All analogue electrical indicating instruments require three essential devices:
(A) A DEFLECTING OR OPERATING DEVICE: A mechanical force is produced by the current or voltage which causes the pointer to deflect from its zero position.
(B) A CONTROLLING DEVICE: The controlling force acts in opposition to the deflecting force and ensures that the deflection shown on the meter is always the same for a given measured quantity. It also prevents the pointer always going to the maximum deflection. There are two main types of controlling device; spring control and gravity control.
(C) A DAMPING DEVICE: The damping force ensures that the pointer comes to rest in its final position quickly and without undue oscillation. There are three main types of damping used; eddy current damping, air-friction damping and fluid friction damping.
There are basically two types of scale; linear and nonlinear. A linear scale is shown in Fig. 1(a), where the divisions or graduations are evenly spaced. The voltmeter shown has a range 0–100 V, i.e. a full-scale deflection (fsd) of 100 V. A nonlinear scale is shown in Fig. 1(b) where the scale is cramped at the beginning and the graduations are uneven throughout the range. The ammeter shown has a fsd of 10 A.
