Electromagnetic InductionThe Experiments of Faraday and Henry

Faraday and Henry did not find induction in one lucky moment, but through a long series of experiments. Here are the three that matter most. In each, a coil of insulated wire is joined to a galvanometer, a meter whose needle swings one way or the other when even a small current flows through it.

Experiment 6.1 (Fig. 6.1). A coil C₁ is connected to a galvanometer G. Drag the bar magnet towards the coil and away from it, and watch the needle.

The needle deflects only while the magnet moves: hold the magnet still, even deep inside the coil, and it reads zero, though the purple bar shows plenty of magnetic field through the coil. Pulling the N end out deflects the needle the other way. The S end gives deflections opposite to those of the N end, and the faster the magnet moves, the larger the deflection. Faraday found one more thing: if the magnet is held still and the coil is moved towards it or away from it, exactly the same happens. It is the relative motion between the magnet and the coil that induces the current.

Experiment 6.2: A Coil Instead of a Magnet

Now replace the bar magnet by a second coil, C₂, connected to a battery. The steady current in C₂ makes a steady magnetic field, just like a bar magnet's. In Fig. 6.2, C₂ is moved towards C₁:

As C₂ moves towards C₁, the galvanometer deflects; as C₂ moves away, it deflects the other way; and the deflection lasts only while C₂ is moving. Now Kabir holds C₂ still and moves C₁ towards it instead. What does the galvanometer do?

Again only the relative motion of the two coils matters, and a coil carrying a current behaves just like a magnet.

Experiment 6.3: No Motion at All

Faraday then showed that even relative motion is not really needed. In Fig. 6.3 two coils, C₁ and C₂, are held still. C₁ is joined to the galvanometer G, and C₂ to a battery through a tapping key K, a switch that stays closed only while you press it. Press the key, release it, and watch G.

Blue: the current in C₂. Amber: the current through G. Both in convenient units.

G gives a momentary kick when the key is pressed, and its needle returns to zero at once; while the key is held down there is nothing; when the key is released, another kick, the opposite way. With an iron rod along the axis of the coils, the kicks are dramatically larger. Compare the two graphs: the current through G appears only where the current in C₂ is rising or falling. Nothing moves in this experiment. What changes is the current in C₂, and with it the magnetic field that reaches C₁.