Prof. J. J. Thomson on Cathode Rays. 305 



To make the error from this cause as small as possible, the 

 inner cylinder was connected to the largest capacity available, 

 1*5 microfarad, and the rays were only kept on for a short 

 time, about 1 or 2 seconds, so that the alteration in potential 

 of the inner cylinder was not large, ranging in the various 

 experiments from about "5 to 5 volts. Another reason why 

 it is necessary to limit the duration of the rays to as short 

 a time as possible, is to avoid the correction for the loss 

 of heat from the thermo-electric junction by conduction along 

 the wires ; the rise in temperature of the junction was of 

 the order 2° C. ; a series of experiments showed that with 

 the same tube and the same gaseous pressure Q and W were 

 proportional to each other when the rays were not kept on 

 too long. 



Tubes of this kind gave satisfactory results, the chief draw- 

 back being that sometimes in consequence of the charging up 

 of the glass of the tube, a secondary discharge started from 

 the cylinder to the walls of the tube, and the cylinders were 

 surrounded by glow ; when this glow appeared, the readings 

 were very irregular ; the glow could, however, be got rid of by 

 pumping and letting the tube rest for sometime. The results 

 got with this tube are given in the Table under the heading 

 Tube 1. 



The second type of tube was like that used for photographing 

 the path of the rays (fig. 4); double cylinders with a thermo- 

 electric junction like those used in the previous tube were 

 placed in the line of fire of the rays, the inside of the bell- 

 jar was lined with copper gauze connected with the earth. 

 This tube gave very satisfactory results ; we were never 

 troubled with any glow round the cylinders, and the readings 

 were most concordant ; the only drawback was that as some 

 of the connexions had to be made with sealing-wax, it was 

 not possible to get the highest exhaustions with this tube, so 

 that the range of pressure for this tube is less than that for 

 tube 1. The results got with this tube are given in the Table 

 under the heading Tube 2. 



The third type of tube was similar to the first, except that 

 the openings in the two cylinders were made very much 

 smaller ; in this tube the slits in the cylinders were replaced 

 by small holes, about 1*5 millim. in diameter. In consequence 

 of the smallness of the openings, the magnitude of the effects 

 was very much reduced ; in order to get measurable results 

 it was necessary to reduce the capacity of the condenser in 

 connexion with the inner cylinder to "15 microfarad, and to 

 make the galvanometer exceedingly sensitive, as the rise in 

 temperature of the thermo-electric junction was in these 

 experiments only about '5° C. on the average. The results 



