It is manifestly impossible for one operator to attend to the calls of all the subscribers in the exchange, and so a number of operators are employed, each one having to attend to the calls of a certain number of subscribers. At the same time it is clear that each operator may be called upon to connect one of her subscribers to any other subscriber in the whole exchange. In order to make this possible the switchboard is divided into sections, each having as many jacks as there are lines in the exchange, so that in this respect all the sections are multiples of each other, and the whole arrangement is called a “multiple switchboard,” the repeated jacks being called “multiple jacks.” Then there are other jacks which it is not necessary to duplicate. We have seen that when a subscriber calls the exchange a lamp glows, and the operator inserts a plug into the jack beside the lamp, in order to answer the call and ascertain what number is required. These are called “answering jacks,” and the lamp is the line signal. It is usual to have three operators to each section of the switchboard, and each operator has charge of so many answering jacks, representing so many subscribers. At the same time she has access to the whole section, so that she can connect any of her subscribers to any other line in the exchange. When a number is called for, the operator must be able to tell at once whether the line is free or not. The jack in her section may be unoccupied, but she must know also whether all the multiple jacks belonging to that number are free, for an operator at another section may have connected the line to one of her subscribers. To enable an operator to ascertain this quickly an electrical test is provided. When two lines are connected, the whole of the multiple jacks belonging to each are charged with electricity, and if an operator at any section touches one of these jacks with a plug, a current through her receiver makes a click, and on hearing the click she knows that the line is engaged. The testing takes an extremely short time, and this is why a caller receives the reply, “Number engaged,” so promptly that he feels inclined to doubt whether the operator has made any attempt at all to connect him up to the number. In order that an operator may have both hands free to manipulate the plugs, her telephone receiver is fixed over one ear by a fastening passing over her head, and the transmitter is hung from her shoulders so as to be close to her mouth.