Michael Faraday, His Life and Work

by Silvanus P. Thompson

It may here be apposite to discuss a fundamental question raised in these researches. In Faraday’s mind there arose the conviction of a connection between the induction of currents by magnets and the magnetic lines which invisibly fill all the space in the neighbourhood of the magnet. That relation he discovered and announced in the following terms:—

THE LAW OF INDUCTION.

“The relation which holds between the magnetic pole, the moving wire or metal, and the direction of the current evolved—i.e. the law which governs the evolution of electricity by magneto-electric induction, is very simple, though rather difficult to express. If in [Fig. 11], P N represent a horizontal wire passing by a marked [i.e. ‘north-seeking’] magnetic pole, so that the direction of its motion shall coincide with the curved line proceeding from below upwards; or if its motion parallel to itself be in a line tangential to the curved line, but in the general direction of the arrows; or if it pass the pole in other directions, but so as to cut the magnetic curves[39] in the same general direction, or on the same side as they would be cut by the wire if moving along the dotted curved line; then the current of electricity in the wire is from P to N. If it be carried in the reverse direction, the electric current will be from N to P. Or if the wire be in the vertical position, figured P´ N´, and it be carried in similar directions, coinciding with the dotted horizontal curve so far as to cut the magnetic curves on the same side with it, the current will be from P´ to N´.”

CUTTING THE MAGNETIC LINES.

When resuming the research in December, Faraday investigated the point whether it was essential or not that the moving wire should, in “cutting” the magnetic curves, pass into positions of greater or lesser magnetic force; or whether, always intersecting curves of equal magnetic intensity, the mere motion sufficed for the production of the current. He found the latter to be true. This notion of cutting the invisible magnetic lines as the essential act necessary and sufficient for induction was entirely original with Faraday. For long it proved a stumbling-block to the abstract mathematicians, since there was, in most cases, no direct or easy way in which to express the number of magnetic lines that were cut. Neither had any convention been adopted up to that time as to how to reckon numerically the number of magnetic lines in any given space near a magnet. Later, in 1851, Faraday himself gave greater precision to these ideas. He found that the current was proportional to the velocity, when the conductor was moving in a uniform magnetic field with a uniform motion. Also, that the quantity of electricity thrown by induction into the circuit was directly proportional to the “amount of curves intersected.” The following passage, from Clerk Maxwell’s article on Faraday in the “Encyclopædia Britannica,” admirably sums up the matter:—

The magnitude and originality of Faraday’s achievement may be estimated by tracing the subsequent history of his discovery. As might be expected, it was at once made the subject of investigation by the whole scientific world, but some of the most experienced physicists were unable to avoid mistakes in stating, in what they conceived to be more scientific language than Faraday’s, the phenomena before them. Up to the present time the mathematicians who have rejected Faraday’s method of stating his law as unworthy of the precision of their science, have never succeeded in devising any essentially different formula which shall fully express the phenomena without introducing hypotheses about the mutual action of things which have no physical existence, such as elements of currents which flow out of nothing, then along a wire, and finally sink into nothing again.

After nearly half a century of labour of this kind, we may say that, though the practical applications of Faraday’s discovery have increased and are increasing in number and value every year, no exception to the statement of these laws as given by Faraday has been discovered, no new law has been added to them, and Faraday’s original statement remains to this day the only one which asserts no more than can be verified by experiment, and the only one by which the theory of the phenomena can be expressed in a manner which is exactly and numerically accurate, and at the same time within the range of elementary methods of exposition.

In the year 1831, which witnessed this masterpiece of scientific research, Faraday was busy in many other ways. He was still undertaking chemical analyses and expert work for fees, as witness his letter to Phillips on [p. 62]. He was also, until November, on the Council of the Royal Society. To the “Philosophical Transactions” he contributed a paper “On Vibrating Surfaces,” in which he solved a problem in acoustics which had previously gone without explanation. It had long been known that in the experiments of obtaining the patterns called “Chladni’s figures,” by strewing powders upon vibrating plates, while the heavier powders, such as sand, moved into the nodal lines, lighter substances, such as lycopodium dust, collected in little circular heaps over the parts where the vibration was most energetic. Faraday’s explanation was that these lighter powders were caught and whirled about in little vortices which formed themselves at spots where the motions were of greatest amplitude.

He also wrote a paper “On a Peculiar Class of Optical Deceptions,” dealing with the illusions that result from the eye being shown in successive glimpses, as between the teeth of a revolving wheel, different views of a moving body. This research was, in effect, the starting point of a whole line of optical toys, beginning with the phenakistiscope or stroboscope, which developed through the zoetrope and praxino-scope into the kinematograph and animatograph of recent date.

LECTURES ON PHYSICAL SUBJECTS.