Cathode and Lenard Rays. 



233 



the curvature of the rays. Of course in the denser medium we 

 would expect the rays to lose their velocity sooner "by impact with 

 the gas particles, and Lenard himself found that in air at atmo- 

 spheric pressure he could only observe the rays at a few centimetres 

 from the window, while at lower pressures he could observe them at 

 ranch greater distances. 



In one of his experiments Lenard reduced the pressure so low in 

 his second or observing tube, that he could not produce rays by 

 connecting the coil directly to terminals in this tube. The pressure 

 was so low that the discharge refused to pass. When the coil was 

 connected to terminals in the primary tnbe the rays, even at this low 

 pressure, were propagated in the second tube away from the alu- 

 minium window. 



To explain this it is necessary to assnme that the electromotive 

 intensity produced by the impact of the charged cathode rays on 

 the window is sufficient to produce discharge in the second tnbe, 

 even when the coil connected to terminals in this tube cannot 

 produce discharge. From the first experiment described above we 

 calculate that at a single break of the primary of the coil the 

 atoms striking on a square millimetre of area of a screen placed in 

 the path of the cathode rays at a distance of 12 cm. from, the 

 cathode, carried a charge of about 60 C.G.S. units. A charge of 

 this magnitude, brought suddenly up to the screen will produce 

 very great electrostatic effects, and the electromotive intensity pro- 

 duced in the second tube may be great enough to dissociate the 

 molecules close to the window, and produce discharge. 



On the whole, the theory that the cathode rays are a stream of 

 charged atoms explains satisfactorily the observed properties of these 

 rays, and also the production and properties of the Lenard rays. 



Seeing that the cathode rays produce some of the most interesting 

 •of the phenomena presented by a discharge tube, it is important to 

 get some idea of the amount of the current actually carried by the 

 rays. It is generally stated that the cathode rays play a compara- 

 tively unimportant part in the actual passage of the discharge, but 

 although this may, to some extent, be true in the earlier stages of 

 exhaustion, it is certainly not so at lower pressures. 



By making use of the fact observed by several experimenters that 

 the discharge does not always take the shortest path between the 

 electrodes, we can separate out a part of the negative rays from the 

 remainder of the discharge, and measure this part directly with a 

 galvanometer, and thus get an inferior limit to the fraction of the 

 whole current carried by the cathode rays. The appearance of the 

 tube justifies us in assuming that the part of the current we thus 

 separate out and measure is carried entirely by cathode rays. 



vol. lxt. s 



