keep on being amplified until the tube reaches the limit of its output.



The Operation of C. W. Telegraph Transmitters With Direct Current—Short Distance C. W. Transmitter.—In the transmitter shown in the wiring diagram in Fig. 76 the positive part of the 110 volt direct current is carried down from the lamp socket through one side of the panel cut-out, thence through the choke coil and to the plate of the oscillator tube, when the latter is charged to the positive sign. The negative part of the 110 volt direct current then flows down the other wire to the filament so that there is a difference of potential between the plate and the filament of 110 volts. Now when the 6-volt battery current is switched on the filament is heated to brilliancy, and the electrons thrown off by it form a conducting path between it and the plate; the 110 volt current then flows from the latter to the former.

Now follow the wiring from the plate over to the blocking condenser, thence to clip 3 of the tuning coil, through the turns of the latter to clip 2 and over to the filament and, when the latter is heated, you have a closed oscillation circuit. The oscillations surging in the latter set up other and like oscillations in the tuning coil between the end of which is connected with the grid, the aerial and the clip 2, and these surge through the circuit formed by this portion of the coil, the grid condenser and the filament; this is the amplifying circuit and it corresponds to the regenerative circuit of a receiving set.

When oscillations are set up in it the grid is alternately charged to the positive and negative signs. These reversals of voltage set up stronger and ever stronger oscillations in the plate circuit as before explained. Not only do the oscillations surge in the closed circuits but they run to and fro on the aerial wire when their energy is radiated in the form of electric waves. The oscillations are varied by means of the telegraph key which is placed in the grid circuit as shown in Fig. 76.

The Operation of the Key Circuit.—The effect in a C. W. transmitter when a telegraph key is connected in series with a buzzer and a battery and these are shunted around the condenser in the grid circuit, is to rapidly change the wave form of the sustained oscillations, and hence, the length of the waves that are sent out. While no sound can be heard in the headphones at the receiving station so long as the points of the key are not in contact, when they are in contact the oscillations are modulated and sounds are heard in the headphones that correspond to the frequency of the buzzer in the key circuit.