Fig. 160
Fig. 161
Fig. 162
We took a telephone induction coil ([Fig. 162]) and fastened it to a board as represented in [Fig. 163], I. One wire of the primary circuit was fastened to the binding post a. The other wire from the primary coil passed to the switch S and then to the battery. From the battery the wire ran to the binding post b. C is a steel tuning fork. The secondary circuit is closed through a telephone receiver. These wires are extended so that the receiver is too far distant for the tuning fork to be heard through the air. When the switch S is closed the tuning fork acts as the interrupter for the primary circuit, and it interrupts according to its time of vibration. If, for instance, the fork gives the tone of middle C on the piano it vibrates 256 times a second. It interrupts the primary circuit 256 times a second. It induces an alternating current of the same frequency in the secondary circuit. The diaphragm of the telephone receiver vibrates in perfect time with the tuning fork and produces the same tone as the tuning fork. We had a series of tuning forks giving a variety of tones, which we could substitute one after another in place of this one. The receiver reproduced accurately the tone of each one of them.