![]() ![]() ![]() 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. 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°. ![]() 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. In this situation the added resistance is known as a shunting resistor. 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. That is, torque is proportional to the coil current and the instrument scale is linear.Īs 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. ![]()
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