THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH $ERIES.] 



V \\ i-j 



^ \. * 



BECEMBER\9n. 



XLIY. The Application of Thermionic Currents to the Study 

 of Ionization by Collision. By Frank Horton, Sc.D., 

 Professor of Physics in the University of London*. 



THE theory of the ionization of gases by collision has 

 been experimentally verified for several gases by 

 Prof. J. S. Townsend and his pupils f. The method used 

 consisted in measuring the currents between two parallel 

 plate electrodes when the ions are set free initially at the 

 surface of the negative electrode by the action of ultra- 

 violet light. Townsend has shown theoretically that under 

 these conditions if n ions are set free at the surface of the 

 negative plate, and if, on the average, each negative ion 

 creates a new pairs of ions, and each positive ion creates 

 /3 new pairs of ions, by collisions in moving through 1 cm. 

 of gas, the number of ions reaching the positive plate is 

 given by 



In this formula d is the distance between the parallel plate 



* Communicated by the Author. 



t J. S. Townsend, Phil. Mag. [6] iii. p. 557 (1902) ; v. p. 389 (1903) ; 



Thi] 



yi. p. 598 (1903). J. S. Townsend and H. E. Hurst, Phil. Mag. [6] 

 viii. p. 738 (1904). H. E. Hurst, Phil. Mag. [6] xi. p. 535 (]"" 

 E. W. B. Gill and F. B. Pidduck, Phil. Mag. [6] xvi. p. 280 (1 

 and xxiii. p. 837 (1912). 



Phil. Maq. S. 6. Vol. 34. No. 204. Dec. 1917. 2 L 



