Lesson 1 of 10 · 6 min

The experiments of Faraday and Henry

NCERT §6.1, §6.2

Arjun's bench starts with a coil wired to a galvanometer and a bar magnet lying beside it. Nothing happens. What does he have to do to get a current out of that coil?

The story this chapter follows: Arjun's induction bench

Arjun's bench holds a 200-turn pickup coil of area 2 × 10⁻³ m² with a galvanometer, a bar magnet, two rails 0.5 m apart in a 0.4 T field with a 2 Ω resistor and a sliding copper rod, a lab solenoid of 1000 turns per metre with a 100-turn outer coil and a soft-iron core, and a hand-cranked coil like NCERT's pedal generator. Every idea in this chapter is tried out on that bench.

The lesson in notes

In short

Oersted and Ampere showed that currents make magnetic fields. Around 1830 Faraday in England and Henry in the USA showed the reverse link: a changing magnetic field drives a current in a closed coil. Producing current this way is called electromagnetic induction.

Experiment 6.1: push the N pole of a bar magnet towards a coil joined to a galvanometer and the pointer kicks. It deflects only while the magnet moves; a magnet held still gives nothing.

Pulling the magnet away reverses the deflection. Using the S pole instead reverses both results. Moving faster gives a bigger deflection.

Holding the magnet still and moving the coil gives the same effects: what matters is relative motion between magnet and coil.

Experiment 6.2: a second coil carrying a steady current from a battery can replace the magnet. Moving either coil relative to the other again gives a current while the motion lasts.

Experiment 6.3: with both coils held still, pressing the key in the battery circuit gives a brief kick in the galvanometer; holding it pressed gives none; releasing it gives a brief kick the other way. An iron rod along the common axis makes the kicks much larger.

So relative motion is not essential. What the three experiments share is a magnetic field through the coil that is changing with time.

Example 6.1: to get a bigger deflection, put a soft-iron rod in the current-carrying coil, use a stronger battery, or move it faster. Without a galvanometer, a small torch bulb in the circuit glows to show the induced current.