The Experiments of Faraday and Henry

Why the chapter exists

• Through the early nineteenth century Oersted, Ampere and others established that moving charges produce magnetic fields: a current deflects a nearby compass needle.
• The obvious converse question — can moving magnets produce currents? — was answered by Michael Faraday in England and Joseph Henry in the USA, working independently around 1830. Their answer was yes: electric currents are induced in closed coils subjected to CHANGING magnetic fields.
• The phenomenon of generating an emf, and hence a current, by a varying magnetic field is called electromagnetic induction. Everything that follows — the generator, the transformer, the inductor — is an application of it.

Experiment 6.1 — bar magnet and coil

• Set-up: a coil $C_1$ whose two ends are connected to a galvanometer $G$. A bar magnet is moved along the axis of the coil.
• Push the N-pole of the magnet TOWARDS the coil: the galvanometer pointer deflects, showing a current. The deflection lasts only as long as the magnet is moving.
• Hold the magnet stationary: no deflection at all, however close or however strong the magnet.
• Pull the magnet AWAY: deflection in the opposite direction, showing the current has reversed.
• Repeat with the S-pole facing the coil: the deflections are opposite to those obtained with the N-pole for the same movements.
• Move the magnet FASTER: the deflection, and hence the current, is larger.
• Hold the magnet fixed and move the COIL instead: exactly the same effects.
• Conclusion: it is the RELATIVE motion between the magnet and the coil that generates the current, and the faster that relative motion, the larger the current.

Experiment 6.2 — two coils, one carrying a steady current

• Set-up: the bar magnet of Experiment 6.1 is replaced by a second coil $C_2$ connected to a battery. The steady current in $C_2$ produces a steady magnetic field, so $C_2$ behaves like a magnet.
• Move $C_2$ towards $C_1$: the galvanometer in the $C_1$ circuit deflects, so a current is induced in $C_1$.
• Move $C_2$ away: deflection in the opposite direction. The deflection lasts only while $C_2$ is in motion.
• Hold $C_2$ fixed and move $C_1$: the same effects again.
• Conclusion: once more, it is the relative motion between the two coils that induces the current.

Experiment 6.3 — no relative motion at all

• Set-up: two coils $C_1$ and $C_2$ held STATIONARY, close together. $C_1$ is connected to a galvanometer $G$; $C_2$ is connected to a battery through a tapping key $K$.
• Press $K$: the galvanometer gives a momentary deflection and the pointer returns at once to zero.
• Keep $K$ pressed: no deflection at all, even though a steady current is flowing in $C_2$.
• Release $K$: a momentary deflection again, this time in the opposite direction.
• Insert an iron rod into the coils along their common axis: the deflection increases dramatically.
• Conclusion: relative motion is NOT an absolute requirement. What matters is that the current in $C_2$ — and hence the magnetic flux threading $C_1$ — is CHANGING. A steady current gives a steady flux and therefore no emf.

What the three experiments together establish

• The common thread, and the sentence that becomes Faraday's law: the time rate of change of magnetic flux through a circuit induces an emf in it.
• Practical caution from NCERT: sensitive electrical instruments near an electromagnet can be damaged by the induced emfs when it is switched on or off.

The NCERT worked example on these experiments (Example 6.1)

• To obtain a LARGE deflection in Experiment 6.2: (i) place a soft-iron rod inside coil $C_2$, (ii) connect $C_2$ to a more powerful battery, and (iii) move the arrangement rapidly towards the test coil $C_1$.
• To demonstrate the induced current WITHOUT a galvanometer: replace the galvanometer by a small torch bulb. The relative motion of the two coils makes the bulb glow.