Chapter XVI

THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.

=119. The Telephone= is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. The part spoken to is called the _transmitter_, and the part which gives sound out again is called the _receiver_. Sound itself does not pass over the line. While the same apparatus can be used for both transmitter and receiver, they are generally different in construction to get the best results.

=120. The Bell or Magneto-transmitter= generates its own current, and is, strictly speaking, a dynamo that is run by the voice. It depends upon induction for its action.

Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the wires being connected to a delicate galvanoscope. If one pole of the magnet H M be suddenly moved up and down near the core, an alternating current will be generated in the coil, the circuit being completed through the galvanoscope. As H M approaches the core the current will flow in one direction, and as H M is withdrawn it will pass in the opposite direction. The combination makes a miniature alternating dynamo.

If we imagine the soft iron core of H, Fig. 125, taken out, and one pole of H M, or preferably that of a bar magnet stuck through the coil, a feeble current will also be produced by moving the soft iron back and forth near the magnet's pole. This is really what is done in the Bell transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being made to vibrate by the voice immediately in front of a coil having a permanent magnet for a core. The disc, or _diaphragm_, as it is called, is fixed near, but it does not touch, the magnet. It is under a constant strain, being attracted by the magnet, so its slightest movement changes the strength of the magnetic field, causing more or less lines of force to shoot through the turns of the coil and induce a current. The coil consists of many turns of fine, insulated wire. The current generated is an alternating one, and although exceedingly small can force its way through a long length of wire.

Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form of compound magnet frequently used in the transmitter. Fig. 129 shows a transmitter with cords which contain flexible wires.

=121. The Receiver=, for short lines, may have the same construction as the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers, either being used as the transmitter and the other as the receiver. As the alternating current goes to the distant receiver, it flies through the coil first in one direction and then in the other. This alternately strengthens and weakens the magnetic field near the diaphragm, causing it to vibrate back and forth as the magnet pulls more or less. The receiver diaphragm repeats the vibrations in the transmitter. Nothing but the induced electric current passes over the wires.

=122. The Microphone.= If a current of electricity be allowed to pass through a circuit like that shown in Fig. 131, which includes a battery, a Bell receiver, and a microphone, any slight sound near the microphone will be greatly magnified in the receiver. The microphone consists of pieces of carbon so fixed that they form loose contacts. Any slight movement of the carbon causes the resistance to the current to be greatly changed. The rapidly varying resistance allows more or less current to pass, the result being that this pulsating current causes the diaphragm to vibrate. The diaphragm has a constantly varying pull upon it when the carbons are in any way disturbed by the voice, or by the ticking of a watch, etc. This principle has been made use of in carbon transmitters, which are made in a large variety of forms.

=123. The Carbon Transmitter= does not, in itself, generate a current like the magneto-transmitter; it merely produces changes in the strength of a current that flows through it and that comes from some outside source. In Fig. 132, X and Y are two carbon buttons, X being attached to the diaphragm D. Button Y presses gently against X, allowing a little current to pass through the circuit which includes a battery, D C, and a receiver, R. When D is caused to vibrate by the voice, X is made to press more or less against Y, and this allows more or less current to pass through the circuit. This direct undulating current changes the pull upon the diaphragm of R, causing it to vibrate and reproduce the original sounds spoken into the transmitter. In regular lines, of course, a receiver and transmitter are connected at each end, together with bells, etc., for signaling.

=124. Induction Coils in Telephone Work.= As the resistance of long telephone lines is great, a high electrical pressure, or E.M.F. is desired. While the current from one or two cells is sufficient to work the transmitter properly, and cause undulating currents in the short line, it does not have power enough to force its way over a long line.

To get around this difficulty, an induction coil, Fig. 133, is used to transform the battery current, that flows through the carbon transmitter and primary coil, into a current with a high E. M. F. The battery current in the primary coil is undulating, but always passes in the same direction, making the magnetic field around the core weaker and stronger. This causes an alternating current in the secondary coil and main line. In Fig. 133 P and S represent the primary and secondary coils. P is joined in series with a cell and carbon transmitter; S is joined to the distant receiver. One end of S can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth.

=125. Various forms= of telephones are shown in Figs. 134, 135, 136. Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form of wall telephone; Fig. 136 shows head-telephones for switchboard operators.