16, in which L 1 represents the line, G a galvanometer, used simply to show that the currents are going to line circuit, when the message is being transmitted, K the trans- - mitting key, B the battery, I the receiving instrument, and E the earth-plate.
The key K (shown in general plan), when worked, sends reversed currents from the battery B.
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 by fig.
Suppose the arm c of the switch S to be in contact with 2; thin when the key is manipulated it sends alternately positive and negative currents into the line.
In order that the line between two stations may be worked on the duplex system it is essential that the receiving instrument shall not be acted on by the outgoing currents, but shall respond to incoming currents.