EXPERIMENT 200. To study the action of the telephone.
478. Directions. (A) Join the ends of coil H ([Fig. 164]) to the astatic galvanoscope. Move magnet M back and forth in front of the soft iron core, while H is held in position. Watch the needle. Imagine that vibrations in the air caused by the voice are strong enough to give M a slight motion to and fro, and you can see how a current would be sent through the galvanoscope by speaking against M.
479. The Telephone is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. The part spoken to is called the transmitter, and the part which gives the sound out again is called the receiver. Sound itself does not pass over the line. Although the same apparatus may be used for both transmitter and receiver, they are generally different in construction.
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.