556 



Prof. J. J. Thomson on the Masses of 



The distance between the carbon filament and the plate 

 connected with the electrometer was 3*5 millim., the 

 strength of the magnetic field 170 C.Gr.S. units. 



Difference of 



Potential between 



wire and plate, 



in Tolts. 



Leak in 5 seconds. 



Ratio 

 of leaks. 



Without 

 magnetic field. 



With 

 magnetic field. 



40 



80 



120 



140 



160 



43 

 170 

 300 

 345 

 400 



1 



50 

 250 

 345 

 430 



■023 

 •29 

 •83 

 10 



1-07 



1 



Taking 140 volts as the critical value of the potential- 

 difference, we find by equation (1) that 



-=7-8xl0 6 . 

 m 



The results of this and similar experiments are given in the 

 following table ; V denoting the critical potential-difference 

 in C.Gr.S. units, and H the magnetic force : 



d. 



V. 



H. 



c/m. 



•35 



140x10' 



170 



7-8x10° 



•35 



220 x10 s 



220 



7-5x10° 



•35 



170 xlO 8 



170 



9-6x10° 



■35 



130 x10 s 



170 



7-2x10° 



•35 



120 x10 s 



120 



11-3x10° 



giving 8*7 x 10 6 as the mean value of e/m. This value 

 does not differ much from that found in the case of ultra- 

 violet light. In the case of the incandescent filament the 

 ions are only produced at a small part of the plate, and not 

 over the whole surface as in the case of ultra-violet light, 

 so the conditions do not approximate so closely to those 

 assumed in the theory. We conclude that the particles which 

 carry the negative electrification in this case are of the same 



