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 $10 k \Omega$ resistance in series
• B. 10 V range with $2000 k \Omega$ 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 = \bigg( \frac{i_g}{i - i_g} \bigg)G $
For option (c)
$ \, \, \, \, \, \, \, \, \, \, S = \bigg( \frac{ 50 \times 10^{-6}}{50 \times 10^{-3} - 50 \times 10^{-6}} \bigg) (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.