764 Prof. R. A. Millikan and Mr. H. Fletcher on the 



§ 3. Ionization by Secondary X rays. 



Table III. represents observations taken on the ionizing 

 effect of secondary X rays under conditions as nearly as 

 possible like those described by Townsend when he found 

 that the positives produced by secondaries were chiefly doubles. 

 Fig. 3 shows the arrangement of the apparatus. 



Fig. 3. 





— 



u 



c 



— 











"- - 



- 





A 



+ 





i) 



4- 





The plate D was covered by a thin coat of oil and only 

 the left-hand portion of it could be hit by the primary X rays. 

 Despite the fact that the secondary rays from the plate D 

 traverse the air on all sides of the drop, we succeeded in 

 catching chiefly positives by holding the drop always within 

 2 or 3 millimetres of the plate C. We had hoped to clear 

 up the apparent contradictions in preceding work on this 

 subject by finding that homogeneous secondary X rays do in 

 fact produce double positives, but it will be seen from 

 Table III. that out of 84 catches there were but 3 which 

 could possibly correspond to multiples of any kind. Of 

 these 3 catches the two followed by an interrogation mark 

 unfortunately happened when Millikan, who was observing 

 at this time, had glanced away to read the stop-watch, and 

 hence may have corresponded to two or three separate 

 catches following in rapid succession. The third catch 

 marked " good " appeared to the observer (Fletcher) to cor- 

 respond to a change in speed which happened all at once 

 rather than in two successive steps. Nevertheless the changes 

 were happening here, when the field was on, at about one or 

 two second intervals, instead of six second intervals as in the 

 work recorded in Tables I. and II. It is not at all per- 

 missible, therefore, to draw the conclusion that this catch 

 corresponded to the advent of a double ion, rather than to 

 the nearly simultaneous advent of two separate single ions. 

 The legitimate conclusion which can be drawn from this 

 table is that if doubles were formed at all by the secondary 

 rays here used, their numbers were certainly exceedingly 

 small in comparison with the number of singles formed. 



