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

English: The connexions for single-current working on the “closed-circuit” system are shown. It differs from the open circuit in only requiring one battery (although, as in the figure, half of it is often placed at each end), in having the receiving instrument between the line and the key, and in having the battery continuously to the line. The battery is kept to the line by the bar c, which short-circuits the keys. When signals are to be sent from either station the operator turns the switch c out of contact with the stop b, and then operates precisely as in open circuit sending. This system is more expensive than the open-circuit system, as the battery is always at work; but it offers some advantages on circuits where there are a number of intermediate stations, as the circuit is under a constant electromotive force and has the same resistance no matter which station is sending or receiving. The arrangement at a wayside station is shown at W. When the circuit is long and contains a large number of stations, the sending battery is sometimes divided among them in order to give greater uniformity of current along the line. When only one battery is used the current at the distant end may be considerably affected by the leakage to earth along the line.