Cathode and Lenard Rays. 



235 



discharge again takes the shortest path AC, so that a field of mag- 

 netic force across the direction of discharge in this case diminishes 

 the resistance of the tnbe, by diminishing the length of it which is 

 occupied by the negative dark space and allowing the positive column 

 to lengthen out. 



A slight variation in the tube (fig. 3) enables us to measure the 

 current carried by the cathode rays in CA. The tube is made as in 

 fig. 4. C is the cathode, A and A x are both connected to earth, and 

 the anode of the coil is also to earth. 



At first the discharge passes between A and C, but when the proper 

 exhaustion is reached the positive column starts from A h and travels 

 round by the longer path. 



We can insert a galvanometer between A or A x and the earth con- 

 nection, and measure the current through AC and A X C. 



The galvanometer used was a very sensitive one shunted with a 

 small resistance negligible in comparison with that of the discharge 

 tube. 



The results obtained were as follows : — 



Before the positive column changed from the large tube to the side- 

 tube B, no current could be detected in the latter. A very small 

 diminution of pressure causes the positive column to travel by A X C, 

 and now the current in AC was about 1/50 of that in AiC. 



The pressure was now gradually diminished and readings taken at 

 every step until very little glow was visible in A;C, while the phos- 

 phorescence in AC was steadily increasing. 



The ratio of the current in AC to that in A X C meanwhile steadily 

 increased, until finally it was 1/4. 



These numbers show that even at a pressure at which there is 

 little phosphorescence a considerable fraction (more than 1/50) of 

 the whole discharge was carried away from the cathode by the nega- 

 tive rays, while at lower pressures these rays carried a large portion 

 of the discharge. 



The fact that the negative rays travel in straight lines away from 

 the cathode, independently of the position of the anode, and there- 

 fore, in a discharge tube in which the anode is not in the line of the 

 rays, do not directly convey their charge to the anode, does not 

 necessarily prove that the part of the discharge which they carry 

 away from the cathode is small. 



In such a tube there will be an excess of negative electricity at the 

 point where the cathode rays are stopped by the glass, and this 

 makes its way to the anode by a current in the gas, which is a good 

 conductor when the primary discharge is passing. 



In conclusion, I wish to thank Professor Thomson for many valu- 

 able suggestions in the course of the above work. 



s 2 



