Page:Electricity (1912) Kapp.djvu/241

Rh we get a system under which each house having an even number would be connected to the positive main and this third main, and each house having an odd number would be connected between the negative and the third main. The condition that there shall be the same number of lamps in use on the positive and the negative side of this third main will be almost naturally fulfilled, and there will be no need of asking householders to agree with their neighbours as to the number of lamps each shall burn at any given hour. With a sufficiently large number of houses connected to this three-wire main there will, by the law of averages, be an almost equal demand at all times on the positive and negative main, and the current which flows in the third or middle wire will be very small.

This is the principle of the "three-wire system" of distribution of electricity invented simultaneously by Mr. Edison and the late Dr. Hopkinson. It is now the system generally. used in public electricity supply. The middle wire has to be connected to corresponding feeders and thus brought back to the central station, where some apparatus for the division of the voltage between the outer wires must be provided. In stations