Experiments of Faraday and Henry

For a long time, electricity and magnetism were thought to be separate and unrelated phenomena. Experiments on electric current by Oersted, Ampere and a few others in the early decades of the nineteenth century established the fact that electricity and magnetism are inter-related. They discovered that moving electric charges generate magnetic fields. An electric current, for example, deflects a magnetic compass needle in its vicinity. The question comes, whether the opposite effect will be possible or not? Can moving magnets generate electric currents? Is such a relationship between electricity and magnetism permitted by nature? The answer is a resounding yes! The experiments performed by Michael Faraday in England and Joseph Henry in the United States demonstrated conclusively that electric currents were induced in closed coils when subjected to changing magnetic fields. 

The phenomenon of electromagnetic induction is of practical utility not only for theoretical or academic interest. Consider a world without electricity – no electric lights, no trains, no phones, and no personal computers. The experiments of Faraday and Henry helped to the development of modern generators and transformers.

Experiments of Faraday and Henry

Faraday and Henry carried out a long series of experiments that led to the discovery and comprehension of electromagnetic induction. We will now go over some of these experiments.

Experiment – 1

A coil C is connected to a galvanometer G in the diagram below. When the North pole of a bar magnet is pushed towards the coil, the galvanometer’s pointer deflects, indicating the presence of electric current in the coil. The deflection is permanent as long as the bar magnet is moving. When the magnet is not moved that is held stationary, then there is no deflection in the galvanometer. The galvanometer deflects in the opposite direction when the magnet is pushed away from the coil, showing that the current has altered.

Furthermore, when the bar magnet’s South-pole is moved towards or away from the coil, the deflections in the galvanometer are the polar opposite of those seen with the North-pole for similar movements. Furthermore, when the magnet is pushed towards or pulled away from the coil faster, the deflection (and thus current) is found to be greater. Instead, when the bar magnet is held stationary and coil C is moved towards or away from the magnet, the same effects are observed. It demonstrates that the relative motion of the magnet and the coil is responsible for the generation (induction) of electric current in the coil.

Experiment – 2

The bar magnet in the below figure is replaced by a second coil C₂ which is connected with a battery. The constant current in coil C₂ generates a constant magnetic field. The galvanometer deflects as coil C₂ is moved towards coil C₁. This indicates that in coil C₁, an electric current is being induced. The galvanometer deflects again when C₂ has moved away but in the opposite direction. The deflection will last as long as coil C₂ is moving. The same effects are observed when coil C₂ is held fixed and coil C₁ is moved. Again, it is the relative motion of the coils that causes the electric current to flow.

Experiment – 3

The previous two experiments involved relative motion between a magnet and a coil, as well as between two coils. Faraday demonstrated in another experiment that relative motion is not an absolute requirement. The figure depicts two coils, C₁ and C₂, that are held stationary. Coil C₁ is connected to the G is galvanometer and coil C₂ is connected with the battery along with a tapping key K.

When the tapping key K is pressed, the galvanometer exhibits a momentary deflection. The galvanometer’s pointer immediately returns to zero. There is no deflection in the galvanometer if the key is held down continuously. When the key is released, a brief deflection is observed, but this time in the opposite direction. When an iron rod is inserted into the coils along their axis, the deflection increases dramatically.

Sample Questions

Question 1: What would you do to get a large galvanometer deflection?

Answer:

One or more of the following steps can be taken to obtain a large deflection-

1. Insert a soft iron rod into the coil C₂,
2. We can connect powerful battery in coil, and
3. By moving faster the arrangement towards the test coil C₁.

Question 2: In the absence of a galvanometer, how would you demonstrate the presence of an induced current?

Answer:

Replace the galvanometer with a small bulb, and the relative motion of the two coils will cause the bulb to glow, indicating the presence of an induced current.

Question 3: In experiment 3 what happens when the tapping key is pressed? 

Answer:

The galvanometer exhibits a momentary deflection and immediately returns to zero.

Question 4: What happens when a magnet is rapidly brought close to a coil?

Answer:

The induced current in the coil will be greater because the number of magnetic fields passing through the coil will change more rapidly, resulting in a greater induced current.

Question 5: What happens if a coil and a magnet move in the same direction and same speed?

Answer:

When the coil and a magnet are moved in the same direction and at the same speed, the magnetic field across the coil does not change, and thus no electric current is induced in the coil.