THE BELL TELEPHONE.
The first telephone that came into general use was that of Bell, shown in Fig. 62. In a central hole of an ebonite casing is fixed a permanent magnet, M. The casing expands at one end to accommodate a coil of insulated wire wound about one extremity of a magnet. The coil ends are attached to wires passing through small channels to terminals at the rear. A circular diaphragm, D, of very thin iron plate, clamped between the concave mouthpiece and the casing, almost touches the end of the magnet.
We will suppose that two Bell telephones, A and B, are connected up by wires, so that the wires and the coils form a complete circuit. Words are spoken into A. The air vibrations, passing through the central hole in the cover, make the diaphragm vibrate towards and away from the magnet. The distances through which the diaphragm moves have been measured, and found not to exceed in some cases more than 1⁄10,000,000 of an inch! Its movements distort the shape of the "lines of force" (see p. 118) emanating from the magnet, and these, cutting through the turns of the coil, induce a current in the line circuit. As the diaphragm approaches the magnet a circuit is sent in one direction; as it leaves it, in the other. Consequently speech produces rapidly alternating currents in the circuit, their duration and intensity depending on the nature of the sound.
Now consider telephone B. The currents passing through its coil increase or diminish the magnetism of the magnet, and cause it to attract its diaphragm with varying force. The vibration of the diaphragm disturbs the air in exact accordance with the vibrations of A's diaphragm, and speech is reproduced.
THE EDISON TRANSMITTER.
The Bell telephone may be used both as a transmitter and a receiver, and the permanent magnetism of the cores renders it independent of an electric battery. But currents generated by it are so minute that they cannot overcome the resistance of a long circuit; therefore a battery is now always used, and with it a special device as transmitter.
If in a circuit containing a telephone and a battery there be a loose contact, and this be shaken, the varying resistance of the contact will cause electrical currents of varying force to pass through the circuit. Edison introduced the first successful microphone transmitter, in which a small platinum disc connected to the diaphragm pressed with varying force against a disc of carbon, each disc forming part of the circuit. Vibrations of the diaphragm caused current to flow in a series of rapid pulsations.