40-4 Prof. J. J. Thomson and Mr. Rutherford on the Passage 



keep the intensity of the rays constant, then the I and E 

 curves for two different gases may intersect. This effect is 

 shown in the following diagram, which represents the I and 

 E curves for hydrogen and air. We see that for small 

 electromotive forces the current is greater in hydrogen than 

 in air, while the saturation current is much greater in air 

 than in hydrogen. The saturation current depends merely 



Figr. 4. 



on the number of conducting particles produced by the rays, 

 while the current in the earlier part of the curve depends on 

 the space described by the conducting particles in the time T 

 (see equation 6), and we infer that more conducting particles 

 are produced by the rays in air than in hydrogen, but that 

 the product of U, the velocity of these particles, and T, a 

 time which is proportional to the time these particles linger 

 after the rays are cut off, is greater for hydrogen than it is 

 for air. 



In fig. 5 we give the curves for air, chlorine, sulphuretted 

 hydrogen, and mercury vapour, the curves being drawn on 

 such scales that the ordinate representing the saturation 

 current is the same in all these cases. It will be noticed that 

 the curves for air, for sulphuretted hydrogen, and for chlorine 

 coincide, mercury vapour falls below, while the hydrogen- 

 curve would be above. This shows that, using the notation 

 of equation (6), UT is the same for air, chlorine, and sul- 

 phuretted hydrogen, and that its value for these gases is 

 smaller than for hydrogen and greater than for mercury 

 vapour. 



It is remarkable that the shapes of the curves for air, 

 sulphuretted hydrogen, and chlorine should agree so closely, 



