Page 824 - Basic Electrical Engineering

If the same instrument is used to measure a voltage, say 150 V, then the
relative limiting error would be

For measurement of 75 V using the same instrument, the relative limiting
error would be equal to

So, we may conclude that measurement of lower voltages using the same

instrument results in less accurate measurement.

11.9 INDICATING-TYPE INSTRUMENTS

In this type of instruments, measurement is indicated by a pointer moving
over a graduated scale. Voltmeters, ammeters, wattmeters, etc., are of
indicating type which are extensively used in laboratories and control panels.

Different types of indicating instruments are described in the following
sections.

11.9.1 Permanent Magnet Moving Coil Instruments

Permanent magnet moving coil (PMMC)-type instrument is the basic dc
measuring instrument. In these instruments a permanent magnet, generally of

horseshoe type, creates a magnetic field in which a coil of fine wire of
number of turns is placed. The coil is wound on a very light aluminium drum

and is pivotted on jewel bearings so that the coil is free to move when current
flows through it. The current-carrying coil placed in the magnetic field
experiences a torque and tries to turn. It’s free turning is restricted by spring

tension attached to it’s shaft. The moving coil produces a deflecting torque
which is opposed by control torque produced by the spring action. A

simplified diagram of a PMMC-type instrument has been shown in Fig.
11.15.