400 Prof. J. J. Thomson and Mr. Rutherford on the Passage 

 Sulphuretted Hydrogen. (Strong radiation.) 



Electromotive 



> Current 



Current calculated 



Force. 



observed. 



by equation 4. 



15*6 



8*7 





34 



18 



17-1 



68 



30-8 



28-5 



126 



40 





Sulphuretted Hydrogen. (Weak radiation.) 



15-6 



3-8 





34 



6-3 



6*2 



68 



8 



8 



136 



8-7 

 Mercury Vapour. 





5-1 



14-2 



14-6 



8*5 



23 





15-6 



35 



36-9 



34 



55 



59 



68 



75 





136 



75 



8'2 



As these measurements require the intensity of the radia- 

 tion to be maintained constant during each series of observa- 

 tions, a condition which it is very difficult to fulfil, we think 

 the agreement between theory and observation is as close as 

 could be expected. 



We have seen how from the measurement of the limiting 

 current we could form an estimate of the proportion which 

 the conducting particles bear to the rest of the molecules of 

 the gas. We can, in addition, get from the curve represent- 

 ing the relation between the current and the electromotive 

 force an estimate of the velocity with which these particles 

 move. Taking equation (3) 



aPc 2 i 



?~e 2 U 2 E 2 U ' 

 we shall endeavour to express the coefficients in terms of 

 quantities which our experiments enable us to estimate. Let 

 I be the limiting current when the electromotive force is 

 infinite, then 



I = qle. 



Let T be the time which elapses after the rays have been 

 stopped for the number of conducting particles to fall to one 

 half the number just before the rays ceased, no current passing 

 through the gas. Then, just before the rays cease to fall on 



