In towns, each person having a telephone is connected to an excnange, and by which he can be connected at will to any other person also connected to the exchange.

The line wires, connecting the transmitting and receiving stations, consist of two copper conductors of high conductivity and great tensile strength. It is found that alloys of copper and silicon, or copper and aluminium, are best for this purpose. The wires have a resistance of about 80 ohms per mile, and their breaking tension is about 150 lbs. With wire weighing about 19 lbs. per mile, spans of 100 to 150 yards are possible. The wires, if overhead, are supported on porcelain insulators, which are attached at a convenient height from the ground to posts or houses; in the country trees often come in handy for the purpose. In cities the best method is to lay them underground, where they are safe from the effects of fire or weather; this, however, is rarely done, as overhead wires are so very much cheaper.

If a telephone line does not run anywhere near any other telephone, telegraph, or electric circuit, the return wire may be replaced with economy by the earth. In this case very careful connection must be made with the earth at each end. A good plan is to solder the connecting wire to a water pipe, but if this is not available, it must be connected to a large metallic plate of copper buried deeply in damp earth, which it is as well to water from time to time. It is best to avoid this earth return for telephones if possible, as it is very seldom satisfactory.

In fixing up the telephone apparatus it should never be attached to thin partitions which are capable of vibration, but should be screwed to a solid wall, and even then it is as well to place a couple of layers of felt between the instrument and the wall.