Vacuum Tube Phenomena.
Fig. 16.—Vacuum tube with cathode rays and a shadow-producing cross.
P and N, conducting wires for the electric current;
a, cathode; b, anode and shadow-producer; c, d, the shadow.
Fig. 17.—Vacuum tube, where a bundle of
cathode rays are deviated by electric forces.
A, anode; K, cathode.
It has previously been said that air is an insulator for electricity, a statement which is, in general, true; however, as has also been said, electric sparks and arcs can pass through air. Moreover, it has been discovered that exhausted air is a very good conductor, so that a strong current can pass between two metal electrodes in a glass tube where the air is exhausted, if the electrodes are connected to an outer conductor by metal wires fused into the glass. In these vacuum tubes there are produced remarkable light effects, at first inexplicable. When the air is very much exhausted, to a hundred thousandth of the atmospheric pressure or less, strong electric forces (large difference of potential between the electrodes) are needed to produce an electric discharge. Such a discharge assumes an entirely new character; in the interior of the glass tube there is hardly any light to be seen, but the glass wall opposite the negative electrode (the cathode) glows with a greenish tint (fluorescence). If a small metal plate is put in the tube between the cathode and the glass wall, a shadow is cast on the wall, just as if light were produced by rays, emitted from the cathode at right angles to its surface (c[f. Fig. 16]). The English physicist, Crookes, was one of the first to study these cathode rays. He assumed that they are not ether waves like the light rays, but that they consist of particles which are hurled from the cathode with great velocity in straight lines; they light the wall by their collisions with it. There was soon no doubt as to the correctness of Crookes’ theory. The cathode rays are evidently particles of negative electricity, which by repulsions are driven from the cathode (the negative electrode). A metal plate bombarded by the rays becomes charged negatively. Let us suppose that we have a small bundle of cathode rays, obtained by passing the rays from the cathode K ([cf. Fig. 17]) through two narrow openings S₁ and S. It can then be shown that the bundle of rays is deviated not only by electric forces, but also by magnetic action from a magnet which is held near the glass. In the figure there is shown a deviation of the kind mentioned, caused by making the plates at B and C respectively positive and negative; since B attracts the negative particles and C repels them, the light spot produced by the bundle of rays is moved from M to M₁. The magnetic deviation is in agreement with Ørsted’s rules for the reciprocal actions between currents and magnets, if we consider the bundle of rays produced by moving electric particles as an electric current. (Since the electric particles travelling in the direction of the rays are negative, and since it is customary by the expression “direction of current” to understand the direction opposite to that in which the negative electricity moves, then, in the case of the cathode rays just mentioned, the direction of the current must be opposite to that of the rays.)
From measurements of the magnetic and electric deviations it is possible to find not only the velocity of the particles, but also the ratio e/m between the charge e of the particle and its mass m. The velocity varies with the potential at the cathode, and may be very great, 50,000 km. per second, for instance (about one-sixth the speed of light), or more. It has been found that e/m always has the same value, regardless of the metal of the cathode and of the gas in the tube; this means that the particles are not atoms of the elements, but something quite new. It has also been found that e/m is about two thousand times as large as the ratio between the charge and the mass of the hydrogen atom in electrolysis. If we now assume that e is just the elementary quantum of electricity 4·77 × 10¹⁰, which in magnitude amounts to the charge of the hydrogen atom in electrolysis (but is negative), then m must have about ¹/₂₀₀₀ the mass of the hydrogen atom. This assumption as to the size of e has been justified by experiments of more direct nature. The experiments with charge and mass of electrons which have in particular been carried out by the English physicist, J. J. Thomson, give reason then to suppose these quite new and unknown particles to be free atoms of negative electricity; they have been given the name of electrons. Gradually more information about them has been acquired. Thus it has been possible in various ways to determine directly the charge on the electron, independently of its mass. Special mention must be made of the brilliant investigations of the American, Millikan, on the motion of very small electrified oil-drops through air under the influence of an electric force. To Millikan is due the above-mentioned value of e, which is accurate to one part in five hundred. Further, the mass of the electron has been more exactly calculated as about ¹/₁₈₃₅ that of the hydrogen atom. Their magnitude has also been learned; the radius of the electron is estimated as 1·5 × 10⁻¹³ cm. or 1·5 × 10⁻⁶ μμ, an order of magnitude one ten-thousandth that of the molecule or atom.
After the atom of negative electricity had been isolated, in the form of cathode rays, the next suggestion was that corresponding positive electric particles might be discharged from the anode in a vacuum tube. By special methods success has been attained in showing and studying rays of positive particles. In order to separate them from the negative cathode ray particles the German scientist, Goldstein, let the positive particles pass through canals in the cathode; they are therefore called canal rays. The velocity of the particles is much less than that of the cathode rays, and the ratio e/m between charge and mass is much smaller and varies according to the gas in the tube. In experiments where the tube contains hydrogen, rays are always found for which e/m, as in electrolysis, is about ¹/₂₀₀₀ of the ratio in the cathode rays. Therefore there can be scarcely any doubt that these canal rays are made up of charged hydrogen atoms or hydrogen ions. The values found with other gases indicate that the particles are atoms (or molecules sometimes) of the elements in question, with charges one or more times the elementary quantum of electricity (4·77 × 10⁻¹⁰ electrostatic units). Research in this field has also been due in particular to J. J. Thomson. From his results, as well as from those obtained by other methods, it follows that positive electricity, unlike negative, cannot appear of its own accord, but is inextricably connected to the atoms of the elements.