File:EB1911 Telegraph - Open Circuit, Single-current System.jpg

English: The arrangement on the “open-circuit” system for single-current working is shown, in which L₁ represents the line, G a galvanometer, used simply to show that the currents are going to line emit when the message is being transmitted, K the transmitting key, B the battery, I the receiving instrument, and E the earth-plate. The complete circuit is from the plate E through the instrument I, the key K, and the galvanoscope G to the line L₁, then through the corresponding instruments to the earth-plate E at the other end, and back through the earth to the plate E. The earth is always, except for some special reason, used as a return, because it offers little resistance and saves the expense and the risk of failure of the return wire. The earth-plate E ought to be buried in moist earth or in water. In towns the water and gas pipe systems form excellent earth plates. It will be observed that the circuit is not in this case actually open; the meaning of the expression “open circuit” is “no battery to line.” In normal circumstances the instruments at both ends are ready to receive, both ends of the line being to earth through the receiving instruments. A signal is sent by depressing the key K, and so changing the contact from a to b, and thus putting the battery to line. On circuits where the traffic is small it is usual to make one wire serve several stations. At an intermediate or wayside station W, a “switch” S, consisting of three blocks of brass fixed to an insulating base, is sometimes used (not in Great Britain). W may be made the terminal station of L1 by inserting plug 3, and of L2 by inserting plug 2, or the instruments may be cut out of circuit by inserting plug 1. In ordinary circumstances the messages from all stations are sent through the whole line, and thus the operator at any station may transmit, if the line is free, by manipulating his key.