First slide

voltmeter

Question

An 80W galvanometer deflects full-scale for a potential of 20 mV. A voltmeter deflecting full-scale of 5 V is to be made using this galvanometer. We must connect

Moderate

A

a resistance of 19.92 k $Ω$ parallel to the galvanometer
B

a resistance of 19.92 k $Ω$ in series with the galvanometer
C

a resistance of 20k $Ω$ parallel to the galvanometer
D

a resistance of 20k $Ω$ in series with the galvanometer

Solution

The current through the galvanometer producing full-scale deflection is
$I = \frac{V}{R} = \frac{20 \times 10^{- 3}}{80} = 2.5 \times 10^{- 4} A$
To convert the galvanometer into a voltmeter, a high resistance is connected in series with the galvanometer.
Therefore, 5 V = (2.5 × 10-4)(R + 80) or R = 19.92 k $Ω$

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