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A moving-coil milliammeter having a full-scale deflection of 1 mA and an internal resistance of 0.5 ohms is to be converted to a voltmeter of 20 volts full-scale deflection. It would be necessary to insert a:
series resistance of 19 999.5 ohms
For full-scale deflection (Im=1mA=0.001A) at V_total=20V: V_total = Im × (Rm + R_series). So, 20V = 0.001A × (0.5Ω + R_series). (0.5 + R_series) = 20/0.001 = 20000Ω. R_series = 20000Ω - 0.5Ω = 19999.5Ω.
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