Experiments of Faraday and Henry

Push a bar magnet into a coil of wire connected to nothing but a galvanometer — no battery, no cell, no external power of any kind — and the needle jumps. Pull the magnet back out and the needle jumps the other way. Hold the magnet perfectly still, even resting right inside the coil, and the needle falls dead to zero. Nothing about this makes sense if you think current needs a battery to push it around a circuit. Yet the effect is real and repeatable, and in the early 1830s it was independently uncovered by two physicists an ocean apart: Michael Faraday in London and Joseph Henry in Albany, New York. Neither had access to the other's notebook; both stumbled onto the same startling fact, that a magnetic field which is changing can drive a current through a wire loop all by itself. This section walks through the actual tabletop experiments that revealed electromagnetic induction, in the order a CBSE Class 12 student meets them: a magnet moving relative to a coil, a current-carrying coil moving relative to a second coil, and — the most surprising of the three — two coils that never move at all, where a current merely switched on or off in one coil is enough to jolt a galvanometer connected to the other. Each experiment isolates a different way of making the picture change, yet all three produce the same kind of transient blip. That common thread — motion or switching produces a deflection, stillness produces none — is exactly what Faraday distilled into the law of electromagnetic induction that the next topic puts into symbols. Understanding these three experiments cold, and predicting what a galvanometer will do in a new variant, is worth real marks and is the conceptual foundation for this whole chapter.

Key takeaways

Michael Faraday (England) and Joseph Henry (USA) independently discovered electromagnetic induction around 1830 to 1831 using coils, magnets, and a galvanometer.
Experiment 1: moving a bar magnet towards or away from a stationary coil deflects its galvanometer only while the magnet is in motion, reversing with the direction of motion.
Experiment 2: moving a current-carrying primary coil relative to a secondary coil gives the identical result, since a current-carrying coil behaves like a magnet.
Experiment 3: two coils are kept completely stationary, one wired to a battery through a tap key. Pressing or releasing the key produces a momentary deflection in the other coil, even though nothing physically moves.
All three are explained by one rule: an EMF is induced whenever the magnetic flux linked with a circuit changes, whether from motion or a changing current.
A steady field, a stationary magnet, or a steady current produces zero induced EMF no matter how strong the field is, since there is no change in flux.
Key idea: relative motion of a magnet and a coil, or a changing current in a nearby coil, induces a transient EMF only for as long as the linked magnetic flux is actually changing, and this shared observation across all of Faraday's and Henry's setups is what becomes the quantitative law of electromagnetic induction.

The core insight

A galvanometer connected to a coil deflects only while the magnetic flux threading that coil is changing — by relative motion of a magnet, by relative motion of a current-carrying coil, or by switching a nearby current on or off — and falls to zero the instant the flux becomes steady again, which is the experimental seed of Faraday's law $\varepsilon=-\dfrac{d\phi_B}{dt}$ .

Where it fits in the syllabus

This is the opening topic of CBSE Class 12 Physics Unit VI, the chapter "Electromagnetic Induction," prescribed as qualitative, observational content: the demonstrations themselves, and the single conclusion that induced EMF requires a changing magnetic flux. It comes before flux is even given a formula, because the historical experiments are exactly how the concept of flux and Faraday's law were motivated in the first place. The next node, "Magnetic Flux," gives $\phi_B=\vec B\cdot\vec A$ its formal definition and the quantitative law $\varepsilon=-\dfrac{d\phi_B}{dt}$ ; this section only needs you to see, physically, why such a law had to exist. Nothing here should be confused with the fuller machinery of motional EMF, eddy currents, or AC generator construction — those sit later in this chapter, and a full Lorentz-force derivation of motional EMF is explicitly outside what this board expects.

The physical picture

Picture a coil of $N$ turns, its two ends joined to a sensitive galvanometer so any current, however small, shows up as a needle deflection. No battery is anywhere in this circuit — the loop is entirely passive. Bring a bar magnet close: while it approaches, the number of field lines threading the coil's area is climbing, and while it recedes, that number falls. The single quantity that tracks "how many field lines thread the coil" is the magnetic flux,

\phi_B=\vec B\cdot\vec A=BA\cos\theta

where: $B$ is the magnetic field's magnitude at the coil, $A$ is the coil's area vector (magnitude equal to the loop's area, direction along its normal), and $\theta$ is the angle between them. Physically, $\phi_B$ is a bookkeeping number for how much field is "caught" by the loop; it grows when the magnet nears, shrinks when it withdraws, and stays fixed the instant nothing about the geometry or field is changing. Faraday's experiments are three different tricks for making $\phi_B$ change without touching the coil's wire directly, and Henry's independent work confirmed the same behaviour before either had published. The observed rule connecting flux change to what the galvanometer shows is the qualitative form of Faraday's law,

\varepsilon=-\dfrac{d\phi_B}{dt}

where: $\varepsilon$ is the induced EMF (in volts) driving the induced current if the circuit is closed, and $\dfrac{d\phi_B}{dt}$ is the rate at which flux is changing (in weber per second). The formula is introduced here only so that "changing flux induces EMF" has a symbol to hang on. What matters for these experiments is the shape of the relationship: no change in $\phi_B$ means $\varepsilon=0$ , however large $\phi_B$ itself is, and a faster change means a larger deflection.

Figure

A coil connected to a galvanometer with a bar magnet approaching from the left, field lines threading the coil, and a small graph showing the galvanometer deflection rising while the magnet moves and dropping to zero the instant it stops

Faraday's and Henry's demonstrations, one at a time

Experiment 1, the magnet and the coil. A coil of insulated wire is connected only to a galvanometer, forming a closed loop with no source of EMF anywhere in it. A bar magnet is pushed towards the coil, held there, then pulled away. While the magnet approaches, the galvanometer deflects one way; the moment its motion stops, however close it sits to the coil, the deflection collapses to zero; as it withdraws, the deflection reappears in the opposite sense. Faster motion produces a larger deflection, and a stronger magnet gives a larger deflection for the same speed — but a stationary magnet of any strength gives nothing at all.

Experiment 2, a current-carrying coil in place of the magnet. Replace the bar magnet with a second coil, the primary, connected to a battery so a steady current flows in it. A current-carrying coil is itself a small electromagnet, so moving this primary towards or away from the secondary (wired to the galvanometer) reproduces Experiment 1 exactly: deflection during relative motion, zero the instant motion stops, reversed deflection on withdrawal. This confirms it is not something special about permanent magnets — any source of magnetic field, moved relative to the coil, does the same job.

Experiment 3, two coils that never move. This surprised contemporaries most, including Faraday himself. Two coils, primary and secondary, are fixed near each other so neither ever moves. The primary is wired to a battery through a tap key; the secondary only to a galvanometer. With the key open, the galvanometer reads zero. Pressing the key: the secondary's galvanometer jumps briefly, then settles to zero even though the primary current is now steady. Holding the key down: nothing happens, however long the steady current continues. Releasing the key: the primary current collapses to zero, and the galvanometer jumps again, this time oppositely. No part of either coil has moved a millimetre — what changed, at the make and break, is the current in the primary, and with it the flux linked with the secondary.

From these three experiments to one law

Derivation AFrom Faraday's and Henry's demonstrations to the law of induction

The goal here is not to prove Faraday's law from first principles — that belongs to the next topic, and a full motional-EMF derivation is explicitly beyond this board's syllabus — but to show, using plain reasoning about the three experiments above, why "changing flux induces EMF" is the only rule consistent with everything Faraday and Henry actually observed, and to attach a rough number to how large that induced EMF can be for a typical laboratory coil. We treat the galvanometer deflection as evidence of an induced EMF, and ask what single physical quantity is doing the same job in all three setups.

Step 1. List what changes and what does not, in each experiment. In Experiment 1, the magnet's distance from the coil changes while it moves, and is constant while it is held still. Experiment 2 is the same, with an electromagnet in place of the permanent magnet. In Experiment 3 nothing physically moves, but the primary's current — and hence the field it produces at the secondary's location — jumps from zero to a steady value at the make of the key, and back to zero at the break.

Step 2. Identify the one quantity common to all three. Distance from a magnet and the field strength of a nearby electromagnet are physically different things, so neither alone can be the common trigger. But all three setups share one derived quantity: the magnetic flux $\phi_B=\vec B\cdot\vec A$ linked with the secondary (or lone) coil. Moving a magnet closer increases $B$ at the coil, hence $\phi_B$ ; switching on the primary current raises $B$ at the secondary's location from zero to a steady value, again raising $\phi_B$ with $A$ fixed. In every experiment, the galvanometer deflects exactly when $\phi_B$ is changing.

Step 3. Read off the qualitative law. Whenever $\phi_B$ linked with a closed circuit is changing, an EMF $\varepsilon$ appears and drives a current; whenever $\phi_B$ is constant — nothing moving, or a current settled to a steady value — no EMF appears, however large $\phi_B$ itself is. This is Faraday's law in qualitative form; its quantitative statement, quoted without derivation since it belongs to the next topic, is $\varepsilon=-\dfrac{d\phi_B}{dt}$ . One signature follows directly: deflection strength scales with how fast the flux changes, not with the field's instantaneous size, so a sudden make-or-break gives a brief, sharp deflection.

Step 4. Attach a rough number. A coil of $N=200$ turns and area $A=4\times10^{-3}\ \text{m}^2$ sits perpendicular to a magnet's field, which rises from $B_1=0.01\ \text{T}$ to $B_2=0.09\ \text{T}$ as the magnet is pushed in over $\Delta t=0.4\ \text{s}$ . The flux per turn changes by $\Delta\phi_B=(B_2-B_1)A=3.2\times10^{-4}\ \text{Wb}$ , the total flux linkage change is $N\Delta\phi_B=6.4\times10^{-2}\ \text{Wb-turns}$ , and the average induced EMF is $\varepsilon=\dfrac{N\Delta\phi_B}{\Delta t}=\dfrac{6.4\times10^{-2}}{0.4}=0.16\ \text{V}$ — easily enough to swing a sensitive galvanometer, and present only for the $0.4\ \text{s}$ of motion; hold the magnet still afterwards and $\varepsilon$ returns to zero. Verified: $\Delta\phi_B=3.2\times10^{-4}\ \text{Wb}$ per turn, $N\Delta\phi_B=6.4\times10^{-2}\ \text{Wb-turns}$ , $\varepsilon=0.16\ \text{V}$ — recomputed directly from $\varepsilon=N\Delta\phi_B/\Delta t$ with the stated $N$ , $A$ , $B_1$ , $B_2$ , $\Delta t$ .

Worked examples

Board level - reading a galvanometer trace

Question: A student pushes the north pole of a bar magnet into a coil connected to a galvanometer, holds the magnet still just inside, then pulls it back out at the same speed. Describe, in words, what the galvanometer does through this sequence, and explain why.

Step 1. While the magnet moves in, the flux $\phi_B$ linked with the coil is continuously increasing, since the field lines threading the coil grow denser as the magnet approaches.

Step 2. A continuously changing flux means $d\phi_B/dt\neq0$ , so an EMF is induced and the galvanometer deflects, say, to the right, for as long as the magnet moves.

Step 3. Once the magnet is held stationary just inside the coil, $B$ there stops changing, so $\phi_B$ becomes constant. With $d\phi_B/dt=0$ , the induced EMF is zero and the needle returns to and stays at zero, however strong the field now is.

Step 4. Pulling the magnet back out at the same speed makes $\phi_B$ decrease at the same rate it previously increased, so the galvanometer deflects again with the same magnitude but now to the left — the mirror image of Step 1.

Answer: The galvanometer deflects one way while the magnet enters, drops to exactly zero while the magnet is held still, and deflects an equal amount the opposite way while the magnet is withdrawn — because induction tracks the rate of change of flux, not the flux itself.

Competitive level - two stationary coils and a tap key

Question: Coil P (primary) connects to a $6\ \text{V}$ battery through a tap key and a rheostat limiting the steady current to $I=2\ \text{A}$ ; coil Q (secondary), placed next to P, connects only to a ballistic galvanometer and has flux linkage $N\phi_B=0.048\ \text{Wb-turns}$ with P's field once steady. The key is pressed and P's current rises from zero to $2\ \text{A}$ in $\Delta t=6\ \text{ms}$ , essentially linearly. Estimate the average EMF induced in Q during that rise, and state what the galvanometer shows once the key is held down and then released.

Step 1. During the rise, the flux linkage with Q grows from $0$ to $N\phi_B=0.048\ \text{Wb-turns}$ , since flux is proportional to the primary's current, which itself rises essentially linearly over $\Delta t$ .

Step 2. Apply the flux-linkage form of the induction law: $\varepsilon=\dfrac{N\Delta\phi_B}{\Delta t}=\dfrac{0.048\ \text{Wb-turns}}{6\times10^{-3}\ \text{s}}=8.0\ \text{V}$ .

Step 3. This $8.0\ \text{V}$ is only an average over the brief make-transient; the galvanometer kicks momentarily and settles back to zero once the primary current becomes steady at $2\ \text{A}$ , since a steady current gives $d\phi_B/dt=0$ in Q.

Step 4. When the key is released, the primary current collapses back to zero over a similarly short interval, so the flux linkage falls by the same $0.048\ \text{Wb-turns}$ , and the galvanometer kicks again — comparable in size — but in the opposite direction, since flux is now decreasing.

Answer: The average induced EMF during the make of the key is about 8.0 V, appearing only for the few milliseconds the current is actually rising; the galvanometer shows zero while the primary current is steady at $2\ \text{A}$ , and a momentary opposite-direction kick when the key is released. Timing note: the trap here is treating the $6\ \text{ms}$ rise time as the duration of a sustained EMF — it is not. The $8.0\ \text{V}$ is an average over a fleeting transient, and a full answer must state explicitly that the deflection vanishes once the current, and hence the flux, stops changing.

Common traps

Assuming a strong magnetic field by itself can induce a current — only a changing flux can; a stationary magnet resting inside a coil, however powerful, gives zero deflection.
Believing physical motion of a magnet is essential — Experiment 3 shows a changing current in a completely stationary coil is equally effective, since it is the flux that must change, not any object's position.
Treating the induced current from a single make or break as sustained, like a battery's output — it is transient, lasting only as long as the flux is in the process of changing.
Mixing up which coil has to change — it is the source (the moving magnet or coil, or the coil whose current is switched) that must change relative to the detecting coil; the detecting coil need not move or carry current of its own.
Forgetting that the deflection direction reverses between increasing and decreasing flux — approach versus recede, and key press versus release, give opposite-sign deflections.

You should be able to…

Describe Faraday's magnet-and-coil experiment and state exactly what the galvanometer shows while the magnet moves, is held still, and withdraws.
Describe the current-carrying-coil version and explain why it behaves identically to the bar-magnet version.
Describe the two-stationary-coils, tap-key experiment and explain why a momentary deflection appears at make and break, with none while the current is steady.
State the single conclusion common to all three experiments: induction requires a change in magnetic flux, not merely the presence of a field.
Compute a rough induced EMF from flux-linkage data using $\varepsilon=N\Delta\phi_B/\Delta t$ , identifying it as an average over a brief transient.
Distinguish an induced (transient, flux-driven) current from an ordinary battery-driven (steady) current in a circuit diagram.

Ask the tutor: want to work through a few more "what does the galvanometer show" scenarios, or see how this leads directly into the formal definition of magnetic flux and Faraday's law in the next topic — just ask the AI tutor to walk it through.

Why does the deflection vanish the instant the magnet stops moving, even though it is still right next to the coil?

Because induction responds to the rate of change of magnetic flux, not to how much flux is present. While the magnet approaches, the flux through the coil is climbing every instant, so $d\phi_B/dt$ is nonzero and an EMF appears. The moment the magnet becomes stationary, the field at the coil stops changing, so $d\phi_B/dt$ drops to exactly zero even though $\phi_B$ itself is now at its largest value. The galvanometer only ever reports the rate, so it reads zero — this is exactly why a strong permanent magnet sitting quietly inside a coil never lights a bulb.

Is the current induced in Experiment 3 the same kind of current as the steady current from the battery in the primary coil?

No. The battery in the primary coil drives a steady, continuous current for as long as the key stays pressed — ordinary circuit current from a source of EMF. The current in the secondary coil is different: it exists only for the brief instant while the primary's current, and hence the flux linked with the secondary, is actually changing. Once the primary settles to its steady value, the secondary's induced current disappears completely, even though the primary keeps driving current through itself. The secondary has no battery at all; its brief current is purely a consequence of the changing flux.

Did Faraday and Henry actually work together, or is calling it "Faraday and Henry" just a syllabus convention?

They worked entirely independently and did not collaborate. Michael Faraday in England and Joseph Henry in the United States each built broadly similar coil-and-magnet demonstrations and reached the same conclusion — that a changing magnetic flux induces an EMF — within roughly the same year or two, around 1830 to 1831, unaware of each other's work. Faraday published first, which is why the effect is usually called Faraday's law, but Henry's independent, near-simultaneous discovery is why his name is honoured alongside Faraday's, and why the SI unit of inductance is named the henry.

If I move the coil instead of the magnet, keeping the magnet fixed, does anything change?

No — only the relative motion between the magnet and the coil matters, not which one you consider to be "moving." If you keep the magnet fixed and move the coil towards it at the same relative speed, the flux linked with the coil changes at exactly the same rate, and the galvanometer shows an identical deflection. This is worth stating explicitly in an answer, because a board question may describe the coil moving rather than the magnet, and a student who has only memorised "move the magnet" can be thrown by the wording even though the physics is unchanged.