Question

The copper coil of a 150 volts moving iron voltmeter has a resistance of 400 $\Omega at{15}^o$ and an inductance of 0.75 H. The current for full-scale deflection is 0.05 A. The temperature coefficient of resistance for copper and eureka at ${15}^{o}C$ are 0.004 and 0.00001 respectively. The percentage increase of resistance of the instrument per degree rise in temperature is __________%.

• A. 0.5
• B. 0.05
• C. 5
• D. 0.005

Answer 1

The correct option is B 0.05

Instrument resistance,
$R=400\Omega$

If r is the swamping resistance in series with the instrument, then

$R+r=\frac{150}{0.05}=3000\Omega$

$or,r=3000-400=2600\Omega$

Total resistance for $1^{o}C$ rise temperature
$=R(1+{\alpha }_c)+r(1+{\alpha }_s)$
$=400(1+0.004)+2600(1+0.00001)$
$=3001.626\Omega$


% rise of resistance per degree rise in temperature,

$=\frac{3001.626-3000}{3000}\times 100=0.0542\%$