SOLUTION

Basically, there are three types of Force or torque act upon the Measuring Instrument

1. Deflecting Torque
2. Controlling Torque
3. Damping Torque

Deflecting Torque: In order to move the pointer from its zero position on the scale deflecting torque is required. The deflecting torque works on the moving system to which the pointer is attached. Obviously, the magnitude of deflecting torque produced is proportional to the magnitude of the quantity being measured, say, the current I flowing through the instrument. A deflecting torque is required to overcome the inertia, damping effect, and controlling effect of the moving system.

This deflecting torque can be produced by any of these effects of current (or of voltage) such as

1. Magnetic effect:-  Suitable for A.C and D.C moving iron instruments such as voltmeter and Ammeter
2. Electrostatic effect:- It utilizes the forces between electrically charged conductors. It is suitable only for both AC and DC Voltmeter
3. Electromagnetic effect:- It utilizes the magnetic effect of electric current. It Is suitable for A.C instruments only such as Ammeter, Energy meter, Wattmeter, Voltmeter, and Watt-hour meter.
4. Thermal effect:- It utilizes the heating effect. It is suitable for both A.C and D.C such as Ammeter and Voltmeter.
5. Chemical Effect:- It is used for D.C instruments such as D.C ampere-hour meter.