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.Fig. 62.--Cross-section of Bell telephone Receiver.

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.Fig. 63.--Wireless Headphone.

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.Fig. 64.--Wireless Headphone.

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.Fig. 65.--Arkay Loud Speaker.

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.Fig. 66.--Amplitone Loud Speaker.

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.Fig. 67.--Amplitron Loud Speaker.

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.Fig. 68.--Magnavox Loud Speaker.