Producing a tone is merely a matter of making something vibrate with the required frequency. It may be a piano string, or a tuning fork, or a reed of an electric buzzer, or the diaphragm of a telephone receiver. If it vibrates 256 times a second, it will produce the same tone as middle _C_ on a piano; if it vibrates 512 times a second it will produce the _C_ which is an octave above, and if 128 times a second an octave below middle _C_. The human voice is produced by vocal cords in the throat, which vibrate with the proper frequency to give any required tone. But how can we make the human voice act as an interrupter of the primary circuit? An examination of the telephone transmitter will supply the answer to this question. [Illustration: Fig. 165] [Illustration: Fig. 166] The boys after taking the transmitter (Fig. 165) apart proceeded to make one which should answer the purpose as follows: A block of wood about one inch thick and three inches square (Fig. 166), _A_, was hollowed out, making a cone-shaped cavity about one half inch deep and one inch broad. This cavity was filled with small pieces of graphite, _G_, made by cutting up a lead pencil. An old tin-type, _D_, was laid over this as a diaphragm and tacked around the edges. A binding post, _E_, passed through the block, its head being buried in the graphite at the bottom of the cavity. The binding post _F_ furnished contact with the tin-type. One dry cell was placed at _B_ and the sensitive ammeter was connected at _C_. The needle showed that although a small current was passing it was constantly varying in strength. Tapping upon the table, walking across the floor of the room, shouting, and particularly whistling, caused variations in the conducting power of the graphite and consequently variations in the current strength. This is precisely the condition we wished to produce in the primary circuit. [Illustration: Fig. 167] We next substitute for the ammeter at _C_ the primary and secondary coil of the telephone. In Fig. 167 _T_ is the transmitter, _B_ is a battery of two dry cells, _P_ is the primary winding of the coils, and _S_ is the secondary winding. To this a telephone receiver, _R_ is connected by wires long enough to reach into another room. A person holding the receiver at his ear could hear everything said or done in the room where the transmitter was almost as plainly as though he were present in the room. [Illustration: Fig. 168]