Question

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 $

Answer 1

Answer: (A)

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 $
$ \therefore $ $ R=\frac{12}{0.1}-2 $
$ =120-2 $ $ =118\,\Omega $