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62 MOTION PICTURE PROJECTION
A regularly recurring series of values of electromotive force, from any point in the series to the corresponding point in the next series, is called a "cycle." The portion of the curve in Fig. 8 from 3 to 7 represents a cycle ; similarly, the portion from 2 to 6. The time required for one cycle is the "period." The number of cycles per second is called the "frequency."
In commercial practice, 60 and 25 cycles per second are the most common frequencies for alternating current circuits. The corresponding periods are 1/60 and 1/25 of a second.
To produce a frequency of 60 cycles per second by the use of a single magnet with two poles requires a speed of rotation of 60 revolutions per second. Such a speed is not practicable for large machines. To get 500 cycles would require 500 r. p. s., or 30,000 r. p. m. (revolutions per minute) . By arranging a number of similar north and south poles alternately, as in Fig. 9, and providing corresponding conductors, a lower speed of rotation may be used. As in Fig. 9, the magnet is supposed to be made to rotate, while the conductors a, b, c, d, e, f remain stationary.
When the upper north pole is coming toward the reader, electromotive forces will be induced in the several conductors in the direction of the arrows. The conductors are all connected in series, except between / and a, where connection is made to an external part of the circuit, X. All are in the same relative position to the several magnetic poles, their electromotive forces are equal, and in the case shown, the total is six times as great as the electromotive force in any one conductor.