Question

A moving coil instrument having a resistance of $10\Omega$, gives a full-scale deflection when the current is 10 mA. What should be the value of the series resistance, so that it can be used as a voltmeter for measuring potential difference up to 100 V?

• A. $9990\Omega$
• B. $990\Omega$
• C. $9\Omega$
• D. $99\Omega$

Answer 1

The correct option is A $9990\Omega$


$V_m=I_m{R}_m$
$=10mA\times 10\Omega$
$=100mV$

$(0-100mV)\Rightarrow (0-100V)$

$m=\frac{V_{ext}}{V_m}=\frac{100V}{100mV}=1000$

$R_{se}=R_m[m-1]$

$R_{se}=10[1000-1]$

$=9990\Omega$