Ionization by Cumulative Action. 533 



this region without loss of energy by radiation is approxi- 

 mately : 



which must at least be of the right order of magnitude. 

 This represents the fraction of the bombarding electrons 

 which attain sufficient speed to ionize a normal atom by a 

 single impact, and is independent of the initial kinetic 

 energy of emission of the electrons from the cathode, 

 provided this does not exceed V,.. 



In Table I. are given values of P calculated for various 

 pressures in mercury vapour and helium, assuming the 

 electrodes to be 1 cm. apart. For mercury: V;=10'4, 

 Vr = 4'9, N = 75; for helium : V* = 25-2, V r = 20'4, N = 8'5. 

 These gases represent extreme cases of small and large 

 values of P, respectively, under given conditions. 



Table T. 



p (mm.). 



PHg. 



PHe. 



001 



0-59 



0-98 



o-i 



3-1 (10r 3 



0-79 



10 



7-8 oor 26 



0-09 



3-0 

 100 



5-0 (10) _76 

 9-6 (10)~ 252 



7-3 (10)- 6 

 2-7 (10) ~ 13 



At the pressures, between 2 mm. and 10 mm., at which 

 the most intense low voltage arcs are obtained, it is evident 

 that any contribution to this ionization due to single impacts 

 against normal atoms is entirely negligible. The same 

 conclusion must be drawn from experimental evidence. In 

 a recent paper * I have shown that the ionization curve in 

 helium shows no discontinuity at the ionizing potential 

 25'2 volts for pressures exceeding three or four millimetres. 

 Further experiments on arcs in helium f have also indicated 

 that the critical voltage for the helium arc is the minimum 

 radiating potential, near 20*4 volts, rather than the ionizing 

 potential. 



It appears, therefore, that while ionization by single 



* Phil. Mag. xL p. 553 (1920). 



t K. T. Conipton, E. G. Lilly, & P. S. Olmstead, Pliys. Rev. xvi. 

 p. 282 (1920) ; K. T. Compton & E. G. Lilly, Astropliys. jour. lii. p. 1 

 (1920). 



