alleryStart learning →A galvanometer of 25.0 ohm resistance gives full-scale deflection at 50.0 microamperes. What voltage across it produces that full-scale reading?
A0.500 mV
B1.25 mV
C1.25 V
D12.5 mV
Answer & Solution
Correct answer: B. 1.25 mV
1. The movement obeys Ohm's law like any other resistance, so V = IR.
2. Take the full-scale current as 50.0 microamperes, which is 0.0000500 A.
3. V = (0.0000500 A)(25.0 ohm) = 0.00125 V.
4. That is 1.25 mV, a very small voltage, which is why a bare galvanometer is useless as a voltmeter.
5. Reading the answer as 1.25 V confuses millivolts with volts and overstates the sensitivity a thousandfold.
_Source: OpenStax College Physics (CC BY 4.0), Ch 21 "Circuits and DC Instruments", section 21.4 DC Voltmeters and Ammeters_
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