ANALOGUE INSTRUMENTS

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 non-linear. A linear scale is shown in Fig. (a), where the divisions or graduations are evenly spaced.

The voltmeter shown has a range 0–100V, i.e. a full scale deflection (f.s.d.) of 100V. A non-linear scale is shown in Fig. (b) where the scale is cramped at the beginning and the graduations are uneven throughout the range. The ammeter shown has an f.s.d. of 10A.

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