A moving coil galvanometer cannot be used directly as an ammeter in a circuit. This is because
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → it is very sensitive, and its large resistance changes the current in the circuit
An ammeter is joined in series with the circuit. So it has to carry the full current, and its resistance must be very small.
Option (1) it is very sensitive, and its large resistance changes the current in the circuit — Correct. A galvanometer gives full-scale deflection for a current of only a few microamperes, so a larger current goes off the scale. Its coil also has a large resistance. When it is joined in series, it lowers the current that it is meant to measure.
Option (2) it is very sensitive, and its very small resistance lets too much current pass through it — Incorrect. The first part is right, but the coil has a large resistance. A small resistance, the shunt, is added in parallel only when the galvanometer is changed into an ammeter.
Option (3) its deflection is not proportional to the current through it — Incorrect. The deflection is proportional to the current, \( \phi=(NAB/k)\,I \). This is why the scale can be marked evenly.
Option (4) it is not sensitive enough for small currents, and its large resistance changes the current — Incorrect. The large resistance is right, but the galvanometer is very sensitive. A current of the order of microamperes is enough for full-scale deflection.
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. This is for two reasons: (i) Galvanometer is a very sensitive device, it gives a full-scale deflection for a current of the order of μA. (ii) For measuring currents, the galvanometer has to be connected in series, and as it has a large resistance, this will change the value of the current in the circuit.”