Now let us see what takes place at the exchange. The subscriber’s instrument is connected to a mechanical arrangement known as a “line switch.” This switch is brought into play by the act of removing the receiver from its hook, and it then automatically connects the subscriber’s line to what is called a “first selector” switch. The group of two impulses sent out by the first turning of the dial raises this first selector two steps, and it then sweeps along a row of contacts connected to “trunks” going to the 2000 section. Passing by occupied trunks, it finds an idle one, and so connects the line to an idle “second selector.” This selector is operated by the second group of impulses, five in number, and after being raised five steps it acts like the first selector, and finds an idle trunk leading to the 2500 section. This places the caller’s line in connexion with still another switch called a “connector,” and this switch, operated by the remaining groups of eight and three impulses, finds the required tens section, and selects the third member of that section. If the number 2583 is disengaged, the connector switch now sends current from the central battery to this instrument, thus ringing its bell, and it also supplies speaking current to the two lines during the conversation, restores the exchange mechanism to its original condition as soon as the conversation is ended and the subscribers have hung up their receivers, and registers the call on the calling subscriber’s meter. If the connector finds the number engaged, it sends out an intermittent buzzing sound, to inform the caller of the fact. All these operations take time to describe, even in outline, but in practice they are carried out with the utmost rapidity, each step in the connecting-up process taking only a small fraction of a second. For ordinary local calls the automatic system requires no operators at all, but for the convenience of users there are usually two clerks at the exchange, one to give any information required by subscribers, and the other to record complaints regarding faulty working. For trunk calls, the subscriber places his finger in the hole marked 0, and gives the dial one turn. This connects him to an operator at the trunk switchboard, who makes the required connexion and then calls him up in the usual way.