198 Prof. Townsend on the Conductivity produced in 



values of r, the square of A is seen to be negligible in com- 

 parison with unity. 



Now B is less than A, for k is less than 1, and a + x is 

 greater than a. Hence, B 2 and AB are negligible in com- 

 parison with 1, and our equation for maximum and minimum 

 reduces to the form 



2ttx 

 cos—- - = +1, 

 A- 



whence 



nX 



where n is any integer. 



Whence it follows that with the arrangement of apparatus 

 adopted in the experiment on stationary wave and index of 

 refraction, the diminished amplitude of the reflected wave and 

 the absorption by the dielectric placed between the oscillator 

 and the reflector could have no appreciable effect on the 

 position of the maxima and minima. 



Jefferson Physical Laboratory) 

 Harvard University, Cambridge, Mass. 



XVII. The Conductivity produced in Gases by the Motion 

 of Negatively charged Ions. By John S. Townsend, ALA., 

 Wykeham Professor of Physics, Oxford*. 



1. TNa paper published in ' Nature/ vol. lxii. August 9th, 

 A 1900, I gave a brief description of some experiments 

 which showed that negatively charged ions, moving through 

 a gas, produce other ions, although the force acting on them 

 is very small compared with the force necessary to produce 

 the ordinary vacuum-tube or spark-diseharges. The present 

 paper contains a more complete account of the principal 

 experiments and also some investigations founded on the 

 theory to which I have been led by the experimental results. 

 In all the experiments with which we are here concerned, 

 a number of ions are generated in the gas by some external 

 source, such as Rontgen or Becquerel rays. The nature of 

 the conductivity produced by these rays has been investigated 

 by several physicists, and it has been generally concluded 

 that when the electric force is increased, the conductivity 

 approaches a maximum. Thus, in the paper f published on 

 this subject by Professors J. J. Thomson and E. Rutherford, 

 it is stated that " for a given intensity of radiation, the 



* Communicated bv the Author. 



t Prof. J. J. Thomson and Mr.- Rutherford, Phil. Mag. Nov. 1890. 



