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