Question

The $80\Omega$ galvanometer, deflects full scale for a potentials of 20 mV. A voltmeter deflecting full scale of 5V is to made using this galvanometer. We must connect

• A. a resistance of $19.92k\Omega$ parallel to the galvanometer
• B. a resistance of $19.92k\Omega$ in series with the galvanoyneter
• C. a resistance of $20k\Omega$ parallel to the galvanometer
• D. a resistance of $20k\Omega$ in series with galvanometer

Answer 1

Answer: (B)

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