COMPASS AND ALTERNATING CURRENT

Up to this point, only direct current examples have been used. What happens with the use of alternating current? Figure 1-11 shows a magnet close to a conductor carrying alternating current at a frequency of 1 hertz.

Figure 1-11. - Compass and conductor with ac.

The compass needle will swing toward the east part of the compass (down) as the current goes positive, as represented in figure 1-11(A). (The sine wave of the current is shown in the lower portion of the figure to help you visualize the current in the conductor.)

In figure 1-11(B), the current returns to zero, and the compass needle returns to magnetic north (right). As the current goes negative, as in figure 1-11(C), the compass needle swings toward the west portion of the compass (up). The compass needle returns to magnetic north as the current returns to zero as shown in figure 1-11(D).

This cycle of the current going positive and negative and the compass swinging back and forth will continue as long as there is alternating current in the conductor.

If the frequency of the alternating current is increased, the compass needle will swing back and forth at a higher rate (faster). At a high enough frequency, the compass needle will not swing back and forth, but simply vibrate around the magnetic north position. This happens because the needle cannot react fast enough to the very rapid current alternations. The compass (a simple meter) will indicate the average value of the alternating current (remember the average value of a sine wave is zero) by vibrating around the zero point on the meter (magnetic north). This is not of much use if you wish to know the value of the alternating current. Some device, such as a rectifier, is needed to allow the compass to react to the alternating current in a way that can be useful in measuring the current.

RECTIFIER FOR AC MEASUREMENT

A rectifier is a device that changes alternating current to a form of direct current. The way in which this is done will be covered later in this training series. For now, it is necessary to know only the information presented in figure 1-12.

Figure 1-12. - Rectifier action.

Figure 1-12 shows that an alternating current passed through a rectifier will come out as a "pulsating direct current."

What happens to the compass now? Figure 1-13 answers that question.

Figure 1-13. - Compass and conductor; rectified ac.

When the compass is placed close to the wire and the frequency of the alternating current is high enough, the compass will vibrate around a point that represents the average value of the pulsating direct current, as shown in figure 1-13.