Why these experiments matter
Through the early nineteenth century, electricity and magnetism were treated as separate phenomena. The work of Oersted, Ampère and others then established one direction of the link: moving electric charges produce a magnetic field, so that a current-carrying wire deflects a nearby compass needle. That result immediately raised the converse question — can moving magnets produce electric currents?
Faraday and Henry answered it experimentally. Working independently around 1830, they demonstrated that electric currents are induced in closed coils when those coils are subjected to changing magnetic fields. The phenomenon by which a current is generated by a varying magnetic field is called electromagnetic induction. The three experiments described below are not separate curiosities; they are a designed sequence that strips the effect down to its essential cause.
Each experiment uses the same detector — a galvanometer, a sensitive instrument whose pointer deflects when current flows and whose direction of deflection reveals the direction of that current. Reading the pointer correctly is the whole skill being tested.
Experiment 1: magnet and coil
A coil C₁ is connected to a galvanometer G, with no battery anywhere in the circuit. When the north pole of a bar magnet is pushed towards the coil, the galvanometer pointer deflects, indicating a current in the coil. The deflection lasts only as long as the magnet is in motion. Hold the magnet stationary near the coil and the pointer returns to zero, however close the magnet is held.
The experiment is then varied systematically, and each variation changes the reading in a definite way:
| Action performed | Galvanometer reading |
|---|---|
| North pole pushed towards the coil | Deflects in one direction |
| Magnet held stationary | No deflection |
| North pole pulled away from the coil | Deflects in the opposite direction |
| South pole moved towards or away | Deflections opposite to the north-pole case |
| Magnet moved faster (in or out) | Larger deflection |
| Magnet fixed, coil C₁ moved instead | Same effects as moving the magnet |
The last row carries the weight of the experiment. Whether the magnet approaches a fixed coil or the coil approaches a fixed magnet, the observations are identical. The conclusion is that it is the relative motion between the magnet and the coil — not the absolute motion of either one — that is responsible for inducing the current.
A strong magnet held still induces nothing
A common error is to assume that a very powerful magnet, simply held close to the coil, will drive a current. It will not. A stationary magnet gives a steady, unchanging flux, and a steady flux induces no emf no matter how strong the field. The same trap appears as a closed loop sitting still between two fixed magnets — NCERT answers this case with a flat "No".
Induction needs a change in flux with time, not a large flux. Static field → zero induced emf.
Experiment 2: two coils in motion
In the second experiment the bar magnet is replaced by a second coil C₂ connected to a battery. The steady current in C₂ produces a steady magnetic field, so C₂ now plays exactly the role the magnet played before. When C₂ is moved towards C₁, the galvanometer connected to C₁ deflects; when C₂ is moved away, it deflects in the opposite direction. The deflection lasts only while C₂ is in motion, and if C₂ is held fixed while C₁ is moved, the same effects appear.
The lesson is the same as before, now generalised: a current-carrying coil behaves like a magnet, and it is again the relative motion between the two coils that induces the current in C₁. The magnet of Experiment 1 was simply one convenient source of a magnetic field.
Every observation here is "flux changed → emf appeared". To turn that into the quantitative tool NEET tests, continue to Magnetic Flux.
Experiment 3: stationary coils and the key
The first two experiments both relied on motion, so a natural objection is that motion itself might be essential. Faraday's third experiment removes motion altogether. Two coils C₁ and C₂ are held stationary side by side. C₁ is connected to the galvanometer, and C₂ is connected to a battery through a tapping key K.
Nothing in the apparatus moves. Instead, the current in C₂ is switched on and off, and the galvanometer is watched closely:
| State of key K | Current in C₂ | Galvanometer in C₁ |
|---|---|---|
| Pressed (just closed) | Rising from zero to maximum | Momentary deflection, then returns to zero |
| Held pressed | Steady at maximum | No deflection |
| Released (just opened) | Falling from maximum to zero | Momentary deflection, opposite direction |
| Iron rod inserted along the axis | — | Deflection increases dramatically |
With no relative motion of any kind, a current is still induced in C₁ — but only at the moments when the current in C₂ is changing. While the current is steady, the galvanometer sits at zero. This is the decisive result: relative motion is one way to produce induction, but it is not a requirement. A changing current, and therefore a changing magnetic field, is enough.
The central inference
Read together, the three experiments point to one cause. In Experiments 6.1 and 6.2, moving the magnet or the coil changes the magnetic flux through C₁. In Experiment 6.3, switching the current in C₂ on or off changes the field, and hence the flux, through the stationary C₁. The common feature in every case is summarised by NCERT in a single sentence:
The time rate of change of magnetic flux through a circuit induces an emf in it.
This is the inference that the entire chapter is built on. The deflections of Experiment 6.3 are explained cleanly by it. When K is pressed, the current in C₂ and its field rise from zero to a maximum in a short time, so the flux through C₁ increases and an emf is induced. When K is held pressed, the current is constant, the flux through C₁ is constant, and the induced current drops to zero. When K is released, the field collapses to zero, the flux decreases, and an emf is again induced — in the opposite sense.
Q. (NCERT Example 6.1, on Experiment 6.2) What would you do to obtain a large deflection of the galvanometer, and how could you show that a current is induced if no galvanometer were available?
To increase the deflection: (i) place a soft-iron rod inside coil C₂, (ii) connect C₂ to a powerful battery, and (iii) move the arrangement rapidly towards the test coil C₁.
Without a galvanometer: replace it with a small torch bulb. The relative motion between the coils makes the bulb glow, demonstrating the induced current directly.
What controls the size of the effect
The experiments also reveal, qualitatively, what makes the induced current larger. Faster motion gives a larger deflection because the flux changes more quickly. A stronger source — a more powerful magnet or a larger battery current — produces a larger field and so a larger flux change. Inserting a soft-iron core sharply increases the deflection because iron greatly increases the field threading the coils. More turns on the coil also add to the effect.
These observations are the experimental seeds of the formal statement that follows in §6.4. There the magnetic flux is defined as $\Phi_B = \vec{B}\cdot\vec{A} = BA\cos\theta$, and the induced emf is written as $\varepsilon = -\dfrac{d\Phi_B}{dt}$, with $\varepsilon = -N\dfrac{d\Phi_B}{dt}$ for a coil of $N$ turns. Every "make it larger" trick above is exactly a way of increasing $\left|d\Phi_B/dt\right|$ or the turns $N$.
Direction reverses when the motion reverses
Pushing the north pole in increases the flux; pulling it out decreases it. Because the sign of $d\Phi_B/dt$ flips, the induced current — and the galvanometer deflection — reverses direction. Switching from the north pole to the south pole reverses it again. Questions that change the pole, the direction of motion, or the on/off state of a key are testing precisely this sign-tracking.
Flux increasing and flux decreasing give opposite current directions. Track the sign of the change, not just its presence.
Faraday and Henry — the takeaways
- Faraday and Henry (around 1830) showed that a changing magnetic field induces a current in a closed coil — the phenomenon of electromagnetic induction.
- Experiment 6.1: relative motion between a bar magnet and a coil deflects the galvanometer; a stationary magnet gives no deflection.
- Experiment 6.2: a current-carrying coil C₂ behaves like the magnet — relative motion between two coils induces a current.
- Experiment 6.3: with both coils stationary, switching the current in C₂ on or off still induces a momentary current in C₁ — motion is not essential.
- Central inference: the time rate of change of magnetic flux through a circuit induces an emf in it.
- Larger effect ← faster change, stronger source, more turns, soft-iron core. Direction reverses when the flux change reverses.