=314. The Telephone Transmitter.=--The telephone receiver just described has great sensitiveness in reproducing sound, but it is not satisfactory as a transmitter or sending apparatus. The _transmitter_ commonly used is represented in cross-section in Fig. 311. In this figure, back of the mouthpiece, is a thin carbon disc, _D_. Back of this disc is a circular compartment containing granular carbon, _g_. The wires of the circuit are connected to the carbon disc and to the back of the case containing granular carbon. The circuit through the transmitter also includes a voltaic or storage cell and the primary coil of an induction coil. (See Fig. 312.) [Illustration: FIG. 311.--A telephone transmitter.] [Illustration: 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. [Illustration: 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.