1022 Occurrence of Ionization by Cumulative Effects. 



remote from the filament. In his reply to this point Prof. 

 Compton admits that some uncertainty exists in the value of 

 R which can be taken as denoting a pure radiation effect, 

 but asserts that the observed variation in R with variation of 

 the pressure of helium is entirely too large to be accounted 

 for in this way, quoting as an instance the result he obtained 

 at 25 mm. pressure. It follows, from the additional data 

 now given by Prof. Compton, that in the case of eight out of 

 the ten results in his table (quoted above) the observed 

 variation in R is not such as to justify the statement that 

 it cannot be adequately accounted for without assuming the 

 presence of ionization. 



The area of the cylindrical surface of the collecting 

 electrode is stated to have been equal to four times the area 

 of the plane end of this electrode. Hence the limiting value 

 of R for pure radiation, a value to which R will approximate 

 when the radiation is so scattered by repeated absorption 

 and re-emission that it is as likely to fall on the curved 



^.^i _1_ rp 



surface as upon the front plane end, would be — tt^-= 1*11, 



1 l 4r + 0"5r 



where r is, as before, the photoelectric current from the foil- 

 covered plane end of the collecting electrode. In deducing 

 this limiting value of R for pure radiation we have neglected 

 the photoelectric effect from the plane end of the collecting 

 electrode remote from the filament, as this is unfavourably 

 situated in regard to the electric field, whereas the cylindrical 

 surface is, according to the diagram of the apparatus, in a 

 situation comparable with that of the front plane end so far 

 as this field is concerned. 



At very low pressures the radiation cannot suffer much 

 deviation from its original direction through absorption and 

 re-emission by helium atoms, and hence the front plane end 

 of the collecting electrode will, in these cases, receive nearly 

 all of the radiation which falls on the outside of this electrode, 

 and the value of P will approximate to the value 2 assumed 

 hj Prof. Compton. At higher pressures, however, the 

 curved surface receives a larger proportion of the total 

 radiation falling on the collecting electrode, and hence, as 

 the pressure is increased, R decreases from the value 2 and 

 approaches more and more nearly to the limiting value I'll. 

 Prom the table already quoted, it will be seen that the 

 decrease in the experimental value of R with increase of the 

 pressure of helium is such as might be anticipated for a 

 pure radiation effect, and that only in the two cases in which 

 the pressure was very considerable do the experimental 

 values of R fall below the limiting value for pure radiation. 

 Thus in eight out of the ten instances the experiments do 



