Read the following passage and answer the questions.
A school laboratory has a moving coil galvanometer. Its rectangular coil has 100 turns, and each turn has an area of 4.0 cm2. The coil can turn between the curved pole pieces of a magnet, with a soft iron cylinder inside it. The magnetic field at the coil is 0.25 T. A spring gives a restoring torque of \(2.0\times10^{-5}\ \mathrm{N\,m}\) for each radian of twist. The coil has a resistance of 50 Ω. The pointer reaches the end of the scale for a current of 2 mA. A student first uses the galvanometer as a current detector in a Wheatstone bridge. Later she plans to change it into an ammeter and into a voltmeter, so that she can measure larger currents and voltages in other circuits.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): When this galvanometer is changed into an ammeter, the resistance of the ammeter is less than 50 Ω, the resistance of its coil.
Reason (R): In an ammeter, the shunt is connected in parallel with the coil of the galvanometer.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are correct and (R) is the correct explanation of (A).
Assertion (A) When this galvanometer is changed into an ammeter, the resistance of the ammeter is less than 50 Ω, the resistance of its coil. — True. The ammeter is the coil and the shunt together. Its resistance is smaller than the coil resistance \( R_G=50\ \Omega \).
Reason (R) In an ammeter, the shunt is connected in parallel with the coil of the galvanometer. — True. The shunt is a small resistance joined across the coil. Most of the current passes through it.
Why Reason (R) explains Assertion (A)?
For the coil \( R_G \) and the shunt \( r_s \) in parallel, the resistance of the ammeter is \( R_Gr_s/(R_G+r_s) \).
Here \( r_s/(R_G+r_s) \) is less than 1. So this resistance is less than \( R_G \), that is, less than 50 Ω.
NCERT (2026-27):
The Moving Coil Galvanometer – “… To overcome these difficulties, one attaches a small resistance rs, called shunt resistance, in parallel with the galvanometer coil; so that most of the current passes through the shunt. The resistance of this arrangement is,”
\( \displaystyle \frac{R_Gr_s}{R_G+r_s} \)\( \simeq r_s \quad \text{if } R_G\gg r_s \)