_=480. The Bell or Magneto-transmitter=_ generates its own current, and is, strictly speaking, a dynamo that is run by the voice. You have seen, by experiments, that a current can be generated in a coil of wire by moving a magnet back and forth in front of its soft iron core. In the telephone this process is reversed, soft iron in the shape of a thin disc (D, Fig. 165) being made to vibrate by the voice immediately in front of a coil having a permanent magnet, M, for a core. The soft iron diaphragm is fixed near, but it does not touch the magnet. The coil consists of many turns of fine insulated wire. The current generated is an alternating one and exceedingly feeble; in fact, it can not be detected by a galvanoscope. _=481. The Receiver=_ has the same construction as the bell transmitter, and receives the currents from the line. As the diaphragm is always attracted by the magnet, it is under a constant strain. This strain is increased when a current passes through the coil in a direction that adds strength to the magnet, and decreased when the current weakens the magnet. When the current through the coil is always in the same direction, but varies in strength, the diaphragm will vibrate on account of the varying pull upon it. [Illustration: Fig. 166.] When the current through the coil is an alternating one, the same result is obtained, as the magnet gets weaker and stronger many times per minute. Fig. 166 shows two bell instruments joined, either being used as the transmitter and the other as the receiver. _=482. 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. 167, X and Y are two carbon buttons, X being attached to the diaphragm, D. Button Y presses gently against X. 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, in which also is the receiver, R. 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. [Illustration: Fig. 167.] _=483. Induction Coils in Telephone Work.=_ As the resistance of telephone lines is large, a current with a fairly high E. M. F. is desired. While the current from one or two cells is sufficient to work the transmitter, it is not strong enough to force its way over a long line. To get around this difficulty an induction coil is used to transform the battery current, that flows through the transmitter and primary coil, into a current with a high E. M. F. that can go into the main line and force its way to a distant receiver.