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20 RICHARDSON'S BLUEBOOK OF PROJECTION
former having no moving parts. Changing" the voltage has no effect on the frequency.
A. c. action usually is expressed diagrammatically as shown in Fig. 4. (65) The vertical line represents voltage (E.M.F.). The horizontal time line represents four alternations. Triangles A and C each represent one alternation; the two together, one cycle. If the current be 60 cycle (60 cycles per second) then the distance from 0 at the left end of horizontal time line to 1 would represent one-half of one-sixtieth, or l/120th of a second of time.
(66) Line B represents voltage or current flow. Current and voltage start from line 0 at zero, raising to 20, 40, 60, 80, 100 and 110 volts in l/240th of a second. It is then at its maximum and instantly begins to sink back to zero at 1.
We now see that line B represents two things : time (l/120th of a second between 0 and 1) and voltage through successive stages up to 110 volts, which is maximum. The same action now takes place, as represented by triangle C, except that polarity is reversed, and so on indefinitely. (67) Where the triangular lines cross the horizontal line is the point where the wires of armature coil AB, Fig. 1, stand above each other and are moving in the same direction as the lines of magnetic force, cutting none of them and generating no voltage.
Two and Three-Phase Current
We have been describing what is known as singlephase a. c, in which, as has been explained, the voltage and amperage both sink to zero at the end of each alternation, resulting in a strong pulsating effect, both in light and power production. (68) This may be avoided in a. c. by the joining of two single phase currents of the same frequency (time per alternation or cycle) and voltage, (69) the two being so coupled together that the low period (0 Fig. 4) of one comes at the high period of the other.
Suppose we have two generator armatures producing exactly equal voltage and frequency (see Fig. 1) so