An alternating current, on the other hand, flows first in one direction, dies out completely, and then flows in the opposite direction. This reversal of the current is not an immediate or instantaneous one, but is a gradual change from one condition to another, similar to the action of wave motion described in Chapter I. The current, beginning at zero, gradually builds up to a maximum value whereupon it dies down to zero, and then reverses in direction and builds up to a similar maximum value in this opposite direction. If we were to draw a figure to represent this action it would be similar to Figure 8. In this figure, the distance along the horizontal line represents the interval of time during which measurements of current or voltage are taken. Distances measured vertically indicate the value of voltage or current at the time indicated by the corresponding horizontal distance at which the vertical measurement was made. This curve shows that the voltage, beginning at "A," increases to a maximum value and then falls to zero. The fact that the voltage is in the reverse direction in the next period is indicated by the portion of the curve drawn below the zero line. This does not indicate that the voltage is less than zero, but shows only that it is reversed. After building up (or down, as shown by the curve) in this reverse direction, the voltage falls (shown by a rise in the second loop of the curve) until it reaches zero again at "B." This action (shown from "A" to "B") represents that of a single cycle of alternating voltage or current. Each cycle consists of two pulsations in opposite directions. Figure 8 illustrates two complete cycles. The number of times per second that this action, as represented from "A" to "B," takes place is called the frequency of the voltage, or current, and is expressed as a certain number of cycles. The frequency of most power systems is 60 cycles, meaning that the current flows back and forth 60 times (120 pulsations) per second. Frequencies of 25, 40, or 50 cycles are supplied by some power companies, necessitating the use of different equipment from that supplied for use on 60 cycle systems. The reasons for these differences will be explained later.

Direct current is more suitable for certain purposes than alternating current, and, conversely, alternating current is better suited for other purposes. Direct current motors are used when variable speed is required, such as in street cars, elevators, etc. For most other purposes AC motors are usually preferable because of their greater simplicity. Direct current is required for vacuum tube amplifiers. The DC required for this type of apparatus can be obtained from batteries, motor-generator sets or rectifiers. A rectifier is a device for changing AC to DC without, ordinarily, the use of rotating machinery.

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