There are two distinct kinds of electric current supplied for lighting and power, one known as direct current and the other as alternating.
A direct current is one which passes in one direction only. It may be represented by a straight line, as A in Figure 88.
An alternating current is one which reverses its direction and passes first one way and then the other. It may be represented by a curved line, shown in Figure 88. It starts at zero, and gradually grows stronger and stronger. Then it commences to die away until no current is flowing. At this point it reverses and commences to flow in the opposite direction, rising gradually and then dying away again.
This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero, it is said to pass through a cycle.
Fig. 88.—Graphic Representation of a Direct and an Alternating Current.
The part of the curved line from a to b in Figure 88 represents the first part of the current, when it is rising. From b to c represents its fall. The point at which the curved line crosses the straight line is zero. At c the current crosses the line and commences to flow in the opposite direction until it reaches d, at which point it dies away and again crosses the line to flow in its original direction and repeat the cycle.
In electrical parlance, that part of the current from a to c or from c to e is known as an alternation. From a to e is called a cycle.
The reason why alternating current is often used in place of direct current is that it can be sent over the wires for long distances more economically than direct current. This is more fully explained farther on in the chapter dealing with a step-down transformer.
The number of cycles taking place in one second is known as the frequency of the current. The usual frequency of commercial alternating currents is 60 cycles per second or 7200 alternations per minute.