Question
Sat April 18, 2015 By: Rakshitreshu1997

The AC voltage across the resistance can be measured using a:
1.potentiometer
2.hot wire voltmeter
3.moving magnet galvanometer
4.moving coil galvanometer

Expert Reply
Jyothi Nair
Mon April 20, 2015
The correct answer is (2) Hot wire voltmeter.
The AC voltage across the resistance can be measured using a hot wire voltmeter
 
Ordinary d.c voltmeter cannot measures alternating voltages. When used in a.c circuits, these instruments record zero reading because average value of alternating voltage over a full cycle is zero. 
Alternating voltage across the resistance can be measured by using an a.c voltmeter. Such an a.c voltmeter is known as a hot wire voltmeter which measure only virtual values(r.m.s values) of the alternating voltages.
Moving coil or magnet galvanometer can measure only d.c values. When a.c is passes through a galvanometer it will not show any deflection, because the impulses to the moving parts of the galvanometer are equal and opposite. To measure ac, the heating effect of current is used beacause heating effect does not depend on the direction of flow of current. The hot wire voltmeter work on the principle of heating effect of current. 
 
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