GROWTH OF THE ELECTRICAL THEORY OF MATTER Franklin.—In the year 1756 Franklin, upon contemplating the phenomenon of electrostatic induction, said with amazing insight, "The electric matter consists of particles extremely subtle since it can permeate common matter, even the densest, with such free dom and ease as not to receive any appreciable resistance." And yet, for fully a hundred years, electric particles were hardly again mentioned.
Faraday.—In 1833 Faraday led one and the same electrical current simultaneously through a solution of a silver salt and through an acid solution (i.e., a solution in which the positive ion was hydrogen) and found that the number of atoms of silver that came out of the one solution was exactly the same as the number of atoms of hydrogen that came out of the other, thus showing conclusively that one and the same quantity of electricity was associated in the electrolytic process with the atom of hydrogen and the atom of silver—a relation that holds for all univalent atoms. And yet Faraday himself, by his discovery of the effect of the medium about a conductor in changing the electrical forces emanating from it, was responsible for starting the period in which electrical phenomena were thought of almost exclusively in terms of stresses and strains in the medium surrounding the electrified body—a period in which Maxwell himself, one of the most outstanding intellects of the 19th century, and the follower and interpreter of Faraday and his work, said, "It is extremely improbable that when we come to understand the true nature of electrolysis we shall retain in any form the theory of molecular charges, for then we shall have secured a sure basis upon which to form a true theory of electrical currents and so become inde pendent of these provisional hypotheses." So that Faraday's experiments certainly did not convince the world of the general correctness of the atomic theory of electricity.
Weber.—Wilhelm Weber (Werke iv., 1871) built up his whole theory of electromagnetism on what was essentially an electron foundation. The hypothetical amperean currents, long before assumed to be circulating around molecules and thus producing the effects of magnetism, he explained as due to the rotation of light, positively charged, particles about a heavy negative nucleus. And yet this idea was not taken up again until the time of Lorentz, and it surely did not get into the consciousness of mankind at the time of Weber.
Crookes.—In 1879 Sir William Crookes, in view of his experi ments on cathode rays—in which he had both proved their ability to set wheels in rotation and to be deflected by a magnet, reached the definite conclusion that these rays consisted of a flying swarm of charged particles. Indeed, he said, "In studying this state of matter we seem to have at last in our grasp and obedient to our control little indivisible particles which with good warrant are supposed to constitute the physical basis of the universe." (Four nier d'Albe, Life of Sir William Crookes, 1924.) And yet his experiments were not convincing to the great body of German physicists who, as late as 1896, were asserting, in view of the great penetrating power which Lenard had shown the cathode rays to possess, that they were clearly of ethereal origin and not cor puscles at all.
Stoney.—In the year 1891, when Dr. G. Johnston Stoney (Scientific Transactions of the Royal Dublin Society, iv., 1891, I Ith series) introduced the word electron to designate the elemen tary electrical charge, and actually computed the value of the electron by a method which is quite comparable in accuracy with any that was used up to 1909. The following quotations bear remarkable testimony to his insight :—"Attention must be given to Faraday's Law of Electrolysis, which is equivalent to the state ment that in electrolysis a definite quantity of electricity, the same in all cases, passes for each chemical bond that is ruptured." The author called attention to this form of the law in a communication made to the British Association in 1874 and printed in the Scien tific Proceedings of the Royal Dublin Society, of February 188i and in the Philosophical Magazine for May 1881. It is there shown that the amount of this very remarkable quantity of electricity is about the twentiethet (that is ) of the usual electromagnetic unit of electricity, i.e., the unit of the Ohm series. This is the same as 3 eleventhets( Tali) of the smaller C.G.S. electrostatic unit of quantity. A charge of this amount is associated in the chemical atom with each bond. There may accordingly be several . such charges in one chemical atom, and there appear to be at least two in each atom. These charges, which it will be convenient to call "electrons," cannot be removed from the atom, but they become disguised when atoms chemically unite. If an electron be lodged at the point P of the molecule which undergoes the motion de scribed in the last chapter, the revolution of this charge will cause an electromagnetic undulation in the surrounding aether.
Also the following remarkable sentences are found in the same paper :—"Finally nature presents us with a single definite quan tity of electricity which is independent of the particular bodies acted on.... This definite quantity of electricity I shall call E,. If we make this our unit of electricity, we shall probably have made a very important step in our study of molecular phenomena. Hence we have very good reason to suppose that in V,, G, and E,, we have three of a series of systematic units that in an eminent sense are the units of nature, and stand in an intimate relation with the work which goes on in her mighty laboratory." It will be noticed from this quotation that the word "electron" was introduced to denote simply a definite elementary quantity of electricity without any reference to the mass or inertia which may be associated with it, and Prof. Stoney implies that every atom must contain at least two electrons, one positive and one negative, because otherwise it would be impossible that the atom as a whole be electrically neutral. As a matter of fact, the evi dence is now altogether convincing that the hydrogen atom does indeed contain just one positive and one negative electron. And yet, despite the clearness of Stoney's vision, the theory of the electrical constitution of matter certainly did not -come into general acceptance in his time.
J. J. Thomson.—In 1897 J. J. Thomson peoved the elec trostatic as well as the magnetic deflectability of cathode rays, and showed by this work that the mass of the particles consti tuting these rays was of the order of a thousandth of the mass of the hydrogen atom. Indeed, it was at this time, and largely as a result of J. J. Thomson's experiments in England and Len ard's similar experiments in Germany in 1898, combined with the nearly simultaneous discovery of the Zeeman effect by Zeeman and Lorentz in Holland (1897), that the theory of the electrical constitution of matter began to be fairly generally accepted, and it is because of this fact that it is customary and proper to fix the birth of the electron theory at about this time; and yet, as late as 1905 or 1906, as eminent an authority as Roentgen would have nothing to do with the electron theory because he regarded it as an unproven and highly speculative hypothesis. What was actually proved in these experiments of Thomson's and Lenard's was that cathode rays are corpuscular in nature, and that if the charge on these corpuscles be assumed to be invariable, and equal to that carried by a hydrogen ion in electrolysis—a natural and, as later experiments proved, a correct assumption—then the mass of each one of them is of the order of one-thousandth of the mass of the hydrogen atom. These experiments also proved that these same cathode rays come out of all kinds of substances, when these different substances are made cathodes, in the passage of a discharge through highly exhausted tubes.
Early Work on the Electron.—Even earlier than the work referred to in the preceding paragraph, a beginning toward the determination of the value of the electron, the unit charge, or the atom of electricity, had been made by G. Johnston Stoney in 1881. The next attempt was made by Townsend in 1897, a third by Sir J. J. Thomson in 1898, a fourth by H. A. Wilson in 1903 and a fifth by Millikan and Begeman in 1908. But all of these workers used methods which involved such uncertain assumptions that no precision of measurement was or could be claimed. Nor were the results themselves thus far of appreciably greater cer tainty than could be obtained from the Faraday constant (Ne= and kinetic theory estimates of the number of molecules N in a given molecule, which yielded a value of about 3X10 1° electrostatic units. Indeed, up to this point there was no method available that could yield anything more than the mean charge on a great number of particles, generally the particles of electrified clouds, and it is now known that the particles of such clouds carry in general widely varying charges, so that there was no certainty at all that this mean charge was the electron itself, and no direct proof that electrons were all exactly alike in charge. This proof was furnished, however, by the development of a technique that was first used in 1909, a technique with the aid of which the elec tron itself was first isolated and accurately measured, its value being soon fixed at 4.774X10-1° absolute electrostatic units of electricity. This means that the electron is so small a quantity of electricity that it takes 1, or about two thousand million of them to make up the very minute unit of electricity defined in the next paragraph.