question_answer

If resistance of voltmeter is 10000 \[\Omega \] and resistance of galvanometer is\[2\,\Omega \], then find R when voltmeter reads 12 V and galvanometer reads 0.1 A.

A)  118 \[\Omega \]         

B)  120 \[\Omega \]

C)  124 \[\Omega \]

D)  114 \[\Omega \]

Correct Answer: A

Solution :

The circuit diagram with current variation can be drawn as : Let current \[{{I}_{g}}\] flows through R and G. For a voltmeter with full scale reading, we have                 \[V={{I}_{g}}(G+R)\] or            \[R=\frac{V}{{{I}_{g}}}-G\] Given, \[G=2\,\Omega ,\,V=12\] volt, \[{{I}_{g}}=0.1\,\,A\]                 \[R=\frac{12}{0.1}-2\]                 \[=120-2\] \[=118\,\,\Omega \]