Page 828 - Basic Electrical Engineering

provide path for entry and exit of current to and from the moving coil. Hair
springs are attached to the spindle and carry current to the moving coil and
also provide the return path.

Damping of the moving system, i.e., the reduction of oscillation under the
deflected condition is reduced because of opposing torque developed due to

induced eddy currents in the aluminium drum. Interaction between eddy
currents and the flux produces opposing torques reducing oscillations of the

moving system. This is known as eddy current damping. Note that
oscillations produce eddy current in the drum, and torque produced by the

induced eddy current and magnetic field flux opposes the oscillation. This is
an example of the application of Lenz’s law.
A properly damped (also called critically damped) moving system will

move reasonably fast but without overshoot and oscillations in the deflected
position. Damping may be of any of the following types, viz (a) eddy current

damping; (b) air friction damping; (c) fluid friction damping.
The pointer fixed on the spindle moves over a graduated scale. To keep the

moving system light, the pointer is also made up of a very light-weight
material with a fine edge for accurate measurement. The weight of the

moving system is balanced by using some counter weight on the pointer on
one side. The scale of a PMMC-type instrument is linear, i.e., its divisions are
equally spaced. This is because the torque developed is directly proportional

to the current flowing through the coil, i.e., the current to be measured. For
low range of currents, say upto 20 mA, the entire current to be measured is

allowed to pass through the moving coil. However, for larger currents to be
measured, the moving coil is shunted by a parallel resistance of very low
value so that the majority of the line current flows through the shunt

resistance. However, reading of the instrument has to be in terms of the total
current. An instrument of lower range can be used for higher range by

connecting a shunt resistance of appropriate value and accordingly changing
the calibration of the scale of measurement.

A PMMC-type milliammeter can be used to measure dc voltage by
connecting a high resistance in series with the instrument. Such series

resistances are called multipliers. In this case the scale has to be calibrated in