Electromagnetism

A curious question which arose out of Faraday’s theory was whether a bar-magnet which is rotated on its own axis carries its lines of magnetic force in rotation with it.

Faraday believed that the lines of force do not rotate[15]: on this view a revolving magnet like the earth is to be regarded as moving through its own lines of force, so that it must become charged at the equator and poles with electricity of opposite signs, and if a wire not partaking in the earth’s rotation were to have sliding contact with the earth at a pole and at the equator, a current would steadily flow through it.

Experiments confirmatory of these views were made by Faraday himself;[16] but they do not strictly prove his hypothesis that the lines of force remain at rest; for it is easily seen[17] that, if they were to rotate, that part of the electromotive force which would be produced by their rotation would be derivable from a potential, and so would produce no effect in closed circuits such as Faraday used.

Three years after the commencement of Faraday’s researches on induced currents he was led to an important extension of them by an observation which was communicated to him by another worker.

William Jenkin had noticed that an electric shock may be obtained with no more powerful source of electricity than a single cell, provided the wire through which the current passes is long and coiled; the shock being felt when contact is broken.[18]

Jenkin did not choose to investigate the matter further. So Faraday took it up, and showed[19] that the powerful momentary current, which was observed when the circuit was interrupted, was really an induced current governed by the same laws as all other induced currents, but with this peculiarity, that the induced and inducing currents now flowed in the same circuit. In fact, the current in its steady state establishes in the surrounding region a magnetic field, whose lines of force are linked with the circuit; and the removal of these lines of force when the circuit is broken originates an induced current, which greatly reinforces the primary current just before its final extinction. To this phenomenon the name of self-induction has been given.

The circumstances attending the discovery of self-induction occasioned a comment from Faraday on the number of suggestions which were continually being laid before him. He remarked that although at different times a large number of authors had presented him with their ideas, this case of Jenkin was the only one in which any result had followed. “The volunteers are serious embarrassments generally to the experienced philosopher."[20]

The discoveries of Oersted, Ampère, and Faraday had shown the close connexion of magnetic with electric science.

But the connexion of the different branches of electric science with each other was still not altogether clear. Although Wollaston’s experiments of 1801 had in effect proved the identity in kind of the currents derived from frictional and voltaic sources, the question was still regarded as open thirty years afterwards,[21] no satisfactory explanation being forthcoming of the fact that frictional electricity appeared to be a surface-phenomenon, whereas voltaic electricity was conducted within the interior substance of bodies. To this question Faraday now applied himself; and in 1833 he succeeded[22] in showing that every known effect of electricity-physiological, magnetic, luminous, calorific, chemical, and mechanical-may be obtained indifferently either with the electricity which is obtained by friction or with that obtained from a voltaic battery. Henceforth the identity of the two was beyond dispute.

Some misapprehension, however, has existed among later writers as to the conclusions which may be drawn from this identification.

What Faraday proved is that the process which goes on in a wire connecting the terminals of a voltaic cell is of the same nature as the process which for a short time goes on in a wire by which a condenser is discharged. He did not prove, and did not profess to have proved, that this process consists in the actual movement of a quasi-substance, electricity, from one plate of the condenser to the other, or of two quasi-substances, the resinous and vitreous electricities, in opposite directions. The process had been pictured in this way by many of his predecessors, notably by Volta; and it has since been so pictured by most of his successors: but from such assumptions Faraday himself carefully abstained.

What is common to all theories, and is universally conceded, is that the rate of increase in the total quantity of electrostatic charge within any volume-element is equal to the excess of the influx over the efflux of current from it. This statement may be represented by the equation

where ρ denotes the volume-density of electrostatic charge, and i the current, at the place (x, y, z) at the time t. Volta’s assumption is really one way of interpreting this equation physically: it presents itself when we compare equation (1) with the equation

which is the equation of continuity for a fluid of density ρ and velocity v: we may identify the two equations by supposing i to be of the same physical nature as the product ρv; and this is precisely what is done by those who accept Volta’s assumption.

But other assumptions might be made which would equally well furnish physical interpretations to equation (1). For instance, if we suppose ρ to be the convergence of any vector of which i is the time-flux,[23] equation (1) is satisfied automatically; 196 Faraday. we can picture this vector as being of the nature of a displacement. By such an assumption we should avoid altogether the necessity for regarding the conduction-current as an actual flow of electric charges, or for speculating whether the drifting charges are positive or negative; and there would be no longer anything surprising in the production of a null effect by the coalescence of electric charges of opposite signs.

Faraday himself wished to leave the matter open, and to avoid any definite assumption.[24] Perhaps the best indication of his views is afforded by a laboratory note[25] of date 1837:—

“After much consideration of the manner in which the electric forces are arranged in the various phenomena generally I have come to certain conclusions which I will endeavour to note down without committing myself to any opinion as to the cause of electricity, i.e., as to the nature of the power. If electricity exist independently of matter, then I think that the hypothesis of one fluid will not stand against that of two fluids. There are, I think, evidently what I may call two elements of power, of equal force and acting toward each other. But these powers may be distinguished only by direction, and may be no more separate than the north and south forces in the elements of a magnetic needle. They may be the polar points of the forces originally placed in the particles of matter.”

Ever since the rise of the mathematical theory of electrostatics, the controversy between the supporters of the one-fluid and the two-fluid theories had become manifestly barren.

The interest was now largely centred on analytical equations. These could be interpreted equally well on either hypothesis.

There was little prospect of discriminating between them by any new experimental discovery.

But a problem does not lose its fascination because it appears insoluble. “I said once to Faraday,” wrote Stokes to his father-in-law in 1879, “as I sat beside him at a British Association dinner, that I thought a great step would be made when we should be able to say of electricity that which we say of light, in saying that it consists of undulations. He said to me he thought we were a long way off that yet."[26]

For his next series of researches,[27] Faraday reverted to subjects which had been among the first to attract him as an apprentice attending Davy’s lectures: the voltaic pile, and the relations of electricity to chemistry.

It was at this time generally supposed that the decomposition of a solution, through which an electric current is passed, is due primarily to attractive and repellent forces exercised on its molecules by the metallic terminals at which the current enters and leaves the solution. Such forces had been assumed both in the hypothesis of Grothuss and Davy, and in the rival hypothesis of De La Rive;[28] the chief difference between these Being that whereas Grothuss and Davy supposed a chain of decompositions and recompositions in the liquid, De La Rive supposed the molecules adjacent to the terminals to be the only ones decomposed, and attributed to their fragments the power of travelling through the liquid from one terminal to the other.

To test this doctrine of the influence of terminals, Faraday moistened a piece of paper in a saline solution, and supported it in the air on wax, so as to occupy part of the interval between two needle-points which were connected with an electric machine. When the machine was worked, the current was conveyed between the needle-points by way of the moistened paper and the two air-intervals on either side of it; and under these circumstances it was found that the salt underwent decomposition. Since in this case no metallic terminals of any kind were in contact with the solution, it was evident that all hypotheses which attributed decomposition to the action of the terminals were untenable.