Electrical Engineering ⇒ Topic : Attracted Disc Type Voltmeter

Attracted Disc Type Voltmeter

Fig. (a) shows the simplified diagram of an attracted disc electrostatic voltmeter. It consists of two mushroom-shaped plates A and B, each mounted on insulated pedestal. The plate B is fixed while the plate A (negative, for direct voltage) has a movable central portion the attracted disc. The movable plate A is attached to a horizontal rod which is suspended by two phosphor bronze strips. When p.d. to be measured is applied across the plates, the plate A moves towards the fixed plate B and actuates the pointer via a pulley or link mechanism. The control force is provided by gravity and damping force by air dash pot. If the plates are too close together or if the applied voltage is too high, a spark discharge may occur. In order to prevent such a possibility, a *ballast resistor is included in the circuit.The function of this resistor is to limit the current if any sparking over occurs. If the applied voltage reverses in polarity, there is a simultaneous change in the sign of charge on the plates so that the direction of deflecting force remains unchanged. Hence such instruments can be used for both d.c. and a.c measurements.

                                                                figure  (a)

Theory. The force of attraction F between the charged plates is given by

Obviously, the scale of the instrument will be non-uniform.