Scale of a PMMC | Connection of PMMC Instruments as Ammeter or Voltmeter

Scale of a PMMC :

~ shows the scale of a PMMC voltmeter. Note that the scale is perfectly linear. i
Fig.

Connection of PMMC Instruments as Ammeter or Voltmeter:

There are many types of instruments which work on different principles. Ihese instruments may be used as ammeters, voltmeters, wattmeters, energymeters etc.
The main coil of the instrument produces deflecting torque when it carnies some current. Such a coil is called as a movement of the instrument.

1. Ammeter :

When the instrument is used as an ammeter, an external low resistance called shunt is connected in parallel with the movement so that net resistance of the instrument is very low and the instrument can be connected in series i with any equipment whose current is to be measured. Due to low resistance, voltage drop across instrument is negligible.
The ammeter is shown in Fig. 4.5.4(a).
Movement
Fig.

Voltmeter :

When the instrument is used as a voltmeter, an external i high resistance is connected in series with the movement and this senes combinaton is connected across supply terminals or load termnals, or in general across any two terminals where terminal voltage is to be measured.
Due to the high senes resistance, current flowing through the instrument is negligible.
This is shown in Fig, 4.5.4(b).

Can we use PMMC meter to measure ac quantities ?

The answer is no. The PMMC instruments are useful only for d.c. measurements which produce a unidirectional torque. The ac quantities (voltage or current) produce a bi-directional deflection torque i.e. a torque that changes its direction at a rapid rate. i
So if a PMMC meter is used, its pointer cannot follow the rapidly reversing torque and the deflection corresponds toi mean torque which is zero.
Hence the pointer appears to be stationary. Hence PMMC
instruments should not be used for measuring ac quantities.