Correct option is A
In an ammeter, the deflecting torque is produced by the current to be measured or by a definite fraction of it whereas in a voltmeter torque is produced by the current proportional to the voltage to be measured.
An ammeter is connected in series with the circuit whose current is to be measured.
Therefore, it should have a low resistance.
On the other hand, a voltmeter is connected in parallel with the circuit whose voltage is to be measured; therefore, it must have high resistance.
Thus, we conclude that the difference is only in the resistance of the instrument an ammeter can be converted into a voltmeter by connecting a high resistance in series with it.
There are four common devices used for the range extension of ammeter and voltmeter namely;
o Shunts.
o Multipliers.
o Current and potential transformers.
A shunt is a low-value resistance having a minimum temperature coefficient and is connected in parallel with the ammeter whose range is to be extended.
The combination is connected in series with the circuit whose current is to be measured.
Multipliers are used for the range extension of voltmeters.
The multiplier is a non-inductive high-value resistance connected in series with the instrument whose range is to be extended.
The combination is connected across the circuit whose voltage is to be measured.
The range of a moving-iron AC voltmeter is extended by connecting a high resistance (multiplier) in series with it.
