Some Experiments with Cathode Rays, 



93 



the fact that at high vacua the cathode rays come off entirely or, at 

 any rate, almost entirely from only a very small portion of the centre 

 of the cathode, explains the observed fact that within limits large 

 cathodes have no advantage over small cathodes in X-ray tubes. 



Daring the carrying out of the above experiments with a carbon 

 cathode, very bright sparks were occasionally seen coming off the 

 cathode and passing through the focus, and it was consequently 

 thought that possibly by placing two concave carbon cathodes facing 

 one another, such particles, by being caused to rebound backwards 

 and forwards continuously between the two, might render the form of 

 cathode stream visible at very high vacua when the stream itself 

 becomes otherwise invisible. 



With this view, a tube was made as shown in fig. 22, in which 

 two concave carbon cathodes CC, similar to those employed in the 

 last experiment, were placed exactly opposite one another, so that a 

 prolongation of the focus of either one would pass through the centre 

 of the other. The anode, D, was placed in an annex, as shown in 

 the illustration, and the two cathodes were connected together by 

 means of a wire outside the tube. At a very high exhaustion, this 

 tube gave very beautiful effects, and showed clearly the form of the 

 cathode discharge at a degree of exhaustion when it is usually in 

 itself quite invisible. Immediately on the current being turned on 

 and the discharge passing, a straight and thin stream of bright golden 

 coloured particles of apparently incandescent carbon passed between 

 small luminescent spots at the centres of each cathode, as shown in 

 fig. 23. This did not last for more than a second, when, owing 

 no doubt, to the rapid fall of vacuum, the appearance changed to that 

 shown in fig. 24, and the incandescent particles of carbon could 

 be seen passing backwards and forwards along the convergent and 



