240 ohms, the current increases to 25 ma, and if the resistance is reduced further to 200 ohms, the current increases to 30

ma. Similarly, if is increased to 400 ohms, the current decreases to 15 ma, and if is increased further to 600 ohms, the

current decreases to 10 ma. If the resistance is varied continuously about its normal value of 300 ohms in a certain

manner, the variations of current about its average value of 20 ma can be tabulated as follows:

(2) Figure 10 shows the graph of current versus time constructed from this tabulation. The current waveshape is

one of pulsating direct current. This consists of an ac (alternating-current) wave superimposed on a dc (direct-current), or

average, value of current. The dc component is 20 ma, and the ac component is a sine wave with an amplitude of 10 ma.

The period of the wave, or the time for 1 cycle, is 4 milliseconds, or .004 second. The frequency of the wave can be

calculated by using the formula:

The rate at which the current varies about its average value depends on the rate at which the resistance is varied about its

normal value.