To convert a galvanometer into an ammeter

The correct answer is Option (3) → a very small resistance is added in parallel with the galvanometer

An ammeter must have a very low resistance and must be able to carry a large current. A small resistance \( S \), the shunt, connected in parallel with the galvanometer takes most of the current, so that only the small current \( I_g \) passes through the coil.

    \( \displaystyle S = \frac{I_g\,G}{I - I_g} \)

Option (1) a very small resistance is added in series with the galvanometer — Incorrect. In series, the whole current still passes through the coil, so the range is not increased.

Option (2) a high resistance is added in series with the galvanometer — Incorrect. A high resistance in series converts the galvanometer into a voltmeter.

Option (3) a very small resistance is added in parallel with the galvanometer — Correct. The small shunt in parallel carries most of the current and keeps the total resistance low.

Option (4) a high resistance is added in parallel with the galvanometer — Incorrect. A high resistance in parallel takes very little current, so the coil still carries almost all of it and the range hardly changes.

NCERT (2026-27):
The Moving Coil Galvanometer – “… The galvanometer cannot as such be used as an ammeter to measure the value of the current in a given circuit.”

The Moving Coil Galvanometer – “… To overcome these difficulties, one attaches a small resistance r_s, called shunt resistance, in parallel with the galvanometer coil; so that most of the current passes through the shunt.”