A moving-coil meter is a very commonly used form of analogue voltmeter becauseof its sensitivity, accuracy and linear scale, although it only responds to d.c. signals.As shown schematically in Figure 6.2, it consists of a rectangular coil wound round asoft iron core that is suspended in the field of a permanent magnet.The signal beingmeasured is applied to the coil and this produces a radial magnetic field.Interactionbetween this induced field and the field produced by the permanent magnet causesa torque, which results in rotation of the coil.The amount of rotation of the coil ismeasured by attaching a pointer to it that moves past a graduated scale.The theoreticaltorque produced is given by:
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