Figure below represents the mechanism of a permanent-magnet, moving-coil instrument. Here, the field produced by the direct current in the moving coil reacts with the field of the permanent magnet to produce torque.
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| Mechanism of Permanent-Magnet, Moving-Coil Instrument. |
The windings consist of several layers of fine enameled wire. Pivot bases are cemented to the ends of the coil frame. These bases carry the hardened steel pivots on which the coil turns as well as the inner ends of the control and current-carrying springs.
In addition, the upper pivot base mounts the pointer and the balance cross. Threaded balance weights, or their equivalents, are adjusted on the balance cross to balance the moving element in its bearing system.
The pivots ride in jewel bearings to keep friction at a minimum. A taut band suspension may be used in place of the pivot and jewel-bearing system. Here the moving coil is supported by two metal ribbons under tension sustained by springs.
Either bearing system allows a properly balanced instrument to be used in any position with little error.
Current is carried to the coil by two springs. These control and current-carrying springs oppose the torque of the moving coil and serve as the calibrating means of the instrument.
The springs are generally made of carefully selected phosphor bronze or beryllium copper specially manufactured to provide stability so that the instrument accuracy will not be affected by time and use. The torque developed by current flowing through the moving coil is a function of the field strength of the permanent magnet and of the current in the moving coil, as well as the dimensional factors of both magnet and coil.
The torque T, in dyne-centimeters, is given by this equation:
T = B x A x I x N
where:
B flux density in lines per square centimeter in the air gap
A coil area in square centimeters
I moving-coil current in amperes
N turns of wire in moving coil
The characteristics of the moving-coil instrument are very desirable. It has a high degree of accuracy, high sensitivity, low cost, and a uniform scale. It can measure extremely small currents because of the fine wire in the moving coil.
The instrument is unique in the variety of accessories that can be used in conjunction with it. The four most commonly used are the series resistor, the shunt, the thermocouple, and the rectifier.
There are two inherent shortcomings of the permanent-magnet, moving-coil mechanism: except in a specially scaled instrument with rectifiers or thermocouples, it cannot measure AC quantities, and without auxiliary shunts or multipliers it can measure only small electrical quantities.
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