Fig. 311.—A telephone transmitter.
Fig. 312.—Telephone instruments at one end of a talking circuit.
315. The action of the transmitter is explained as follows: When the sound waves of the voice strike upon the carbon disc, the latter vibrates, alternately increasing and decreasing the pressure upon the granular carbon. When the pressure increases, the electrical resistance of the granular carbon is lessened, and when the pressure upon it is decreased, its resistance increases. This changing resistance causes fluctuations in the electric current that correspond exactly with the sound waves of the voice affecting it.
316. A complete telephone system operating with a local battery is shown in Fig. 312. A person speaking into the transmitter causes a fluctuation in the electric current in the transmitter as described in Art. 315. This fluctuating current passes through the primary coil of the induction coil Ic. This fluctuating current produces a fluctuating magnetic field in its core. This fluctuating field induces an alternating current in the secondary coil which alternates just as the primary current fluctuates, but with a much higher E.M.F. than the latter. The alternating current passes to the receiver which reproduces the speech as described in Art. 313. The line circuit includes the secondary of the induction coil, the receiving instrument and the receiver of the sending instrument so that the voice is reproduced in both receivers. An electric bell is placed at each station to call the attention of parties wanted. The movement of the receiver hook when the receiver is lifted, disconnects the bell and closes the talking circuit. The latter is opened and the bell connected when the receiver is hung up again.
Fig. 313.—Diagram of a telephone system as used in a large exchange.
In cities and towns, the telephone system in use differs from the one described in usually having one large battery placed in the central exchange, instead of dry cells at each instrument. (See Fig. 313.) Also the operator at central is called by simply taking the receiver from the hook instead of being "rung up" by the subscriber. The operations of the transmitter, induction coil and receiver, however, are the same in all telephones.
Important Topics
1. Receiver: parts, action.
2. Transmitter: parts, action.