All the subscribers' lines are connected in order to jacks on the first two or three or four operators' positions, and these connexions are repeated or " multipled " upon each succeeding similar group of positions.
Each subscriber's circuit is further connected to another spring-jack directly associated with the calling-drop. These springjacks, known as answering jacks, are distributed along the switchboard, a certain number being terminated upon each position and placed in the care of the operator assigned to that position.
In the arrangement first introduced the line wire is connected in series through the various spring-jacks, the circuit finally passing through the answering jack to the calling-drop. This arrangement is liable to give trouble, as disconnexions may arise in the spring-jacks in consequence of the failures of the springs to make contact.
In this arrangement, instead of the circuit being made through the jacks in series, each jack is connected to an independent branch from the main circuit.
This requirement is usually met by connecting a third or " test " wire to each of the jacks associated with a subscriber's line, and by making the circuit arrangements such that this wire is either disconnected or at earth potential when the line is not in use, and at some potential above or below that of the earth, when the circuit is engaged.