Fig. 131.
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.
Fig. 132.
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.
Fig. 133.
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.
Fig. 134.