Diagram of Lenard’s Apparatus. pp. [53] to [69].
CHAPTER VI
65. Lenard’s Experiments. Cathode Rays Outside of the Discharge Tube. Wied. Ann., Jan., ’94, Vol. LVI., p. 225; The Elect., Lon., Mar. 23 and 30, ’94, Apr. 6, ’94; and Elect. Rev., Lon., Jan. 24, ’96, p. 99.—Of more importance in connection with X-rays is the consideration of Lenard’s experiments than any others. The reader must bear in mind that his exhaustive investigations resulted from his discovery (founded upon a hint from Hertz) that the cathode rays might be transmitted to the outside of the generating discharge tube. His interest, therefore, in the discovery was so great that his researches extended to the minutest details. Passing from these introductory remarks, the characteristics of the tube that he employed will be explained first. Reference may now be made to the accompanying Fig. [A]. He employed several different kinds of tubes, but finally settled upon one of which the essential elements are shown in the said figures. It was permanently connected to the pump, [§ 53], so that the pressure within could be varied. Opposite the cathode, which consisted of a thin disk of aluminum, the end of the tube was provided with a thick metal cap, having a perforation, which in turn was closed by a thin aluminum sheet secured by marine glue in an air-tight manner, and called a window. The anode was a heavy brass cylinder, shown in section, within the discharge tube and surrounding the leading-in wire of the cathode. The anode and the aluminum window were connected to each other, electrically, and to earth, as well as two a secondary terminal of an induction coil, whose electrodes were in shunt to those of the discharge tube, in order that the operator might adjust the sparking distance which rapidly increased with the exhaustion. The induction coil had a mercury interrupter.
65. Properties of Cathode Rays in Open Air.—In all directions around the window upon the outside and in the open air, a faint bluish glow ([§ 11] and [140]) extended and vanished at a distance of 5 cm., as indicated by dotted lines in Fig. [B] at beginning of this chapter. The degree of luminosity may be judged by saying that it was not sufficient to admit of investigation by the ordinary pocket spectroscope. A new window was void of luminosity; but with use, bluish gray and green and yellow spots occurred thereon.
66. Phosphorescence by Cathode Rays.—Substances which generally phosphoresced by light and cathode rays in the generating bulb, [§ 55], also phosphoresced under the influence of the rays in open air, excepting eosin, gelatin, both phosphorescent in light, were not so in cathode rays; so also with solutions of fluorescein, magdala red, sulphate of quinine and chlorophyll. Phosphorescence was less if the rays first passed through a tube of glass or tinfoil lengthwise. The phosphorescent light of the phosphides of the alkaline group, uranium glass, calcspar and some other substances, was so great that the luminosity of the air was invisible by contrast. The maximum distance at which phosphorescense was discernable in open air was about 8 cm. The best phosphorescent screen consisted of paper saturated with pentadecylparatatolylketone. In order to prepare it, he laid a sheet of paper upon glass and applied the fused chemical with a brush. As to the color of the phosphorescence and fluorescence of different substances, and as to the degree of luminosity outside of the vacuum tube, they were about the same as reported by Crookes when located within the discharge tube. §55. Baric and potassic and other double cyanides of platinum, common flint, glass, chalk and asaron all exhibited the same property as when exposed to ultra-violet light, that is, fluoresced or phosphoresced. Sulphide of quinine in the solid state fluoresced, but not in solution. Petroleum spread on a piece of wood fluoresced, and also fluorescent-hydrocarbons generally.
66a. The cathode rays were not easily transmitted by tinfoil or glass, because the degree of phosphorescence on the screen was greatly reduced by interposing such sheets. The phosphorescense ceased also by deflecting internal cathode rays from the window by a magnet. For full treatment of the phenomena of phosphorescence, see Stokes’ experiments, described in Phil. Trans., 1852, Art. “Change of Refrangibility of Light.” In brief, Stokes’ theory assumes that such substances have the power of reducing the refrangibility. Example: Ultra-violet light, highly refractive, is changed to yellowish green, less refrangible, by reflection from uranium glass.
67. The Aluminum Window, a Diffuser of Cathode Rays. [§ 63b]. The conclusion arrived at by mounting the phosphorescent screen in different positions and at different angles as well as by observance of the gaseous luminosity, was that the aluminum window scattered the rectilinear parallel cathode rays in all directions, [§ 57].