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A galvanometer (microammeter) has resistance 100 ohm and full-scale deflection at 50 microA. By adding a suitable resistor it can be turned into a voltmeter or a higher-range ammeter. Which of the following range-and-resistor combinations is correct?

1. 10 V range with about 200 kohm resistance in series
2. 50 V range with 10 kohm resistance in series
3. 5 mA range with 100 ohm resistance in parallel
4. 10 mA range with 1 kohm resistance in parallel

Correct answer: 10 V range with about 200 kohm resistance in series

Solution

For a 10 V range, series R = 10/(50e-6) - 100 = 200000 - 100 ~ 200 kohm, which matches the stated combination.

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