In view of this remarkable confirmation, Fresnel’s hypothesis was considered fairly established, so that the situation confronting science was the following: The ether was stagnant in that the earth moved through it without disturbing it in any way. Velocity through the ether had therefore a definite physical significance, and a real ether hurricane must be blowing over the earth’s surface in various directions according to the time of day. Experiments had failed to detect it, not because it did not exist, but on account of this compensating effect of the partial drag of the ether by dielectrics in motion. Such a failure, however, could only be regarded as temporary; for Fresnel’s hypothesis specifically showed that this compensating effect was but partial, and that experiments of a still more refined order ought certainly to detect the ether drift on the earth’s surface, revealing thereby the earth’s absolute velocity.
Obviously Fresnel’s hypothesis would fall if more refined experiments should also fail to detect the ether hurricane. Furthermore, even to the order of precision contemplated by Fresnel, if in our experiments we should appeal to phenomena in which dielectrics played no part, his showed that this compensating effect was but partial, and that experiment of Michelson and Morley was of this type. Not only was it far more precise than the ones hitherto attempted, but in addition refraction played no part in it. Yet, in spite of all, the Michelson and Morley experiment again gave a negative result.
The next great advance in our understanding of the problem was due to Lorentz. He, however, like all his predecessors, dared not take the great step, but still endeavoured to explain the negative results of experiment while holding to the view of a real stagnant ether. As might have been expected, Lorentz was compelled to appeal to new compensating influences. But before going farther it appears indispensable to devote a few pages to the general subject of electrodynamics; for as we proceed, we shall see that theoretical considerations are destined to play a part of ever-increasing importance.
CHAPTER XII
THE EQUATIONS OF ELECTROMAGNETICS AND LORENTZ’S THEORY
WHEN amber is rubbed, it develops the peculiar property of attracting small bodies, such as bits of paper, particles of dust and the like. This phenomenon, which was known to the Greeks, was described by saying that the amber had become electrified (electron meaning amber in Greek). To-day, however, we should say that it had received an electric charge. Soon it was found that electrified bodies did not always attract one another, but that in many instances they appeared to exert a repulsive action. For this reason it was assumed that there existed two different species of electricity, the positive and the negative; and the laws governing the phenomena involved were compressed into the statement that like charges repelled whereas opposite charges attracted. Similar conditions were found to endure between magnetic poles, so that the existence of two different types of magnetism was also assumed. But magnetism and electricity remained entirely distinct; no reciprocal action appeared to exist between an electric charge and a magnetic pole.
We may illustrate these phenomena of attraction and repulsion in a more concrete way by assuming that invisible fields of electric and magnetic forces surround electrified bodies and magnetic poles. To these fields we may ascribe the mechanical actions of attraction and repulsion detected by experiment; the electric fields act on electrified bodies, and the magnetic ones on magnetic poles. It was not until the early years of the nineteenth century that Coulomb submitted the mutual attractions and repulsions of charged bodies to a quantitative test. He found them to be expressed by a law (Coulomb’s law) which was of the inverse-square variety; the same as Newton’s law of gravitation, except that the electric charges took the place of masses, and that the actions could be either attractive or repulsive.
But with the invention of the electric cell by Volta a new manifestation of electricity presented itself for study and experiment, in the form of the electric current. Oersted discovered that an electric current was able to deflect a magnetized needle placed in its vicinity. This was of great significance, as showing that electricity and magnetism were closely allied phenomena. The nature of their relationship became better understood when, by placing iron filings round a wire, it was shown that an electric current was surrounded by a magnetic field. The deviation of the magnet in Oersted’s experiment was accordingly ascribed to the existence of the magnetic field surrounding the current. In the following year Ampère gave an exact quantitative formulation of the laws involved.
Now the fact that an electric current generated a magnetic field rendered it legitimate to suspect that conversely a varying magnetic field should generate an electric current. If this were the case, a magnet displaced near a closed wire should generate or induce an electric current in the wire. This most important phenomenon, known as electromagnetic induction, was discovered by Henry and Faraday, and as a result the connection between magnetism and electricity became still more pronounced. Our present-day dynamos and generators are nothing but machines constructed with a view to utilising this phenomenon of induction to generate electric currents on a commercial basis.
Faraday was the first scientist to realise the enormous importance of the electromagnetic field. He saw in it a reality of a new category differing from matter. It was capable of transmitting effects from place to place, and was not to be likened to a mere mathematical fiction such as the gravitational field was then assumed to be. In his opinion, the phenomena of electricity and magnetism should be approached via the field rather than via the charged bodies and currents. In other words, according to Faraday, when a current was flowing along a wire, the most important aspect of the phenomenon lay not in the current itself but in the fields of electric and magnetic force distributed throughout space in the current’s vicinity. It is this elevation of the field to a position of pre-eminence that is often called the pure physics of the field. Faraday was not a mathematician and was unable to co-ordinate the phenomena he foresaw in a mathematical way, and derive the full benefit from his ideas. Before dying, however, he entrusted this task to his colleague Maxwell; and one of the most astounding theories of science, eclipsed only in recent years by Einstein’s theory of relativity, was the outcome.
In order to appreciate the nature of Maxwell’s contributions, let us recall how matters stood in his day. If we confine-our attention to regions of space where no electric currents, no charged bodies or magnets are present, so that solely the electric and magnetic fields need be considered, the fundamental law of electromagnetism is Faraday’s law of induction. This law states that a variable magnetic field generates an electric field.