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.
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,[206] 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.
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.
The battery current in the primary coil is undulating, but always in the same direction, the magnetic field around the core getting weaker and stronger. This causes an alternating current in the secondary coil and main line.