Roentgen Rays and Phenomena of the Anode and Cathode.

by Edward P. Thompson

57. Cathode Rays Rectilinear. Radiate Normally From the Surface of the Cathode. In his lecture he had, side by side, two bulbs, one, in which the vacuum was of such a degree, that a blue stream of light existed between the negative pole and positive pole, [§ 54], at centre. It is evident that the vacuum in this bulb was not very high. Fig. 9, p. [17], shows a stream extending from the negative to the positive pole, Fig. 10, p. [17], is the same kind of a tube only the vacuum is about 1 to 2 m. In other words, the vacuum in the latter was just so high that a discharge took place, and instead of the luminous effect being like that with a low vacuum, there was a patch of green light directly opposite the concave negative pole. The radiations from this pole were rectilinear, crossing each other at a focus within the bulb and producing upon the glass a phosphorescent spot. It should be remembered that the word radiations is used as a mere matter of convenience. Directly opposite the concave cathode, there was a green patch of light on the inner surface of the glass. It was shown that it made no difference where the anode was. This fact becomes useful in carrying on experiments in connection with Roentgen rays, and it may have a great deal to do with the solution of the theoretical problems in connection with electrical discharges in vacuum tubes. In regard to the three streams shown in Fig. 9, p. [17], it may be stated that only one occurred at a time in the experiment, for, first one anode was connected in circuit, and then the next by itself, and then the third one by itself, while the concave pole was always negative. Each time the anode was changed, the stream changed, and connected that pole which was in circuit, [§ 43], but similar changes made upon the tube with a high vacuum, did not alter the position of the phosphorescent spot. This and other experiments show that the radiations took place perpendicularly from the surface of the cathode.

58. Shadow Cast Within the Discharge Tube. This is illustrated in Fig. 11, p. [17], where there is a negative polar disk at the small end of the egg shaped tube, and a cross near the large end, the same forming the positive pole. The cross is made of aluminum. There was a novel action, however, discovered in addition to the mere casting of a shadow. The glass which had become phosphorescent except within the shadow, became after a while, less phosphorescent. Its property to phosphoresce became less as proved by removing the cross, which was arranged to fall down upon tipping the bulb. Immediately, the part which was within the shadow became brighter than the rest of the glass, thereby reversing the appearances, by making a luminous picture of the cross upon only partially phosphorescent glass. A remarkable feature is that the glass never recovered its first exhibited power of phosphorescence, neither did this power entirely become nothing, however many times the tube was employed. Was the deposit of metal from the cathode the cause?

58a. Mechanical Motion Produced by Radiations from the Negative Pole. It occured to Crookes that the radiations from the cathode might perhaps cause a wheel to turn around. He therefore had a minute wheel made by Mr. Gimingham, like an undershot water wheel, and its axle rested on two rails of glass, so that it might roll along from one end of the tube to the other. The vanes were exactly opposite to the plane surface of the cathode. The molecular stream or radiations, or whatever they may be, possibly vibrations, from the cathode, were so powerful mechanically that the wheel was caused to run up hill, the tube being inclined very slightly. On the principle that action and reaction are equal, he built another device in which the negative electrode was movable, and he observed that when the current was on, the negative electrode moved slightly. Upon these principles he built the well-known Crookes radiometer in which the vanes rotated by reaction of the radiations. The vanes in this form of radiometer were made of aluminum, and a cup of hard steel served as the bearing, Fig. 12, p. [17]. One side of each disk was coated with a thin scale of mica. The aluminum disks formed the cathode, while the anode was located at the top. The operation consisted in connecting the terminals as stated, so that the vanes were the negative poles and it was observed that the little wheel rotated. The vacuum was not as high as that for obtaining phosphorescence. With a low vacuum, an envelope of violet light existed near the surface of the aluminum vanes. Effects were carefully studied by maintaining connection with the pump. At the pressure of .5 mm. there was a dark cylinder opposite the aluminum extending to the glass, and this was the pressure at which the vanes began to rotate. The dark spaces opposite each vane became larger and larger in width, until they appeared to be opposed or resisted by the inner surface of the glass, and then the rotation became very rapid. He modified this experiment by having vanes entirely of mica, and by having the cathode disconnected electrically from the vanes, Fig. 13, p. [17]. A coil of metal near the vanes served as the cathode. The anode was at a distance in the top of the tube as in Fig. 12, p. [17]. During the electrical discharge, the wheels rotated by radiations from the coil which formed the cathode. He made the discovery that when this coil was heated red hot conveniently by a current from a primary battery, the vanes also rotated, showing that there is probably some relation between the radiations from the cathode and heat rays. The fact remains however, that both kinds of rays produced rotation, directly or indirectly.

59. Action of Magnet Upon Cathode Rays.—He had two tubes, one of which is shown in Fig. 14 and the other in Fig. 15, on page [17]. In the former, the vacuum was so low that a violet stream of light existed between the electrodes. In the other, the rays were invisible, but were converted into luminosity by projection at an exceedingly slight angle, upon a phosphorescent screen arranged along the length of the tube and inside thereof. Inasmuch as the whole surface of the cathode in the latter case radiated parallel and invisible rays, he cut off some of them by a mica screen having a hole in the centre and located near the negative pole, so that only a pencil of invisible rays could go through the mica screen and act upon the phosphorescent screen. In both cases, there was visible a straight pencil of light. Now notice the effect which took place upon locating a magnet as indicated in the figures. With the low vacuum, the pencil was bent out of its course but returned again to the line of its original path. [§ 28]. With the high vacuum, the rays were bent but did not return to their original direction nor parallel thereto. In the former case, the magnet acted as upon a very delicate flexible conductor, while in the latter, it acted, as Crookes said, like the earth upon projectiles. He modified the latter experiment in order to determine if the similarity between this phenomenon and gravitation existed in other respects. He anticipated that if the molecular resistance to the rays were increased they would be bent more out of their course like a horizontally projected bullet. He therefore heated the caustic potash sticks slightly, and in view of the liberation of molecules of water within the vacuum tube, the rays, he thought, would be resisted; and such was the case to all appearances, for then the pencil of light was bent out of its course to a greater extent, although the magnetic power remained the same as well as the E. M. F. producing the electric discharge. He therefore established, apparently, the principle that the magnetic actions upon cathode rays vary somewhat in their nature according to the degree of vacuum. In either case, it may be stated incidentally, that when the magnet was moved to and fro, the pencils of light waved back and forth.

In the modified form of construction over that shown in Fig. 15, p. [17], he caused a wheel to rotate that was located in the high vacuum. The vanes of the wheel were so located that the faces of the same were perpendicular to the direction of the pencil of the rays radiating from the cathode. When the magnet deflected the rays, the wheel ceased rotation.

60. Mutual Repulsion of Cathode Rays.—If the little mica screen, as shown in Fig. 16, p. [17] has two holes, and if there are two cathodes instead of one, there will also be two pencils of light. He performed an experiment involving the latter modification, and the result was something that could not have been predicted. The two pencils, as displayed by the long fluorescent screen, repelled each other like molecules similarly electrified. The white pencils, it will be noticed, were repelled from each other and showed their condition when both of the negative poles were in circuit. The black pencils show the location of both of the pencils when only one pole is in circuit at a time, the direction being perpendicular to the plane of the cathode disc ([§ 57]) at end.

61. Heating and Lighting Power of Cathode Rays. Heat of Phosphorescent Spot.—By making the cathode concave as in Fig. 10, p. [17], and so locating it that the focus of the cathode rays falls upon some substance, the latter becomes very hot. In this way Crookes melted wax on the outside of the bulb at the phosphorescent spot. Further than this, the heat was so great that it cracked the glass without at first injuring the vacuum; next the glass at this point softened, and the air, by its pressure, rushed into the bulb, forcing a hole through the soft part. He performed an experiment also which illustrated the intensity of the heat when the rays were brought to a focus. He used an unusually large electrode like a concave mirror, and in the focus, which was near the centre of the bulb, he supported a small piece of iridio-platinum. At first, with a moderately low E. M. F., the metal was made white hot. When a magnet was caused to approach, the rays were drawn to one side, [§ 59], and the little piece of metal cooled. He then put in all the coils of an inductorium, and allowed the metal not only to become white hot, but to become so heated that it melted. How little did Prof. Crookes know about the most important phenomena associated with his experiment. Although he was so exceedingly enthusiastic and ingenious in planning his experiments, and in reasoning, yet it seems almost mysterious that he should have been subjected to what have become known as X-rays, which passed into his body, and would have photographed portions of his skeleton, and which would have performed outside of the tube many of the acts that were noticed within. Seventeen years elapsed between the time of Crookes on the one hand, and Lenard and Roentgen’s discoveries on the other. Dr. Lodge, F.R.S., (The Elect., Lon., Jan. 31, ’96, p. 438,) and Lenard, in his first paper, attributed to Hittorf the discovery of the mere existence of cathode rays, but credited to Crookes the full establishment of their properties, deduction of their principles and formulation of an ingenious theory.

61a As an appropriate conclusion to Crookes’ work, I cannot do better than to let Lord Kelvin repeat what he said in his Pres. Addr., Ro. So., Nov. ’93, see also The Elect., Lon. Feb. 14, ’96, p. 522, showing that a small portion of the credit is due not only to Hittorf, [§ 53], but to Varley. “His short paper of 1871, which, strange to say has lain almost or quite unperceived in the Proceedings during the 22 years since its publication, contains an important first instalment of discovery in a new field, the molecular torrent [§ 53], at centre, from the ‘negative pole,’ the control of its course by a magnet, [§ 59], its pressure against either end of a pivoted vane of mica, [§ 59], at end, and the shadow produced by its interception by a mica screen, [§ 58]. Quite independently of Varley, and not knowing what he had done, Crookes (Roy. Inst. Proc., April 4, ’79, vol. LX, p. 138. Ro. So. Trans., ’74, “On attractions and repulsions resulting from radiation” Part II, ’76, parts III and IV, ’76, part V, ’78, part VI, ’79) was led to the same primary discovery, not by accident and not merely by experimental skill and acuteness of observation.” * * * * “He brought all his work more and more into touch with the kinetic theory of gases; so much so, that when he discovered the molecular torrent he immediately gave it its true explanation—molecules of residual air, or gas or vapor projected at great velocities (probably, I believe not greater in any case than 2 or 3 kilometers per second, [§ 61b]), by electric repulsion from the negative electrode. This explanation has been repeatedly and strenuously attacked by many other able investigators, but Crookes has defended (Presidential address to the Inst. Elect. Eng., 1891.) it, and thoroughly established it by what I believe is irrefragable evidence of experiment. Skillful investigations perseveringly continued brought out more and more wonderful and valuable results; the non-importance of the position of the positive electrode, [§ 57], near end, the projection of the torrent perpendicularly from the surface of the negative electrode, [§ 57], at end; its convergence into a focus and divergence thenceforward when the surface is slightly concave, [§ 47], near beginning; the slight but perceptible repulsion, [§ 60], between two parallel torrents due, according to Crookes, to negative electrifications of their constituent molecules; the change of the direction of the molecular torrent by a neighboring magnet, [§ 59]. the tremendous heating effect of the torrent from a concave electrode when glass, metal or any ponderable substance is placed in the focus, [§ 61]. the phosphorescence procured on a plate coated with sensitive paint by a molecular torrent skirting along it, Fig. 15, p. [17]; the brilliant colors—turquoise blue, emerald, orange, ruby-red—with which grey, colorless objects, and clear, colorless crystals glow on their struck faces when lying separately or piled up in a heap in the course of a molecular torrent, [§ 55]. “electrical evaporation” of negatively electrified liquids and solids, [§ 59]. (Ro. So. Proc., June 11, ’91.) the seemingly red-hot glow, but with no heat conducted inwards from the surface, of cool solid silver kept negatively electrified in a vacuum 1/1,000,000 of an atmosphere, and thereby caused to rapidly evaporate, [§ 40] and [139a]. This last named result is almost more surprising than the phosphorescent glow excited by molecular impacts on bodies not rendered perceptibly phosphorescent by light, [§ 55], at centre. Both phenomena will usually be found very telling in respect to the molecular constitution of matter and origination of thermal radiation, whether visible as light or not. In the whole train of Crookes investigations on the radiometer, the viscosity of gases at high exhaustion, and the electro-phenomena of high vacuums, ether seems to have nothing to do except the humble function of showing to our eye something of what the molecules and atoms are doing. The same confession of ignorance must be made with reference to the subject dealt with in the important researches of Schuster and J. J. Thomson on the passage of electricity through gases. Even in Thomson’s beautiful experiments, showing currents produced by circuital electro-magnetic induction in complete poleless circuits, the presence of molecules of residual gas or vapor seems to be the essential. It seems certainly true that without the molecules, electricity has no meaning. But in obedience to logic, I must withdraw one expression I have used. We must not imagine the “presence of molecules is the essential.” It is certainly an essential. Ether is certainly also an essential, and certainly has more to do than merely to telegraph to our eyes to tell us what the molecules and atoms are about. If the first step towards understanding the relations between ether and ponderable matter is to be made it seems to me that the most hopeful foundation for it is knowledge derived from experiment on electricity in high vacuum; and if, as I believe is true there is good reason for hoping to see this step made, we owe a debt of gratitude to the able and persevering workers of the last 40 years who have given us the knowledge we have; and we may hope for more and more from some of themselves and from others encouraged by the fruitfulness of their labors to persevere in the work.”

61b. Thomson’s Experiment. Velocity of Cathode Rays. The Elect., Lon., Oct. 5, ’94, p. 762; Phil. Mag., ’94.—The object of the experiment of J. J. Thomson was to determine whether the velocity approached that of light or that of molecules. The apparatus he employed involved the rotating mirror, which was fully described in Proc. Royal So., ’90, slightly modified. The rays were caused to produce phosphorescence, while the mirror was so adjusted that when at rest, the two images on the phosphorescent strips appeared in the same rectilinear line. Many other elements comprised the apparatus. All the steps were performed carefully and according to the best methods, but the results are those which in this experiment are of particular interest, for by knowing the velocity of the rays, their nature is better appreciated and that of the X-rays can be better deduced. The velocity bore a close relation to that of the mean square of the molecules of gases at temperatures zero ° C. or in the case of hydrogen, 1.8 × 10⁵ cm. per second. As compared with such a velocity, that of the cathode rays was found to be in the neighborhood of 100 times as great, and this agrees very nearly with the velocity of a negatively electrified atom of hydrogen acquired under the influence of the potential fall, which occurred at the cathode. In further evidence of the verity of this statement, he made a rough calculation upon the curve or displacement produced by a magnet upon the rays. [§ 59]. He stated: “The action of a magnetic force in deflecting the rays shows, assuming that the deflection is due to the action of a magnet on a moving electrified body, that the velocity of the atom must be at least of the order we have found.”