Electrical Indicating and Test Instruments 1 Introduction 161

Moving Coil Meter

A moving coil meter is a very commonly used form of analogue voltmeter because of its sensitivity, accuracy, and linear scale, although it only responds to d.c. signals. As shown schematically in Figure 7.3, it consists of a rectangular coil wound round a soft iron core that is suspended in the field of a permanent magnet. The signal being measured is applied to the coil, which produces a radial magnetic field. Interaction between this induced field and the field

Figure 7.2. Eltime analogue panel meter (reproduced by permission of Eltime Controls).



Figure 7.3. Mechanism of a moving coil meter.

produced by the permanent magnet causes torque, which results in rotation of the coil. The amount of rotation of the coil is measured by attaching a pointer to it that moves past a graduated scale. The theoretical torque produced is given by

where B is the flux density of the radial field, I is the current flowing in the coil, h is the height of the coil, w is the width of the coil, and N is the number of turns in the coil. If the iron core is cylindrical and the air gap between the coil and pole faces of the permanent magnet is uniform, then the flux density B is constant and Equation (7.2) can be rewritten as

that is, torque is proportional to the coil current and the instrument scale is linear.
As the basic instrument operates at low current levels of one milliamp or so, it is only suitable for measuring voltages up to around 2 volts. If there is a requirement to measure higher voltages, the measuring range of the instrument can be increased by placing a resistance in series with the coil, such that only a known proportion of the applied voltage is measured by the meter. In this situation the added resistance is known as a shunting resistor.
While Figure 7.3 shows the traditional moving coil instrument with a long U-shaped permanent magnet, many newer instruments employ much shorter magnets made from recently developed magnetic materials such as Alnico and Alcomax. These materials produce a substantially greater flux density, which, in addition to allowing the magnet to be smaller, has additional advantages in allowing reductions to be made in the size of the coil and in increasing the usable range of deflection of the coil to about 120^◦. Some versions of the instrument also have either a specially shaped core or specially shaped magnet pole faces to cater for special situations where a nonlinear scale, such as a logarithmic one, is required.