68. Transmission of External Cathode Rays Through Metals.—The phosphorescence was not diminished apparently by an intervening gold-leaf or silver or aluminum foil, while it was extinguished by quartz .5 mm. thick which also cut off the atmospheric glow beyond itself. The leaves and foil did not so act. The difference of thickness should be borne in mind, as metal, as thick as the quartz did not transmit. As to other substances, tissue paper cast a slight shadow, which was darker with an additional sheet; but the shadow was independent of color and blackness, [§ 154]. Ordinary writing paper was roughly, proportionally opaque, while the shadow was black with cardboard .3 mm. thick. Glass films as made by blowing glass, cast faint shadows when .01 mm. thick. He proved that there was little difference as to the transmitting power of conductors and dielectrics when thin. Mica and collodion sheets .01 mm. thick cast scarcely any shadow. The reader may bear in mind the striking differences between these properties of cathode rays, and X-rays, [§ 135], it being assumed always that the generating devices are the same; for example, water permitted the cathode rays (were these simply feeble X-rays?) to be transmitted only when in very thin layers. Even soap water films which were only .0012 mm. thick cast shadows, although very faintly. The shadows of drops of water were black, while water several feet thick has been traversed by X-rays from a small set of apparatus. By careful measurements he found that the law of transmission must be different from that of light, for in the latter, many substances are opaque although exceedingly thin, while with cathode rays, the same will traverse all films. Goldstein and Crookes reported that thin mica, glass and collodion films made very dark shadows, [§ 58], within the discharge tube, whereas Lenard found that outside of the vacuum tube, in open air, the transparency was greater than according to the earlier experimenters, but he acknowledged that Crookes and Goldstein were inconvenienced and limited in the number of observations because it is so difficult to carry on such experiments within an hermetically sealed tube. Again, he acknowledged that perhaps the cathode rays of those experimenters were of a different kind. The construction shown in the above figures was modified by using a very thin glass window instead of aluminum, and the results were the same allowing for the different opacity, to ordinary light, of aluminum and glass.
The cathode rays acted upon the sense of smell and taste as the nose and mouth could detect ozone, [§ 84], at end.
69. Propagation. Turbidity of Air. Upon studying the shadows on the phosphorescent screen, it was noticed that the rays were bent around the edges of the object. Again, when the object had a slit, diffusion could be noticed by the shape (as in Crookes Ex., Fig. 15, p. [17],) of the luminous portion of the phosphorescent screen. In Fig. [B], at beginning of this chapter, the spatter work represents the shape of the luminous portion, the darker part representing the most luminous surface of the screen, the latter being held at right angles to the thick plate, having the slit and opposite the aluminum window. By varying these experiments, especially by changing the angle of the screen he found that not the all rays were diffused, but as in the passage of light through milk, some were transmitted in rectilinear lines.
70. Photographic Action.—He performed with sensitive silver compound papers, an experiment somewhat similar to those with phosphorescent bodies and also others. Behind a rather thick opaque plate the chemical film was not acted upon, but the rate of blackening near the aluminum window without obstruction of intermediate bodies was about the same as that with befogged sunlight. The former, moreover, was acted upon at a much greater distance than that at which phosphorescence was exhibited and beyond the atmospheric luminosity. By means of shadow pictures or sciagraphs, he compared the shadows produced by the external cathode rays with those which would have been obtained by light. Referring to Fig. [C], beginning of this chapter, the sensitive plate was half covered with a plate of quartz, Q, and half with a plate of aluminum, A´ overlapping the quartz. With light, the shadows would have appeared as in said figure, that is, one-half black as produced by aluminum, a quarter rather light as produced by quartz, and the other quarter bright, or a similar arrangement, according to whether the negative or the positive photograph is considered; but with the cathode rays, the appearance of the developed plate was as in Fig. [D]., beginning of this chapter. The quartz cast the black shadow, while the aluminum, the lighter one. Furthermore, the luminosity of the air produced a variable light on the other quarters. A similar appearance was produced by casting shadows of such plates upon the phosphorescent screen; but, of course, the picture was not a permanent one. The photographic plate served to accumulate the power, for the cardboard which cast a faint shadow upon the phosphorescent screen, showed a black shadow upon the photographic paper by sufficiently long exposure. At the same time, strips of thin metal were placed side by side between the chemical paper and the cardboard, and they showed different degrees of shading. The cardboard was quite thick, being .3 mm. Prof. Slaby (see Elect. Rev., Lon., Feb. 7, ’96), after Röntgen’s discovery, produced sciagraphs of the bones of the hand at the window of the Lenard tube. Lenard doubted whether the cathode rays produced direct chemical action. Iodine paper became bluish, but he could not obtain other chemical effects usually produced by light, and other agencies, for example, oxygen and hydrogen mixed together in the proportion to form water, and which were in their nascent state, and which were located in a soap-bubble, did not explode or ignite. No effect was produced upon carbon bi-sulphide nor hydrogen-sulphide, although the exposure was very long. Ammonia was not formed when the rays acted upon a mixture of three parts hydrogen and one part nitrogen, as to volume. He thought that he noticed a small expansion of air, hydrogen and carbonic acid separately located in a vessel having a capillary tube and water to indicate the expansion. He attributed the slight expansion to an indirect action, although very slight, caused by heat produced by the cathode rays, [§ 27], and yet neither the thermopile nor the thermometer showed any calorific effects although the thermopile responded to the flame of a candle 50 cm. distant.
71. Cathode Rays and Electric Forces Distinguished. The earth connection heretofore mentioned with the aluminum window was for the purpose of dispensing with sparking, but even then the approach of another conductor connected to earth would cause some sparking. Sparks could be drawn when the cathode rays were deflected from the aluminum window by a magnet. Fig. [E], at beginning of chapter. He argued that the rays and the electric forces of the spark are non-identical. He was not satisfied with this as an absolute proof, and he instituted others. He enclosed the whole generator in a large metal box. In the observation space, that is, around and near the window, he located another box, having an aluminum front facing the window. See Fig. [E], at beginning of chapter. It was within this second box that he took the sciagraph shown in Fig. [D], at beginning of chapter. It is important to notice that sparks could not be drawn at points within the said second box, shown at the left, even by a metallic point shown projecting thereinto. No spark occurred whatever, not even from the aluminum front. Sparking occurred when the pointed wire was extended to a considerable distance outside of the back of the small box, but it was remarked that the electric force did not enter through the front wall but was introduced “from behind into the box, by the insulation of the wire.” No one can, therefore, enter the objection that the cathode rays experimented with, were generated from the aluminum window as a cathode. They came from the cathode referred to entirely within the vacuum tube. Prof. J. J. Thomson, F. R. S., had at an early date conjectured that cathode rays did not pass through thin films of metal, but that these films acted as intermediate cathodes themselves. See his book on “Recent Researches,” p. 26, also The Elect., Lon. March 23, ’94, p. 573, in an article by Prof. Fitzgerald, who names that citation.
72. Cathode Rays Propagated, but not Generated in a High Vacuum.—The proposition was proved by having two tubes, one called the generating tube and one the observation tube, the former being like that shown in Fig. [A], at beginning of chapter, which is partly repeated in Fig. [F], at beginning of chapter, combined with the observation tube, which contains the two electrodes for casual use; but the one on the right is a disk extending nearly throughout the cross sectional area, and having a small central opening. Although both tubes were connected to the air pump, yet, by means of stop-cocks, the vacuum in one tube could be maintained at a maximum degree for hours, while the other was at a minimum. The first experiment was performed with a vacuum, about as high as that employed in Crookes’ phosphorescent experiments, [§ 53]. There was a patch of green light, [§ 57], at the extreme left end of the observation tube and the glass was green at the right, [§ 54], and a little to the left of the perforated disk electrode a. The other electrode of this tube was located at the upper left and lettered k.
72a. The magnet deflected the rays in the observing tube as indicated by the partial extinction of the phosphorescent patch. He noticed that with the rarefied atmosphere the amount of turbidity was enormously reduced, or in other words, that the rays were propagated more nearly in rectilinear lines. All the experiments on the cathode rays, in this observing tube, were of about the same nature as those which could be produced in the discharge tube.
From Sciagraph of Cat’s Leg, by Prof. William F. Magie.
Copyright, 1896, by William Beverly Harison, pub. of X-ray pictures, 59 Fifth Ave., New York City.
72b. The principal experiment consisted in exhausting the observing tube to such a degree that cathode rays could not be generated therein. The vacuum was so perfect that when used as a discharge tube all phosphorescence gradually died away until it disappeared, and no current passed ([§ 25]) except on the outside surface of the glass. The coil was so large, electrically, that the length of the spark between spheres was 15 cm. Upon charging the right hand tube and generating cathode rays, it was determined by means of magnetic deflection, phosphorescence and other effects, that the cathode rays traversed the highest possible vacuum ([§ 19], near end, where energy must have passed through the high vacuum to produce luminosity in the inner bulb). The external and internal rays were certainly different forms of energy. Inasmuch as he noticed that rarefied air was less turbid and less absorptive than air at ordinary pressures, it occurred to him to make a very long tube, namely, 1 m, or a little over 3 feet. He employed very severe steps for obtaining an exceedingly high vacuum, the operation occupying several days. The pump used was a Toepler-Hagen, while a Geissler pump was employed separately for the discharge tube. The pencil of cathode rays traversed the whole length of the long tube. See a portion of the apparatus in Fig. [G], at beginning of this chapter. One disk was of metal and perforated with a pin hole and the other was a phosphorescent screen, so that when the cathode pencil passed through the hole in the plate a patch was seen upon the phosphorescent screen. The phosphorescent spot was always, no matter what the relative distances of the disks were from each other, and from the end of the tube, substantially the same as it would have been by calculation assuming that there was no turbidity effect. The patches, in each instance, were a little smaller in diameter than the calculated ones. For example with one measurement, at certain distances, the actual diameter of the patch was 2.5 mm., while the calculated diameter was 2.9 mm. In his experiments with light under the same conditions, the luminous spots were also a little smaller than the calculated or geometrical. The disks had iron shoes and were moved to different positions by a magnet. He concluded, therefore, that in what may be called a perfect vacuum, light and cathode rays have a common medium of propagation, namely, the assumed ether. Prof. Fitzgerald, in The Elect., Lon. Mar. 23, ’94, does not agree broadly with him in this; neither does he contradict him. He argues rather on the point that the cathode rays and light rays are not identical, but Lenard does not affirm this, because the magnet will attract the former and not the other. Prof. Fitzgerald admits this and calls to mind that even in a vacuum, as obtained by Lenard, there were still ten thousand million molecules per cu. mm. and therefore he thinks it is better to look to matter rather than ether as the medium of propagation of cathode rays. [§ 61b]. On the other hand, Lenard agrees with certain other predecessors, Wiedemann, Hertz and Goldstein, in favor of cathode rays being etheric phenomena. See Wied. Ann., IX., p. 159, ’80; X., p. 251, ’80, XII., p. 264, ’81; XIX., p. 816, ’83; XX., p. 781, ’83. The vacuum with which Lenard operated, was .00002 mm. pressure, obtained by cooling down the mercury to minus 21° C. This vacuum was so high that all attempts to prove the presence of matter failed. Neither did the exceedingly high vacuum deaden the cathode rays. On the other hand, as noted, they were assisted rather than hindered. [§ 135].