Case: Read the passage and answer the question.
Consider a coil C1 connected to a galvanometer G, as shown. Another coil C2, connected to a battery through a tapping key K, is placed next to the first coil.
What does the galvanometer show at the instant the tapping key K is pressed?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A momentary deflection
An emf is induced in coil C1 only while the magnetic flux through it is changing. This flux is set up by the current in C2, so it changes only while that current is changing.
Option (1) A steady deflection that lasts as long as the key is held pressed — Incorrect. While the key is held pressed, the current in C2 is steady and the flux through C1 is constant. So no emf is induced, and the pointer stays at zero.
Option (2) A momentary deflection — Correct. On pressing the key, the current in C2 rises from zero to its steady value in a very short time. During this rise the flux through C1 changes, so an emf is induced only for that brief moment. The galvanometer deflects and returns to zero at once.
Option (3) A deflection only after the key is released — Incorrect. Releasing the key does give a momentary deflection, in the opposite direction, as the current falls to zero. The rise of current on pressing the key changes the flux too, so there is a deflection on pressing as well.
Option (4) No deflection at any time — Incorrect. The coils are at rest, but relative motion is not needed. The changing current in C2 changes the flux through C1 at the instant of pressing and induces an emf.
NCERT (2026-27):
The Experiments of Faraday and Henry – “… Coil C1 is connected to galvanometer G while the second coil C2 is connected to a battery through a tapping key K. It is observed that the galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately.”