CBSE 2026 · Region 1 · Set 1 · Q29 · 4 marks

A galvanometer is used to detect or/and measure small currents in an electrical circuit. It essentially works on the fact that a current-carrying coil experiences a deflecting torque when placed in a magnetic field. This deflection in the coil can be measured and it is related to the current flowing in the coil, the number of turns in the coil, area of the coil and the magnetic field. A hair spring attached to the coil provides a counter torque and helps in measuring the deflection. A galvanometer can be converted to an ammeter or a voltmeter of desired range by using suitable resistances.

(I)

The torque on the coil remains constant irrespective of the coil's orientation during rotation due to

(A)

use of soft iron core which increases the magnetic field.

(B)

radial magnetic field

(C)

hair spring which provides the counter torque

(D)

eddy current in the iron core which causes damping.

(II)

The best way to increase current sensitivity of a galvanometer is by

(A)

increasing number of turns of the coil

(B)

increasing area of coil and magnitic field strength

(C)

decreasing area of coil and magnetic field strength

(D)

increasing torsional constant of the hair spring

(III)

A moving coil galvanometer has a coil with area of cross-section $\displaystyle 4.0 \times 10^{-3} \mathrm{~m}^{2}$ and number of turns 50. The coil is rotating in a magnetic field of $\displaystyle 0.25$ T. The torque acting on the coil when a current of $\displaystyle 5$ A passes through it is

(A)

1.$\displaystyle 0$ N m

(B)

2.$\displaystyle 0$ N m

(C)

0.$\displaystyle 50$ N m

(D)

0.$\displaystyle 25$ N m

(D)

OR A galvanometer coil has a resistance of $\displaystyle 15 \Omega$ and the meter shows full scale deflection for a current of $\displaystyle 3$ mA . The value of resistance required to convert it into a voltmeter of range ($\displaystyle 0$-$\displaystyle 12$ V) is

(A)

$\displaystyle 4015 \Omega$

(B)

$\displaystyle 3985 \Omega$

(C)

$\displaystyle 415 \Omega$

(D)

$\displaystyle 385 \Omega$

(IV)

A galvanometer with coil of resistance $\displaystyle 20 \Omega$ shows full scale deflection for a current of $\displaystyle 5$ mA . To convert it into an ammeter of range ($\displaystyle 0$-$\displaystyle 10$ A), a resistance of

(A)

$\displaystyle 0.05 \Omega$ should be connected in series with it.

(B)

$\displaystyle 0.05 \Omega$ should be connected in parallel with it.

(C)

$\displaystyle 0.01 \Omega$ should be connected in parallel with it.

(D)

$\displaystyle 0.01 \Omega$ should be connected in series with it.

CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.