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

Options

A. a resistance of `19.92 k Omega` parallel to the galvanometer
B. a resistance of `19.92 k Omega` in series with the galvanometer
C. a resistance of `20 k Omega` parallel to the galvanometer
D. a resistance of `20 k Omega` in series with the galvanometer

Answer

B

Explanation

The current through the galvanometer producing full-scale deflection is
`I = (V)/(R ) = (20xx 10^(-3))/(80) = 2.5 xx 10^(-4) A`
To convert the galvenometer into a voltmeter, a high resistance is connected in series with the galvanometer. Therefore,
`5 V = (2.5 xx 10^(-4)) (R + 80)` or `R = 19.92 k Omega`