The Experiments of Faraday and Henry

What the Figure Shows

The diagram depicts a coil $C_1$ — drawn as a stack of concentric ellipses seen edge-on — connected by two wires to a galvanometer $G$ (a circle with a deflected needle). To the right of the coil, along a horizontal dashed axis, lies a bar magnet (shaded rectangle) with its North pole (N) facing the coil and its South pole (S) at the far right. A short arrow above the magnet points left, indicating the magnet is being pushed toward the coil. Blue magnetic field lines emerge from the N-pole and from the coil, curving outward to the upper-left and lower-left, with arrowheads showing direction. The right wire from the coil carries a downward arrow labelled I (induced current).

The Physical Idea

The figure illustrates Faraday’s first experiment: when the North pole of a bar magnet is moved toward a stationary coil, the galvanometer needle deflects — showing that an electric current is induced in the coil. The deflection lasts only while the magnet is in motion; if the magnet is held still, the needle returns to zero. This demonstrates that relative motion between the magnet and the coil is essential for generating induced current. The direction of deflection (and hence current) reverses when the magnet is pulled away, or when the South pole is used instead.

Key Formula Developed from This Figure

From this experiment, Faraday’s law of electromagnetic induction is derived:

where:

• $\mathcal{E}$ is the induced emf (electromotive force) in the coil (in volts),
• $\Phi_B$ is the magnetic flux through the coil (in webers), given by $\Phi_B = \vec{B} \cdot \vec{A} = BA\cos\theta$,
• $\frac{d\Phi_B}{dt}$ is the rate of change of magnetic flux with time,
• The negative sign (Lenz’s law) indicates that the induced emf opposes the change in flux.

For a coil with $N$ turns, the formula becomes: …

What the diagram shows

The figure depicts two coaxial disc‑shaped coils placed along a common dashed horizontal axis. On the left is a larger coil labelled C₁, drawn as a stack of ellipses. On the right is a smaller coil labelled C₂. Blue magnetic field lines radiate from C₂ toward C₁, fanning outward in the upper‑left and lower‑left directions. A black arrow pointing leftward between the coils indicates that C₂ is moving toward C₁. Below C₁, two leads drop to a rectangular box containing a circular galvanometer G, with a downward‑pointing arrow labelled I (the induced current). Below C₂, its leads form a loop that includes a battery symbol (two unequal parallel lines). The arrangement mirrors Fig. 6.1, with the bar magnet replaced by the current‑carrying coil C₂.

Physical idea taught

The figure illustrates Faraday’s experiment 6.2: a steady current in coil C₂ produces a steady magnetic field. When C₂ is moved toward C₁, the magnetic flux through C₁ changes, inducing an electric current in C₁ that is detected by the galvanometer. The deflection lasts only while C₂ is in motion; when C₂ is stationary, no current is induced. Reversing the direction of motion (moving C₂ away) reverses the deflection, showing that the induced current’s direction depends on whether the flux is increasing or decreasing. The key insight is that relative motion between the coils — not the presence of a permanent magnet — is sufficient to induce current.

Key formula developed from this figure

The textbook uses this experiment to introduce Faraday’s law of induction. The induced electromotive force (emf) in coil C₁ is given by:

where:

• $\mathcal{E}$ is the induced emf (in volts),
• $\Phi_B$ is the magnetic flux through coil C₁ (in webers),
• $\frac{d\Phi_B}{dt}$ is the rate of change of that flux with time,
• the negative sign indicates the direction of the induced emf (Lenz’s law). …

What the Figure Shows

The diagram depicts two stationary coaxial coils — a large multi-turn coil $C_1$ on the left and a smaller coil $C_2$ on the right. A long dashed horizontal line runs through their centres, indicating they share a common axis. There are no motion arrows anywhere; both coils are fixed in place.

Below $C_1$, two leads connect to a rectangular box containing a galvanometer $G$ (drawn as a circle with an upward-pointing needle). Below $C_2$, the leads form a loop that includes a battery symbol on the left and a tapping key $K$ (an open switch with a contact dot) on the right. The key $K$ is the only active element — it can be pressed (closed) or released (opened).

The Physical Idea

This figure illustrates Faraday’s Experiment 6.3, which shows that relative motion between a magnet and a coil is not necessary to induce an electric current. Instead, a changing magnetic field is the essential requirement.

When the key $K$ is pressed, current from the battery begins to flow through $C_2$, building up its magnetic field. This changing magnetic field (from zero to a steady value) passes through $C_1$, inducing a momentary current in $C_1$ — seen as a brief deflection of the galvanometer needle. Once the current in $C_2$ becomes steady, the magnetic field stops changing, and the galvanometer returns to zero.

When the key is released, the current in $C_2$ drops to zero, causing the magnetic field to decrease. This change again induces a momentary current in $C_1$, but in the opposite direction — the galvanometer needle deflects the other way.

The effect is dramatically enhanced when an iron rod is inserted along the axis of both coils, because iron increases the magnetic field strength and concentrates the flux.

Key Formula Developed from This Figure

The textbook uses this experiment to introduce Faraday’s law of induction. The induced electromotive force (emf) in coil $C_1$ is given by:

where:

• $\mathcal{E}$ is the induced emf (in volts) in coil $C_1$,
• $\Phi_B$ is the magnetic flux through coil $C_1$ (in webers),
• $\frac{d\Phi_B}{dt}$ is the rate of change of that flux with time, …