The net resistance of a voltmeter should be large to ensure that
A. it does not get overheated
B. it does not draw excessive current
C. it can measure large potential differences
D. it does not appreciably change the potential difference to be measured.
Voltmeter is always connected in parallel to measure the voltage or potential difference across the elements. When the voltmeter is connected across some element in a circuit, it will change the overall resistance in the circuit. It will effect the current values also.
To minimize the error voltmeter should have the large net resistance. Then large resistance in parallel with small resistor will have only a very slight change.
The net resistance of a voltmeter should be large to ensure that it does not appreciably change the potential difference to be measured. Option D is correct.
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