Rays of Positive Electricity and Their Application to Chemical Analyses/Double Cathodes

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Thomson RoPE 1913 Fig3.jpg

Goldstein1 found that positive rays came freely from the space between two parallel plates metallically connected together and used as a cathode for the discharge through gas at a low pressure. Cathode rays also come from this region, and the discharge from a cathode of this kind, through a gas where there is a marked difference in colour between the luminosity produced by the cathode and positive rays, presents some very interesting features. Hydrogen, and to a still greater degree helium and neon, are suitable gases for this purpose. When a cathode formed of two parallel equilateral triangles connected together by a wire is used for the discharge through helium at a low pressure, the discharge near the cathode has the appearance represented in Fig. 3. From the points of the triangle stream pencils of luminosity showing the characteristic red colour of the positive rays in helium, while the middle points of the sides are the origin of streams of greenish luminosity, the colour of the path of the cathode rays through helium. The difference In the character of the rays is also made evident by bringing a small magnet near the discharge tube; the green rays are visibly deflected by the magnet but no appreciable effect is produced on the red rays. By using polygons instead of triangles, or scalene triangles Instead of equilateral ones, very interesting distributions of the red and green pencils can be obtained. Researches on these parallel cathodes have been made by Kunz2 and Orange,3 and they are often useful for giving strong pencils of positive rays in definite directions; we shall have to consider later on examples of their use for this purpose.

1Goldstein, " Phil. Mag." VI, p. 372,1908.

2Kunz, " Phil. Mag,," VI, xvi, p. 161,1908.

3Orange, " Proc. Camb. Phil. Soc.," XV, p. 217.