Section III., 1915 [13] Trans. R.S.C. 



Secondary Cathode Rays from Gases. 



By A. Norman Shaw, D.Sc, Macdonald College, McGill University. 



Presented by Professor H. T. Barnes, F.R.S., F. R.S.C. 



(Read May Meeting, 1915). 



1. Introduction. It is well known that the impact of cathode 

 rays against the molecules of a gas in a "vacuum" tube produces 

 a luminous haze which extends beyond the boundary of the primary 

 beam. In 1908 Sir Joseph Thomson examined this secondary radia- 

 tion and made a determination of its velocity^ finding that it consisted 

 of slow electrons the energy of which was apparently independent 

 of that of the main beam, and possibly also of the nature of the gas. 

 The present paper gives an account of some experiments performed 

 with the object of repeating and extending this investigation. 



2. Apparatus. The apparatus described below has been set 

 up for this purpose. A section of the fundamental parts of the 

 apparatus is shown in the figure. W represents a Wehnelt cathode, 

 A the anode, and B a brass water cooler containing a cylindrical 

 tube through which the pencil of rays may be directed. Suitable 

 joints M M M make the adjustment of the position of W an easy matter. 

 C is a small brass box with a wire gauze top and four wire gauze plates 

 arranged as shown. The box is placed so that the pencil of cathode 

 rays from W just passes over the top. Connections from the gauze 

 terminals E, F, D, H pass to a set of mercury cups, J, and thence 

 through an ebonite plug K to electrical connections for charging the 

 gauzes to definite potentials. D represents a window through which 

 the luminosity in C may be observed. The end of the apparatus 

 is closed with a glass plate. A tube N leads to a Topler pump, McLeod 

 gauge, charcoal bulb and drying flask. Another tube, not shown 

 in the figure, can be used for introducing various gases into the appa- 

 ratus. 



3. The Velocity of the Radiation. The gauze covering 

 to the box, C, was removed and the interior gauzes were insulated. 

 On passing the cathode beam over the box it was found that for a 

 short interval of time, the surrounding haze or glow completely 

 enveloped the gauzes. If the four gauzes (E, F, G, and H) were 

 earthed, the glow was repelled to a distance of from one to two milli- 



^Thomson, Proc. Camb. Phil. Soc, XIV, p. 541 (1908). 



