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A galvanometer of resistance 100 ohm has 50 divisions on its scale, with a sensitivity of 20 microampere per division. It is to be turned into a multi-range voltmeter reading 0-2 V, 0-10 V and 0-20 V using series resistors R1, R2 and R3. Which set of series resistance values is appropriate?
- R1 = 1900 ohm, R2 = 9900 ohm, R3 = 19990 ohm
- R1 = 2000 ohm, R2 = 8000 ohm, R3 = 10000 ohm
- R1 = 19900 ohm, R2 = 9900 ohm, R3 = 1900 ohm
- R1 = 1900 ohm, R2 = 8000 ohm, R3 = 10000 ohm
Correct answer: R1 = 1900 ohm, R2 = 9900 ohm, R3 = 19990 ohm
Solution
The full-scale deflection current is Ig = 50 * 20 microA = 1 mA, so each range requires a total resistance V/Ig with the galvanometer's 100 ohm subtracted to give the series resistor.
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