Which famous experiment was done by Michael Faraday in 1831?

Correct — B. In 1831 Michael Faraday showed that a changing magnetic field through a coil drives a current in it. That is electromagnetic induction.

The experiments were plain ones: push a bar magnet into a coil wired to a galvanometer, or switch the current on and off in one coil wound beside another. The needle moves only while the field is changing — hold the magnet still and it falls back to zero.

Generators, transformers and induction motors all descend from that result, which option (b) names.

Why the others are wrong

• (a)
  Discovery of law of elasticity — The law of elasticity — stress proportional to strain within the elastic limit — is Hooke's, from the seventeenth century, and it belongs to mechanics rather than to electricity.
• (c)
  Discovery of quantum magnetometers — Quantum magnetometers are twentieth-century instruments built on quantum effects. Nothing of the kind could have been built in 1831.
• (d)
  Discovery of natural radioactivity — Natural radioactivity was found by Henri Becquerel in 1896, when uranium salts fogged a photographic plate that had been kept in the dark.

Faraday's law ties the induced emf to the rate of change of magnetic flux through a circuit. A stationary magnet induces nothing; a moving one does, and moving it faster induces more.

Lenz's law supplies the direction: the induced current opposes the change that produced it, which is why a generator gets harder to turn as you draw more current from it.

Every machine that turns motion into electricity, or steps a voltage up or down, runs on this one result — which is why 1831 is a date worth carrying.

Key facts

• Michael Faraday demonstrated electromagnetic induction in 1831.
• An emf is induced in a circuit only while the magnetic flux through it is changing.
• Lenz's law states that the induced current opposes the change that caused it.
• Henri Becquerel discovered natural radioactivity in 1896.

Common traps

• Attaching 1831 to Faraday's laws of electrolysis, which are a separate part of his work.
• Reading the item as a discovery of magnetism itself rather than of induction.

No directly related past PYQ was found.