Michael Faraday's Magnetic Fields

Towards the end of 1812, Michael Faraday was a 21-year old bookbinder’s journeyman. He wrote to the great chemist Davy asking to obtain employment in a scientific laboratory to escape from trade.

In the letter, he enclosed a neatly written copy of notes which he had made from Davy’s own public lectures.

Davy arranged an interview and learned that Faraday educated himself by reading 2 books that he got for binding:

1. The Encyclopaedia Britannica

From this, he got his first notions of electricity

2. Mrs. Marcet’s Conversations on Chemistry

This gave me my foundation in Chemistry.

Before applying to Davy, he had already performed a number of chemical experiments. He had made for himself a voltaic pile which he used to decompose several compound bodies.

In the following spring, at Davy’s recommendation, Faraday was appointed to a post in the laboratory of the Royal Institution. It had been established at the end of the 18th century under Count Rumford.

Here, he remained during his active life.

• He first was an assistant
• Then he became the laboratory director
• From 1833, he was the chair of chemistry which was founded for his benefit.

For many years Faraday was directly under Davy’s influence, and was occupied chiefly in chemical investigations.

But in 1821, when the new field of inquiry opened by Oersted’s discovery was attracting attention, he wrote an Historical Sketch of Electro-Magnetism,[1] as a preparation for which he carefully repeated the experiments described by the writers he was reviewing.

This seems to have been the beginning of the researches to which his fame is chiefly due.

The memoir which stands first in the published volumes of Faraday’s electrical work[2] was communicated to the Royal Society on November 24th, 1831.

The investigation was inspired, as he tells us, by the hope of discovering analogies between the behaviour of electricity as observed in motion in currents, and the behaviour of electricity at rest on conductors.

Static electricity possessed the power of “induction”. It caused an opposite electrical state on bodies in its neighbourhood.

Could electric currents show a similar property?

If in any circuit a current were made to flow, then any adjacent circuit would be traversed by an induced current, which would persist exactly as long as the inducing current.

Faraday found that this was not the case.

A current was induced. But it lasted only for an instant, being in fact perceived only when the primary current was started or stopped. It depended, as he soon convinced himself, not on the mere existence of the inducing current, but on its variation.

Faraday tried to determine the laws of induction of currents. For this, he devised a new way of representing the state of a magnetic field.

Philosophers had been long illustrated magnetism by strewing iron filings on a sheet of paper, and observing the resulting curves from a magnet underneath.

These curves suggested to Faraday[4] the idea of lines of magnetic force, or curves whose direction at every point coincides with the direction of the magnetic intensity at that point.

The curves in which the iron filings arrange themselves on the paper resemble these curves so far is possible subject to the condition of not leaving the plane of the paper.

Faraday conceived all space to be filled with these lines of magnetic force.

Every line of force is a closed curve, which in some part of its course passes through the magnet to which it belongs.[5] Hence if any small closed curve be taken in space, the lines of force which intersect this curve must form a tubular surface returning into itself, such a surface is called a tube of force.

From a tube of force we may derive information not only regarding the direction of the magnetic intensity, but also regarding its magnitude; for the product of this magnitude[6] and the cross-section of any tube is constant along the entire length of the tube.[7]

From this result, Faraday conceived the idea of partitioning all space into compartments by tubes.

Each tube being such that this product has the same definite value.

For simplicity, each of these tubes may be called a “unit line of force”; the strength of the field is then indicated by the separation or concentration of the unit lines of force,[8] so that the number of them which intersect a unit area placed at right angles to their direction at any point measures the intensity of the magnetic field at that point.

Faraday constantly thought in terms of lines of force writing in 1851:

Faraday found that a current is induced in a circuit either when the strength of an adjacent current is altered, or when a magnet is brought near to the circuit, or when the circuit itself is moved about in presence of another current or a magnet.

He saw from the first[11] that in all cases the induction depends on the relative motion of the circuit and the lines of magnetic force in its vicinity.

The precise nature of this dependence was the subject of long-continued further experiments.

In 1832, he found[12] that the currents produced by induction under the same circumstances in different wires are proportional to the conducting powers of the wires. This showed that the induction consists in the production of a definite electromotive force.

It was independent of the nature of the wire, and dependent only on the intersections of the wire and the magnetic curves.

This electromotive force is produced whether the wire forms a closed circuit (so that a current flows) or is open (so that electric tension results).

How does the electromotive force depend on the relative motion of the wire and the lines of force?

The answer to this inquiry is, in Faraday’s own words,[13] that "

The induced electromotive force is, in fact, simply proportional to the number of the unit lines of magnetic force intersected by the wire per second.

This is the fundamental principle of the induction of currents.

Faraday is undoubtedly entitled to the full honour of its discovery.

Many years elapsed before all the conceptions involved in Faraday’s principle became clear and familiar to his contemporaries. In the meantime, the problem of formulating the laws of induced currents was approached with success from other points of view.

There were many obstacles to the direct appropriation of Faraday’s work by the mathematical physicists of his own generation.

He himself was not a mathematician. He was unable to address them in their own language.

His favourite mode of representation by moving lines of force repelled analysts who had been trained in the school of Laplace and Poisson.

Moreover, the idea of electromotive force itself, which had been applied to currents a few years previously in Ohm’s memoir, was, as we have seen, still involved in obscurity and misapprehension.