In cases where “universal battery” working, i.e. the working of several instruments from one set of batteries or accumulators, is adopted, the positive and negative currents have to be sent from independent batteries, as shown. The stop "a" of the key K is connected through a switch S with one pole of the battery B, and the stop "b" in the usual way with the other pole. Suppose the arm c of the switch S to be in contact with 2; then when the key is manipulated it sends alternately positive and negative currents into the line. If the positive is called the signalling current, the line will be charged positively each time a signal is sent; but as soon as the signal is completed a negative charge is communicated to the line, thus hastening the discharge and the return of the relay tongue to its insulated stop.