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.
In telegraphy it is the rule to employ the earth for the return part of the circuit, but this is not customary in telephony. The telephone is a much more sensitive instrument than the telegraph, and a telephone having an earth return is subject to all kinds of strange and weird noises which greatly interfere with conversation. These noises may be caused by natural electrical disturbances, or by the proximity of telegraph and other wires conveying electric currents. On this account telephone lines are made with a complete metallic circuit. As in telegraphy, protection from lightning flashes is afforded by lightning arresters. The current for the working of a telephone exchange is supplied from a central battery of accumulators, and also from dynamos.
PLATE XII.
By permission of
Craven Brothers Ltd.
LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.
Although the manual exchange telephone system of to-day works with remarkable efficiency, it has certain weak points. For instance, if an operator cares to do so, she can listen to conversations between subscribers, so that privacy cannot be assured. As a matter of fact, the operators have little time for this kind of thing, at any rate during the busy hours of the day, and as a rule they are not sufficiently interested in other people’s affairs to make any attempt to listen to their remarks. The male operators who work through the slack hours of the night are occasionally guilty of listening. Some time ago the writer had to ring up a friend in the very early morning, and during the conversation this gentleman asked what time it was. Before the writer had time to get a word out, a deep bass voice from the exchange replied, “Half-past two.” Little incidents of this sort remind one that it is not wise to speak too freely by telephone. Then again operators are liable to make wrong connexions through faulty hearing of the number called for, and these are equally annoying to the caller and to the person rung up in mistake. Many other defects might be mentioned, but these are sufficient to show that the manual system is not perfect.