Fig. 165
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.
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.
