Rays of Positive Electricity and Their Application to Chemical Analyses/Rays of Positive Electricity
The positive rays were discovered by Goldstein in I886.1 His apparatus is represented in Fig. 1 ; the cathode K which stretched right across the tube r was a metal plate through which a number of holes were drilled, the diameter of the holes being considerably less than the thickness of the plate; the axes of the holes were at right angles to the surface of the plate; the anode a was at the end of the lower part of the tube. The pressure of the gas in the tube was so low that when the electrodes K and a were connected with the terminals of an induction coil and a discharge passed through the tube, the dark space below the cathode was well developed. Under these circumstances Goldstein found that slightly diverging bundles of a luminous discharge streamed through the holes in the cathode into the upper tube. The colour of the light in these bundles depended on the kind of gas with which the tube was filled: when it was air the light was yellowish, when it was hydrogen, rose colour. These rays can be shown very conveniently by the use of the tube represented in Fig. 2 ; a form also used by Goldstein in his earlier experiments. The cathode which fills the middle of the tube is a flat disc with a hole in It; a metal tube fitting into the hole is soldered on to the cathode, the length of the tube should be several times the diameter of the hole and Its axis perpendicular to the plane of the cathode; the anode is a wire fused Into the upper part of the tube. When the pressure of the gas Is pro- perly adjusted, the positive rays stream through the tube into the lower part of the vessel while the cathode rays shoot upwards. The contrast between the colour of light due to the positive rays and that due to the cathode rays is, when some arc In the tube, exceedingly striking. Of all the gases I have tried for this purpose neon gives the most striking results, for with this gas the light due to the positive rays is a most gorgeous red, while that due to the cathode rays is pale blue; with helium the positive rays give a reddish light while that due to the cathode rays is green. The spectroscopic examination of the light due to the positive and cathode rays reveals interesting differences which we shall have to consider later; we may anticipate, however, so far as to say that the character of the light produced by the positive rays is similar to that of the velvety glow which, in an ordinary discharge tube with an unperforated cathode, spreads over the surface of the cathode.
As in Goldstein's experiments these rays were observed streaming through holes or channels In the cathode; he called them " Kanalstrahlen". Now that they have been proved to be streams of particles, the majority of which are positively electrified, it seems advisable to call them positive rays, as indicating their nature; the name Kanalstrahlen only suggests the methods of demonstrating them.
Many important properties of the positive rays can be easily demonstrated by the use of a tube like that shown in Fig. 2. For example when the rays strike against the glass sides of the tube they make the glass phosphoresce. The phosphorescence produced by the positive rays is of a different colour from that produced by the cathode rays and is in general not nearly so bright. With German glass the positive and cathode rays both produce a greenish phosphorescence, though the greens are of different shades. With some substances the contrast is much more striking, for example with fused lithium chloride the phosphorescence produced by the positive rays is an Intense red showing when examined by the spectroscope the red lithium line; the phosphorescence due to the cathode rays is a light blue giving a continuous spectrum. The phosphorescence due to the positive rays is a most valuable aid for studying the way the rays are deflected by arid magnetic forces, and it is important to find the which gives the brightest phosphorescence. The substance which I have found most useful is wlllemite, a natural of zinc. The mineral should be ground into as fine a as possible, the powder shaken up In alcohol so as to form a suspension, which is allowed to deposit slowly on a plate; by this means the glass is covered with an exceedingly even deposit of the willemite. After continued exposure to the positive rays the brightness of the phosphorescence and ultimately disappears, so that for the detection of rays the willemite must be renewed from time to time. Some substances deteriorate more rapidly than others, for example zinc blende phosphoresces very brightly under the positive rays, but, as far as my experience goes, it deteriorates more quickly than willemite, so that when the observations have to last for any considerable time the willemite is preferable. A more sensitive, and for many purposes more convenient, way of registering the deflection of the positive rays is to take advantage of the fact that, when these rays strike against a photographic plate, they affect the plate at the place of impact and thus a permanent record of the position of the rays can be obtained. The action of the rays on the plate differs from that of light, since these rays do not use the whole thickness of the film but only a layer close to the surface, so that it does not follow that the most " rapid " photographic plates are the most sensitive to the positive rays. The most sensitive plates for the detection of the positive rays would be having very thin films containing as much silver as possible. I have tried the old Daguerreotype process instead of the usual dry plate method, but without much success. It is probable that if Schumann plates (Baly's " Spectroscopy," p. 359) could be prepared as uniform and free from streaks as ordinary commercial plates they would be the most suitable for the study of the positive rays; all the plates of this kind I have tried, however, have been too streaky to permit the determination of faint lines with any certainty. The plates known as " Imperial Sovereign " give very good results.
The positive rays gradually remove a thin deposit of metal from the part of walls of the tube against which they strike, Such thin deposits can readily be produced by running an electric discharge through the tube when it contains gas at a low pressure, using for the cathode a piece of the metal it is wished to deposit on the glass. The metal cathode " splutters" and the metal is deposited as a thin layer on the glass near the cathode.
1. Uber eine noch nicht untersuchte Strahlungsform an der Kathode inducirter Entladungen. "Berl. Ber.," xxxix, p. 691, 1886; " Wied. Ann.," LXIV, p. 38, 1898.