558 Prof. K. T. Compton on Ionization and Production 



respectively, and let c be the ratio of closed area to total area 

 of the gauze end. 

 Then 



Jdj i + cr 



whence 



i 1 — cr 

 r ~ H^Y 



The ratio c is a constant of the apparatus, and the ratio R 

 is found by comparing the electrometer deflexions obtained 

 with the foil and gauze ends respectively of the cylinder 

 turned to face the filament. For pure radiation R should 

 equal 1/c, while for pure ionization R should equal unity. 

 Any change in the type of the effect due to the electron 

 impacts should be indicated by a change in the value 

 ofR. 



The value of the ratio c was found to be 0*495 when calcu- 

 lated from geometrical considerations. This value is subject 

 to small corrections because some electrons, photoelectrical!)' 

 excited on the inside of the cylinder, may escape through 

 the gauze; because also the platinum surfaces of the foil and 

 gauze may be intrinsically different in photoelectric sensitivity 

 owing to different modes of manufacture; and because the 

 surface in one case is curved and in the other case flat. 

 Therefore, an independent experimental determination of e 

 was made as follows : — The ground-glass cone J whicli sup- 

 ported the filament was removed and a quartz window sealed 

 over the end. After thorough evacuation of the apparatus,, 

 a quartz-mercury arc was placed before the window and the 

 photoelectric currents from E were measured with a field 

 between E and G of the same general magnitude as in the 

 ionization experiments. The ratio of these currents for the 

 two positions of E gave R=l/c = l*82 / . whence g' = 0*55. 

 This value agrees as well as could be expected with the 

 calculated value. 



The remaining features of the apparatus require little 

 explanation. A large tube of coconut charcoal is attached 

 directly to the ionization tube. The helium is stored in a 

 reservoir, from which it can be pumped into or out of the 

 ionization tube through two U-tubes. The U-tube nearest 

 the ionization chamber contains coconut charcoal, while the 

 other serves as a mercury trap, into which the mercury vapour 

 can be condensed by applying liquid air before the liquid air 



