13 - Instruments and measurements

Abstract

This chapter discusses the principles of working of moving coil and moving iron instruments and their limitations and the basic methods of current and voltage measurement. The principle underlying the operation of moving coil instrument is the motor principle: when a coil carrying a current is suspended in a strong magnetic field, a deflecting torque acts on the coil and it rotates until the torque is balanced by a restoring couple, usually provided by the suspension system. The field is provided by a permanent magnet which has shaped pole pieces, between which is positioned a soft iron cylinder so making the field in the air gap radial in form. The coil that is suspended to rotate in this gap thus moves in a field that acts always at right-angles to the direction of current flow in the sides of the coil located within the air gap. The deflection is thus proportional to the current in the coil, and so the scale of the instrument will be linear. Moving coil meters are available with full scale deflections as low as 10 μA, but their coil resistance is quite high. The measurement of resistance is made through Avometer. The basic principle of this sort of ohmmeter is when switched to the Ohms' range, an internal battery is connected in series with the moving coil and an adjustable resistor usually marked “Set zero ohms.” The higher the resistance, the greater must be the battery voltage to provide adequate indication. The scale is nonlinear, but resistors can be measured to quite reasonable accuracy.