An ammeter is convertible to a voltmeter by
A. Changing the scale
B. Putting a large resistance in parallel with the actual measuring part of the instrument
C. Putting a large resistance in series with the actual measuring part of the instrument
D. Simply installing the instrument in parallel with the circuit
Show Answer
Answer: C
Explanation:-
An ammeter is used to measure current in amperes. It is a moving coil galvanometer in which a low resistance wire is connected in parallel. An ammeter is converted into voltmeter by putting a large resistance in series with the actual measuring part of the instrument.
Let
Vr - Voltage range you required to measure (KNOWN VALUE)
If - Current required to full-scale deflection of your Ammeter (KNOWN VALUE)
Ra - Internal Resistance of Ammeter (Ideally Ra=0)
Rs - Resistance Should be Connected in Series with your existing Ammeter=?
Vr = (Ra + Rs) × (If)Vr = (Ra+Rs) × (If)
Vr = (Rs×If)Vr=(Rs × If)(We know that Ra=0 )
Rs = Vr/If
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