﻿812 Mr. A. J. Saxton on Impact Ionization by 



different from that operating with low-speed impact where 

 the striking particle does not penetrate the atom. 



In the case where the kinetic energy of the positive ion 

 is great enough for it to penetrate the atom (a-ray), the 

 amount of ionization per cm. path increases as the speed of 

 the ion decreases. The amount of perturbation increases 

 with the time taken for the ion to cross the atom. Thus 

 a-rays produce most ionization near the end of their range. 

 Glasson's * experiments on ionization by cathode rays indi- 

 cate a similar effect with high-speed electrons. 



(6) Ionization by Collision. — Townsend f measured the 

 current between two metal plates with different field- strengths 

 between them, the negative plate being illuminated by ultra- 

 violet light. For small distances between the plates the 

 results could be readily explained on the assumption that 

 the photo-electrons emitted by the negative plate (and the 

 electrons they produced in the gas by ionization) when 

 accelerated through the field produced a-ions per cm. by 

 collision with the molecules of the gas. For distances 

 between the plates above a certain value he obtained currents 

 which were larger than would be expected on the above 

 simple theory, and ascribed this increase to ionization by 

 positive ions which produced /3-ions per cm. 



For example t, in hydrogen at 8 mm. pressure with a 

 distance between the plates of *3 cm. and a field of 700 volts 

 per cm. giving a P.D. of 210 volts between the plates, he 

 obtained an increase ascribed to the action of positive ions. 

 Thus positive ions falling through 210 volts in hydrogen at 

 8 mm. pressure ionize by impact. The M.F.P. of a positive 

 ion would be very small at this high pressure, so that it 

 could not obtain an unimpeded fall through more than a 

 fraction of a volt. This suggests that the ionization pro- 

 duced may be due to (a) either successive collision or (6) an 

 accelerated positive ion does not lose the whole of its kinetic 

 energy on every collision, and so may acquire a velocity 

 corresponding to a P.D. which is greater than that along 

 its M.F. path. 



(c) Cathode Fall in Discharge- tubes.' — It is not yet clear 

 whether the positive ions accelerated through the cathode 

 fall of potential produce electrons (cathode rays) by impact 

 with the molecules of the gas or with the metal or occluded 

 gas of the cathode itself §. The values of the minimum 



* J. L. Glasson, Phil. Mag. (6) xxii. p. 647 (1911). 

 f J. S. Townsend, 'Electricity in Gases/ Chapter IX. 

 t Ibid. p. 317. 

 § Eatner, Phil. Mag., Dec. 1920, p. 785. 



