Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is

Assertion (A): The deflection in a galvanometer is directly proportional to the current passing through it.

Reason (R): The coil of a galvanometer is suspended in a uniform radial magnetic field.

CBSE CLASS XII - 2025
Physics
The Moving Coil Galvanometer

Galvanometer:

A galvanometer is an instrument used to show the direction and strength of the current passing through it. In a galvanometer, a coil placed in a magnetic field experiences a torque and hence gets deflected when a current passes through it.

The name "galvanometer" is derived from the surname of Italian scientist Luigi Galvani, who in 1791 discovered that electric current makes a dead frog’s leg jerk.

A spring attached to the coil provides a counter torque. In equilibrium, the deflecting torque is balanced by the restoring torque of the spring, and we have the relation:

\[ NBAI = k\phi \]

Where:

\( N \) is the total number of turns in the coil
\( A \) is the area of cross-section of each turn
\( B \) is the radial magnetic field
\( k \) is the torsional constant of the spring
\( \phi \) is the angular deflection of the coil

As the current \( I_g \) that produces full-scale deflection in the galvanometer is very small, the galvanometer alone cannot be used to measure current in electric circuits.

To convert a galvanometer into an ammeter (to measure larger currents), a small resistance called a shunt is connected in parallel to the galvanometer.

To convert it into a voltmeter (to measure potential difference), a high resistance is connected in series with the galvanometer.