432 Mr. E. Rutherford on the Velocity and Rate 



Velocity of the Ions. 



It is a question of considerable interest to determine the 

 velocity with which the ions travel through a gas under the 

 influence of an electromotive force, as it indirectly gives us 

 some information in regard to the nature and size of the 

 carrier of electricity in conduction under the Rontgen rays. 



The method of determining the velocities was based on an 

 investigation given in a previous paper (J. J. Thomson and 

 E. Rutherford. Phil. Mag. Nov. 1896), where it is shown that 



* EUT 



VT(I37)- l 2 ' 



where i is the current between two parallel plates, I cms. 

 apart, when there is a potential-difference E acting between 

 the plates. 

 I = the maximum current through the gas, when a saturating 



E.M.F. is applied. 

 U = sum of the velocities of the positive and negative ions 



for a potential gradient of one volt per cm. 

 T = time taken for the number of ions to fall to half its 



original value after the rays have ceased. 



If E is chosen so that — is small this equation very ap- 

 proximately reduces to 



i EUT 

 I - l l ' 



Now all the quantities in this equation can be measured, 

 so that the velocity is readily calculated. In practice the 

 value of E was taken of such an order that the value of the 

 current i corresponding to it was about y 1 ^- of the maximum 

 current. 



The ratio ■= is not a constant for the same gas for the same 



potential gradient, as it depends on the intensity of the radia- 

 tion, and also on the rate of recombination of the ions, which 

 is in turn largely dependent, for the same intensity of radia- 

 tion, on the amount of dust and other solid matter in the 

 gas. 



Although these quantities are variable, the velocity U is a 

 constant, for its value is quite independent of the value of T 

 or of the intensity of the radiation. 



The steepness of the curves showing the relation between 

 the current and the E.M.F. for air and other gases, which 

 are given in a previous paper (loc. cit.), is very variable, 



