Question

A microammeter has a resistance of $100\Omega$ and full scale range of $50\mu A$. It can be used as a voltmeter or as a higher range ammeter provided a resistance is added to it. Pick the correct range and resistance combination (s)

• A. 50 V range with $10k\Omega$ resistance in series
• B. 10 V range with $2000k\Omega$ resistance in series
• C. 5 mA range with $1\Omega$ resistance in parallel
• D. 10 mA range with $1\Omega$ resistance in parallel

Answer 1

Answer: (B), (C)

To increase the range of ammeter a parallel resistance (cal led shunt) is required which is given by
$S=\left(\frac{i_g}{i-i_g}\right)G$
For option (c)
$S=\left(\frac{50\times 10^{-6}}{50\times 10^{-3}-50\times 10^{-6}}\right)(100)\approx 1\Omega$
To change it in voltmeter, a high resistance R is put in series,
where R is given by $r=\frac{V}{i_g}-G$
For option (b), $R=\frac{10}{50\times 10^{-6}}-100\approx 200k\Omega$
Therefore, options (b) and (c) are correct.