Question

An ammeter has a resistance of 50 $\Omega$ and a full scale deflection current of 50 $\mu A$. It can be used as a voltmeter or as a higher range ammeter provided that a resistance is added to it. Choose the correct range and resistance combinations from the following

• A. 20 V range with approximately 400 K$\Omega$ resistance in series
• B. 100 V range with approximately $2\times {10}^6\Omega$ resistance in series
• C. 1 mA range with 50 $\Omega$ resistance in parallel
• D. 0.1 mA range with 50$\Omega$ resistance in parallel

Answer 1

Answer: (A), (B), (D)

20 V range with approximately 400 K$\Omega$ resistance in series
100 V range with approximately $2\times {10}^6\Omega$ resistance in series
0.1 mA range with 50$\Omega$ resistance in parallel

We know by Ohm's Law that V=IR..

With 400Kohm resistor in series, $R_{eq.}\approx 400k\Omega$. Hence range = $50\mu X400k=20V$.

With $2X{10}^6\Omega$ in series, range= $2X{10}^{6}X50\mu =100V$.

With 50ohm resistance in parallel, range of ammeter will double, hence new range = 0.1mA.