rapid for the needle to follow and it would stand perfectly still at the zero line.
But alternating currents must be measured, and Weston adapted the D’Arsonval principle to A.C. use in a very simple and ingenious way. He made use of the principle that “like poles repel.” See Fig. 25a.
The working elements of the A.C. meter are a coil and two iron plates inside the coil, one fastened to the coil, the other free to move. The pointer is attached to the free plate and when the pointer is at zero, the plates are together and in line--the free plate held in place by a fine spring, as in Fig. 25b.
When an A.C. current is passed through the coil, the iron plates are magnetized by the magnetic field about the coil. Both plates are magnetized alike. Because the north poles and south poles of these two plates, which are now magnets, are near each other, the free plate tends to move away from the fixed plate. When the current reverses, the ends of the plates which had been the north poles become the south poles and vice versa. But the repelling force is still there--the free plate will still be repelled by the fixed plate, and the greater the current, the greater will be the magnetism set up in the plates and the farther away the free plate will move, causing the needle to show a correspondingly larger reading.
A.C. meters read only about 70% of the maximum voltages--this being known as the “effective” voltage. Now you will ask, “How about the time when no current is flowing, twice during each cycle?” You will say that there is bound to be some change or flicker in the reading as the current rises and drops to zero. This is taken care of by a damping vane on the needle. It is a thin plate attached to the pointer needle, and moves in a closed air compartment. Thus, when the maximum repelling effect has taken place, the damping vane will tend to hold the pointer in position momentarily, just long enough so that the next half of the cycle will catch it before it has a chance to drop back toward zero.
Meters working on this principle are “repulsion” type meters. They can, by special construction, also be used to measure direct currents in which case the polarity of the plates does not change but the repelling effect is still present.
And now we come to the end of our fourth lesson. In this lesson, we have learned quite a great deal of the way an electric current
