In a standard d’Arsonval meter, movement may have a

additional circuitry is required to accomplish the task. This additional circuitry is a simple shunt resistor. The purpose of the shunt resistor is to bypass current that exceeds the 1 mA limitation of the meter movement. To illustrate this, assume that the 1 mA meter in question is needed to measure 10 mA. The shunt resistor used should carry 9 mA while the remaining 1 mA is allowed to pass through the meter. [Figure 12-149]

To determine the proper shunt resistance for this situation:

in parallel, the voltage drop across both of them is the same.

Using Ohm’s Law, this relationship can be rewritten as:

Simply solve for RSH

Substituting the values

To make meter readings quickly and accurately, it is desirable that the moving pointer overshoot its proper position only a small amount and come to rest after not more than one or two small oscillations. The term “damping” is applied to methods used to bring the pointer of an electrical meter to rest after it has been set in motion. Damping may be accomplished by electrical means, by mechanical means, or by a combination of both.

A common method of damping by electrical means is to wind the moving coil on an aluminum frame. As the coil moves in the field of the permanent magnet, eddy currents are set up in the aluminum frame. The magnetic field produced by the eddy currents opposes the motion of the coil. The pointer therefore swings more slowly to its proper position and comes to rest quickly with very little oscillation.

Air damping is a common method of damping by mechanical means. As shown in Figure 12-150, a vane is attached to the shaft of the moving element and enclosed in an air chamber. The movement of the shaft is retarded because of the resistance that the air offers to the vane. Effective damping is achieved if the vane nearly touches the walls of the chamber.