Ques. Define the term "period."
Ans. This is the time of one cycle of the alternating current.
Ques. What is periodicity?
Ans. A term sometimes used for frequency.
Frequency.—If a slowly varying alternating current be passed through an incandescent lamp, the filament will be seen to vary in brightness, following the change of current strength. If, however, the alternations take place more rapidly than about 50 to 60 per second, the eye cannot follow the variations and the lamp appears to burn steadily. Hence it is important to consider the rate at which the alternations take place, or as it is called, the frequency, which is defined as: the number of cycles per second.
Fig. 1,223.—Diagram of alternator and engine, illustrating frequency. The frequency or cycles per second is equal to the revolution of armature per second multiplied by one-half the number of poles per phase. In the figure the armature makes 6 revolutions to one of the engine; one-half the number of poles = 8 ÷ 2 = 4, hence frequency = (150 × 4 × 6) ÷ 60 = 60. The expression in the parenthesis gives the cycles per minute, and dividing by 60, the cycles per second.
In a two pole machine, the frequency is the same as the number of revolutions per second, but in multipolar machines, it is greater in proportion to the number of pairs of poles per phase.
Thus, in an 8 pole machine, there will be four cycles per revolution. If the speed be 900 revolutions per minute, the frequency is