Question

A galvanometer has a resistance of $50\Omega$ and a full scale deflection current of $50\mu A$. It can be used as a volmeter or as a higher range ammeter provided that a resistance is added to it. Choose correct options.

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

Answer 1

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

The correct options are
A 20 V voltmeter with $400k\Omega$ in series
B 100 V voltmeter with $2\times {10}^6\Omega$ in series
D 0.1 mA Ammeter with $50\Omega$ in parallel


If r = 50 $\Omega$
$i_r=50\mu A$
$\therefore I=i+i_r=100\mu A=0.1mA$

$V_{AB}=50\mu A[R+50\Omega ]$

If R = $400k\Omega \to V_{AB}\approx 20V$
If R = $2\times {10}^6\Omega \Rightarrow V_{AB}\approx 100V$