Roentgen Rays and Phenomena of the Anode and Cathode.

by Edward P. Thompson

73. Cathode Rays. Phenomena in Different Gases.—The apparatus consisted of an observing tube having a tubular gas inlet and outlet both in one end and arranged in line with the cathode of the discharge tube. See construction in Fig. [H], at beginning of this chapter, the tube being about 40 cm. long and 3 cm. in diameter. He was very careful in every case to chemically purify and dry the particular gas. He omitted the perforated disk and provided an opaque strip of the phosphorescent screen on the side toward the window and made his observations from the other side, the object of the experiment being particularly to test the transmission of cathode rays in different gases. With any particular gas, he moved the phosphorescent screen along by means of a magnet until the shadow on the screen became invisible. It is evident that the distances of the screen from the window for different gases would indicate the relative transmitting powers. He also modified the experiment by varying the density of the gases, hydrogen being taken as 1 as usual, nitrogen 14, and so on. The transmitting power of hydrogen was nearly five times as great as that of nitrogen, air, oxygen and carbonic acid gas, which did not much differ. [§ 10] and [18]. Sulphurous acid was a very weak transmitter. All the gases became luminous near the window as in air. [§ 65]. The colors were all about the same as far as distinguishable, [§ 11], which was difficult in view of the brightness of the phosphorescence on the glass. It was a universal rule, that when the density decreased, the transmitting power increased. In high vacua, in all gases, the rays went through the space in rectilinear lines in all directions from the window, and generally it made no difference what gas was employed provided the vacuum was as high as hundredths of a millimetre. At this pressure all gases acted the same. To be sure, the phosphorescence did not occur at this high vacuum at a great distance as might be expected, but it should be remembered that the intensity of the rays varied as the square of the distance, and, therefore, at very great distances, the action was very weak.

74. Cause of Luminosity of Gas Outside the Discharge Tube.—At ordinary pressures, in the cases of hydrogen and air, as has been noted, the gas became luminous in the observing tube, the effect being, of course, the same as entering open air, represented in Fig. [A], beginning of this chapter. In order to determine the luminosity at less pressures, the gas, of whichever kind, was enclosed in a rather long observing tube and only at rather high vacua did the bluish and sometimes reddish gaseous luminosity disappear. Upon grasping the tube with the hand or approaching any conductor connected to earth, of large capacity, the column stopped at that point so that the remainder of the tube, beyond the hand, measured from the discharge, was dark. The phosphorescence on the glass wall of the tube produced by the cathode rays was not influenced in any way by outside conductors, such as the hand. Cathode rays themselves were not stopped apparently by the hand, because the phosphorescent screen and glass, located beyond the hand, became luminous. He concluded, therefore, that the glowing of the gas had no close connection with the cathode rays. He proved this also by deflecting the cathode rays in the discharge tube from a certain space, and yet the gaseous luminosity remained. As an exception, the cathode rays sometimes appeared to be closely associated with the light column. He attributed the luminosity of the gas in general, at low pressures, not to the cathode rays, but directly to the electric current or some kind of electric force, [§ 11] and [14], which, as already remarked, permitted sparks to be drawn from the aluminum window and surrounding points.

The negative glow light in Geissler tubes, [§ 30], is also to be regarded as gas illuminated by cathode rays. (Compare Hertz, Wied. Ann., XIX., p. 807, ’83.) Between that phenomenon and the glow observed here and attributed to irradiation, there exists a correspondence, inasmuch as in both cases the light disappears at high exhaustions, [§ 53], appears fainter and larger when the pressure increases, [§ 54], and then becomes brighter and smaller, [§ 54]. But, whereas, the glow in the Geissler tube has become very bright and small at 0.5 mm. pressure, the gas in our experiment remains much darker up to 760 mm. pressure, and yet the illuminated spot is much larger. This difference cannot, therefore, be attributed to an inferior intensity of the rays here used. But it will be explained, [§ 76], as soon as we can show that at higher pressures cathode rays of a different kind are produced, which are much more strongly absorbed by gases than the rays investigated hitherto and produced at very low pressures.

Use of Stops in Sciagraphy. (Perch.) [§ 107]., p. [101].
By Leeds and Stokes.

Fig. I, p. [52], illustrates the apparatus by which he studied the rectilinear propagation and whereby he found that it was rectilinear only in a very high vacuum. In the figure, the gas is at ordinary pressure, and it will be noticed that the turbidity of the same is indicated by the curved lines while the dotted lines show the volume that would be occupied by light or other rectilinear rays, unaccompanied by any kind of diffusion. In the observing tube, there was a disc having a central hole at a. Beyond this disc, measured from the aluminum window, was a fluorescent screen which, as well as the perforated disc, could be moved to different distances by means of a magnet acting on a little iron base. It is evident that upon moving the fluorescent screen to different distances, the diameter of the luminous patch would be a measure of the amount of turbidity. The curved lines intersecting the peripheries of the luminous spots indicate, therefore, the field of the cathode rays, so that said field would appear like a kind of curved cone if the same were visible. Although hydrogen is the least turbid gas, yet the phosphorescent patches were all larger except with a high vacuum than they could have been with rectilinear propagation. An additional characteristic of the phosphorescent spot, was its being made up of a central bright spot and a halo less luminous, appearing like some of the pictures of a nebula, see Fig. I´, p. [52], the darker or centre indicating the brighter portion. In a perfect vacuum the halo did not exist. He performed a similar experiment with ordinary light. No halo occurred on a paper screen which was used instead of the phosphorescent screen, but upon introducing a glass trough of dilute milk between the window and the perforated disc, or between the disc and the paper screen, nuclei and halos were obtained, illustrating a case of the effect of a turbid fluid upon light, and assisting in proving that gases act as a turbid medium to cathode rays as milk and similar substances do to light; also in other gases than hydrogen, and by the use of cathode rays, nuclei and halos were not obtained at high exhaustion, all the gases becoming limpid. Taking into account pressure and density, all gases behaved the same as to the power of transmission when they were of the same density, without any regard whatever to their chemical nature. Density alone determined the matter, according to Lenard.

75. Cathode Rays of Different Kinds are Variably Diffused.—He discovered the remarkable property, contrary to his expectation, that if the rays are generated at high pressures, they are capable of more diffusion than when generated at lower pressures. This can be easily proved by any one, for it will be noticed that upon increasing the pressure in the discharge tubes the spots on the phosphorescent screen will not only grow darker but larger and more indefinite as to the nucleus and halo. He called attention to the agreement with Hertz, who also found that there were two different kinds of rays, see Wied. Ann., XIX, p. 816, ’83, also see Hertz’s experiment. Lenard also pointed out the analogue in respect to light, which, when of short wave length, is more diffused in certain turbid media than that of greater wave length. Although Lenard held that his experiment proved that cathode rays were phenomena in some way connected with the ether, yet he pointed out an important difference in connection with the property of deflection of the rays by the molecules even of elementary gases like hydrogen, producing diffusion of the rays, which accordingly may be considered as behaving like light in passing through, not gases, but vapors, liquids and dust. In the case of the cathode rays the molecules of a gas acted as a turbid medium, but in the case of light, turbidity is only exhibited by vapors or certain liquids, as so eloquently explained by Tyndall, in “Fragments of Science,” 1871, where it is shown that aggregation of molecules, like vapors or dust in the presence of light, make themselves known by color and diffusion, whereas the substances in a molecular or atomic state do not serve to show the presence of rays of light.

76. Law of Propagation.—Lenard recognized continually that there were two kinds of cathode rays. One of them may have been X-rays without his knowing it. In the latter part of ’95, he made some experiments especially of a quantitative nature as to the principle of absorption of the rays by gases. By mathematical analysis, based upon experiments, he arrived at the principle that the absorptivity of a gas is proportional to its pressure, or what is the same thing, to its density, or as to another way of stating the law, “the same mass of gas absorbs at all pressures the same quantity of cathode rays.” See Elect. Rev., Lon., as cited, p. 100.

77. Charged Bodies Discharged by Cathode Rays.—An insulated metallic plate was charged first with positive electricity and in another experiment with negative electricity. In each instance, the plate was discharged rapidly by the cathode rays as indicated by the electroscope, and the same held true when a wire cage in contact with the aluminum window, surrounded the electroscope and the metallic plate. The effect was stopped by cutting off the cathode rays by quartz .5 mm. thick. The discharge took place, however, through aluminum foil. A magnet was made to deflect the internal cathode rays, whereupon the discharge did not take place, all showing that the discharge of the insulated plate was directly due to those rays. A remarkable occurrence was the accomplishment of the discharge at a much greater distance than that at which phosphorescence was exhibited. See also Roentgen’s experiment—who suggested that Lenard had to do with X-rays in this experiment, but thought they were cathode rays. The maximum distance for the discharge was about 30 cm. measured normally to the aluminum window. He caused a discharge of a plate also in rarefied air. He admitted that the experiments were not carried far enough to know whether the effect was due to the action of the cathode rays upon the surface of the window, or upon the surrounding air, or upon the plate. The author could not find in Lenard’s paper any positive or negative proof that he had actually deflected the external cathode rays by a magnet while passing through air or gas at ordinary pressure. He had deflected them while passing through a very high vacuum in the observing tube. Dr. Lodge, who briefly reviewed Lenard’s experiments, expressed the same opinion. See The Elect., Lon., Jan. 31, ’96, p. 439. For theoretical considerations of the electric nature of light, the discharge law in the photo-electric phenomena, the simple validity of the discharge law, the occurrence of interference surfaces in the blue cathode light, the cathode rays in the axis of symmetry, the necessary degrees of longitudinal electric waves, the frequency of the cathode rays, and proof of longitudinal character of cathode rays, see Jaumann in The Elect., Lon., Mar. 6, ’96; translated from Wied. Ann., 571, pp. 147 to 184, ’96, and succeeding numbers of The Elect., Lon., which were freely discussed in foreign literature contemporaneously.

78. De Kowalskie’s Experiment. Source, Propagation and Direction of Cathode Rays. Acad. Sci., Paris, Jan. 14, ’95; So. Fran. Phys. Jan. ’95; Nature, Lon. Jan. 24, ’95; Feb. 21, ’95.—The conclusions he arrived at are, 1. The production of the cathode rays does not depend on the discharge from metallic electrodes across a rarefied gas, nor is their production connected with the disintegration of metallic electrodes. 2. They are produced chiefly where the primary illumination attains suitable intensity, that is, where the density of the current lines is very considerable. 3. Their direction of propagation is that of the current lines at the place where the rays are produced, from the negative to the positive poles. They are propagated in the opposite direction to that in which the positive luminosity is supposed to flow. [§ 43]. He employed a Goldstein tube reduced at the centre. [§ 41]. It was found that the cathode rays are formed not only at the negative electrode, but also at the constriction, directly opposite the cathode. De Kowalskie carried on further experiments in this line in order to be satisfied with the principles named above, which he formulated. In one tube, he was able to produce cathode rays at either end of the capillary tube forming the constricted part of a long vacuum tube. No electrodes were employed. The tube was merely placed near a discharger through which “Tesla currents” were passed? He seems to have been working with X-rays without knowing it; for his results agree with those of Roentgen and later experimenters that the source of X-rays is the surface of a substance where it is struck by cathode rays. The statements were about as definite as could be expected at that date.