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Figure 1-9.—Assembled meter movement. | Up
Neets Module 03-Introduction to Circuit Protection, Control, and Measurement | Next
RECTIFIER FOR AC MEASUREMENT |
1-10
Q8. What is the effect of current flow through the coil in a d’Arsonval meter movement?
Q9. What are three functions of the hairsprings in a d’Arsonval meter movement?
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.
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