The d’Arsonval meter movement has a small, rectangular coil of wire. The coil is suspended in a magnetic field created by a permanent magnet. When current is applied to the coil, it becomes an electromagnet.
Before you use a meter, check its instruction manual. Each meter is a little different and you should be aware of any important instructions in the instruction manual.
The energized coil then lines up with the poles of the permanent magnet. The amount of current applied to the coil controls its movement. An electromagnet that is free to move will align its axis with the magnetic axis of a fixed magnet.
The coil must be free to rotate in order to align itself with the magnetic axis. The coil in the meter is mounted on pivots that permit easy rotation.
Two small springs are mounted on the top and bottom. These springs offer slight resistance to the rotation of the coil. The springs control the position of the coil when there is no current flowing.
When current flows in the coil, it produces a magnetic field around the coil. This magnetic flux overcomes the force of the springs and moves the coil.
A pointer on top of the coil rotates to mark the amount of movement. The greater the current through the coil, the more it turns, and the further the pointer moves. The pointer then stops in front of a marked scale on the face of the meter.
As you look closer at the d’Arsonval meter movement, you will find that it can be extremely sensitive to small currents. For instance, a relatively inexpensive meter can give a full-scale deflection of 10 microamperes (A).
Full-scale deflection is the total range of a meter scale. A microampere is one-millionth (0.000001) of an ampere.
Meter deflection is determined by three factors:
1. The number of turns of wire in the coil and the amount of current flowing in the coil.
2. The strength of the magnetic field produced by the permanent magnet affects the positioning of the coil.
3. The tension of the springs and the friction of the bearings determine the sensitivity of the meter movement. The amount of current flowing through the meter determines its pointer deflection since it is the only variable in the meter movement circuit.
The scale of the meter is calibrated (marked) to show the type of reading (volts, ohms, or amperes).