[Illustration: FIG. 439.--Power house showing alternators, direct connected to horizontal hydraulic turbines. Note the direct current "exciter" on end of shaft of alternator. (_Courtesy of General Electric Co._)] =439. Single-phase Currents.=--There are several kinds of a.-c. currents. One of the most common is the _single-phase_. It is simply the common a.-c. current used for light and power in the average home, and uses a two-wire circuit around which the current is rapidly alternating. Fig. 440 illustrates the changes of e.m.f. in an a.-c. single-phase current. It may be produced by a single coil rotating in a magnetic field. The curve of Fig. 440 represents one _cycle_, that is, one complete series of changes in the electromotive forces. At the end of the cycle the armature is in the same condition as at the beginning so far as the magnetic field is concerned. It then begins a new cycle. The ordinary commercial alternating current has a frequency of 60, that is 60 cycles per second. One rotation produces as many cycles as there are pairs of poles. For example, if there are 48 poles in the generator field, one rotation produces 24 cycles. [Illustration: FIG. 440.--Graph showing the e.m.f. changes of a single-phase current for one "cycle."] =440. Three-phase Currents.=--Now suppose we have three coils as in Fig. 441, the coils being evenly spaced, or 120 degrees apart, at _A_, _B_, and _C_. If the coils are rotated in a magnetic field, each will produce an electromotive force. The result produced by three such coils is called a _three-phase_ current. Ordinarily six wires, or three circuits, would be required to carry the current produced by three separate coils; for when coil "_C_" is in the 90 degree position, where its e.m.f. is a maximum, coil "_B_" is 120 degrees past its maximum, and coil "_A_" is 240 degrees past its maximum. The graph (Fig. 441) shows the maximum points of the three e.m.f's. separated by intervals of 120 degrees. In practice, however, it is found possible to use _three wires_ instead of six, as explained in Art. 441. [Illustration: FIG. 441.--Graph showing the e.m.f. changes of a three-phase current for one "cycle."]