Frequency

Author: J.B. Hoag

The faster the loop of Fig. 4 A rotates, the shorter the "period" T and the greater the number of cycles of e.m.f. and current that will be produced each second. If the loop rotates sixty times each second, sixty complete cycles of current will be generated each second. The frequency, f, is the number of complete cycles per second. The loop will require 1/60 of a second to rotate once. The time for one rotation or cycle is called the period. It is easy to see that f = 1/T, i.e., the frequency is the reciprocal of the period.

For power lines, the frequencies in common use are 25, 50, and 60 Hz, the latter being by far the most widespread. Incidentally, the direct current from a battery or d.c. generator has a frequency of zero.

There are other methods of producing alternating currents than that of the rotating machine described above. In particular, vacuum tubes have been used in oscillator circuits to good advantage. With these, it is possible to produce much higher frequencies, into the billions of Hz.

Frequencies from 15 to 15,000 are known as audio frequencies because they can operate telephone receivers and loud speakers to produce sound waves which are audible to the ear.

Frequencies in the range from 15,000 to 100,000 cycles per second (= 15 to 100 kHz or kilohertz (kilocycles)) are called intermediate or low radio frequencies. From 100 to 1,500 kHz they are called medium r.f. (radio frequency). From 1.5 to 6 megahertz (mega = million) they are called medium-high frequencies. From 6 to 30 MHz they are h.f. (high frequencies); from 30 to 300 MHz they are ultra-high frequencies (u.h.f.). Above this they are referred to as centimeter-waves, quasi-optical waves, or microwaves.