through the machine. These waves are relatively to each other displaced by a quarter period. A machine of this kind, which from the same armature gives two independent currents displaced by a quarter period, is called a "two-phaser" or a "two-phase machine." We may also provide the armature with three distinct phase windings, each displaced against the others by one-third of a full period, or 120 degrees. Such a machine is called a "three-phaser" or "three-phase machine." The use of three-phase current results in certain technical and financial advantages in the supply of electricity, and it is on this account that most modern electricity works, if they use alternating current at all, use it in the shape of three- phase current.
But suppose an electricity works does not want to supply alternating current to its customers, but continuous current. What sort of machinery will it have to use in this ease? If we wish to produce continuous, that is uni-directed, current by electromagnetic induction, we must obviously add to our machine some organ which reverses the current in every second half-wave. But this is not enough. Even after we have reversed every
