Question

A moving coil galvanometer has 150 equal divisions. Its current sensitivity is 10 divisions per milliampere and voltage sensitivity is 2 divisions per millivolt. In order that each division reads 1 volt, the resistance in ohm needed to be connected in series, with the coil will be
A. 1000
B. 100000
C. 99995
D. 9995

Answer 1

Hint: A galvanometer is an electrical instrument which detects the presence of current in the circuit. A moving coil galvanometer is a sensitive electromagnetic device which can measure low currents even of the order of a few microamperes. It consists of a metal piece on which wire is wound and a magnet field is applied, due to which it gets rotated on passing even the smallest current.

Complete answer:
Given that the moving coil galvanometer has 150 equal divisions.
Current sensitivity = 10 div per milliampere.
Voltage sensitivity = 2 div per millivolt.
The maximum current that can flow in the galvanometer for full scale deflection = $I_g = \dfrac{150}{10}=15mA$
Also the maximum voltage $V_g$that can flow through the galvanometer for full scale deflection = $V_g = \dfrac{150}{2}= 75mV$
So, resistance of galvanometer $R_g=\dfrac{V_g}{I_g}=\dfrac{75}{15}=5\Omega$
Thus the resistance needed in series with the galvanometer to get converted into voltmeter of range V = $150 \times 1 = 150V$ is

$R=\dfrac{V}{I_g}-R_g$
$R = \dfrac{150}{15\times 10^{-3}}-5$
$R=10000-5 = 9995 \Omega$

So, the correct answer is “Option C”.

Additional Information:
A galvanometer is used to detect the presence of current in the circuit. It can be of many types. Some of them are tangent galvanometer, mirror galvanometer, astatic galvanometer, ballistic galvanometer and moving coil galvanometer. The most important and widely used galvanometer is the moving coil galvanometer.

Note:
Moving coil galvanometer works on the principle that whenever a current carrying wire lies inside a magnetic field, it experiences a force. Due to this principle, wire is rolled onto a piece on which needle is also fixed. The magnetic field is provided by using permanent magnets, which is radial in direction. Hence when a current is passed through the wire, a force starts acting on the wire which rotates the wire.