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ELECTRICAL ACTION 21
coupled together with chain or gearing that the low zero period of one came at the high period of the other. If these two currents are used together in a motor, each current actuating a different field, it is evident that the pull on the motor armature will be much more than with a single a. c. current. This is known as 2-phase current. The action or relation o\ the two currents is diagram matically represented at A, Fig. 5. The voltage of such a system is never at zero.
Figure 5
If we couple a third generator in such manner that the alternations of the three are equally spaced, we shall have 3-phase current action as shown at B, Fig. 5, which will approach the effect of d. c. in its non-pulsating characteristics.
(70) Two-phase current is transmitted ordinarily by two entirely separate two-wire circuits. This method provides a much more even pull to motor armatures, and consequently a more even effect to any apparatus to which it may be connected, than can be obtained from singlephase current.
(71) Three-phase current requires only three wires for its transmission. It forms an ideal system for the distribution of electrical power. Tt provides an even pull on motor armatures.
Following are definitions of electrical terms which users of the bluebook will find valuable and instructive.
Polarity
(72) Wires attached to opposite terminals of an electrical power source are capable, when connected to some suitable device, of producing power to perform