a central cavity of an ebonite seating is a carbon block, C, with a face moulded into a number of pyramidal projections, P P. The space between C and a carbon diaphragm, D, is packed with carbon granules, G G. C has direct contact with line terminal T, which screws into it; D with T1 through the brass casing, screw S, and a small plate at the back of the transmitter. Voice vibrations compress G G, and allow current to pass more freely from D to C. This form of microphone is very delicate, and unequalled for long-distance transmission.
GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT.
In many forms of subscriber's instruments both receiver and transmitter are mounted on a single handle in such a way as to be conveniently placed for ear and mouth. For the sake of clearness the diagrammatic sketch of a complete installation (Fig. 64) shows them separated. The transmitters, it will be noticed, are located in battery circuits, including the primary windings P P2 of induction coils. The transmitters are in the line circuit, which includes the secondary windings S S2 of the coils.
We will assume that the transmitters are, in the first instance, both hung on the hooks of the metallic switches, which their weight depresses to the position indicated by the dotted lines. The handle of the magneto-generator at the left-end station is turned, and current passes through the closed circuit:—Line A, E B2, contact 10, the switch 9; line B, 4, the other switch, contact 5, and E B. Both bells ring. Both parties now lift their receivers from the switch hooks. The switches rise against contacts 1, 2, 3 and 6, 7, 8 respectively. Both primary and both secondary circuits are now completed, while the