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A resistor of true value 30 ohm is measured using a voltmeter and an ammeter, applying R = V/I. The voltmeter is connected across the resistor so it draws part of the current. If the measured resistance comes out 5% lower than 30 ohm, find the internal (coil) resistance of the voltmeter.

1. 350 ohm
2. 570 ohm
3. 35 ohm
4. 600 ohm

Correct answer: 570 ohm

Solution

The measured resistance is the parallel value of the true resistance and the voltmeter resistance; requiring this to be 95% of 30 ohm gives R_V = 570 ohm.

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