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per conducting mains becomes so great as to seriously interfere with the economical introduction of the system upon a commercial basis. It was to obviate this defect, that the three-wire system was invented by Mr. Thomas A. Edison.
In practical electric lighting, the number of lamps receiving their current from a common central station^ often runs up into thousands; and, as it is neither practicable nor desirable, to run these lamps all from one generator, a number of generators have to be combined together to furnish the necessary current, each generator having a capacity of a given number of lamps. These being the conditions under which lighting must be done, we will endeavor to make clear by comparison the advantages secured by the employment of the three-wire system. In. Fig. 11 A represents two dynamos having a capacity of five lamps each. Supposing these lamps to require one ampere of current each, and a total electro-motive force of pressure of ninety volts at the lamps. If these
