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but whatever be the size of the station it is formed of two or more of these units, that is^ of two or more couples of generators each couple being composed of two dynamos as near alike as it is possible to have them, and ran from one and the same engine. It will be seen that by this arrangement a station is secured against a general break-down; for the disablement of any one unit will in no way prevent the operation of the others, and as a station rarely runs at its full capacity there is always machinery enough at hand to supply the current cut-off by the failure of any unit to operate in a satisfactory manner.
It is important in operating the three-wire system that the number of lamps on either side of the middle or neutral wire should be the same. When this condition exists, the neutral wire has no current flowing through it and the system is said to be balanced; but as the lights are turned on and off at will, by the parties using them, it is obvious that the lights on different sides
