Experiments on Residual Ionization. 399 



emanation had had ample time to decay, and so the number 

 o£ ions made per c.c. per second was quite low. 



To make certain of the source of the emanation the calcium 

 carbide was tested for radioactivity in the following manner. 

 A metal cap, fig. 3, was made which could be screwed on 



Fi°r. 3. 



in place of the drying-tube of" the Wolff electrometer. A 

 small brass cup, which was to contain the carbide, was sup- 

 ported on a rod screwed into the end of the cap. The figure 

 shows the cap in place, the top of the cup being flush with 

 the wall of the electrometer. The following measurements 

 were made. The cup was put in place, and the electrometer 

 filled with fresh, dry air. The number of ions made per c.c. 

 per second, n, was then measured. The cup was removed, 

 filled with powdered carbide, and the carbide was covered 

 with aluminium foil '0003 cm. thick. The cup was then 

 replaced in the electrometer, and the number of ions made 

 per c.c. per second again measured. The next day two more 

 measurements of the ionization were made, the first with 

 both brass cup and carbide in place, and the second with them 

 both removed. The air in the electrometer was the same 

 as that used on the first day, and had probably received 

 slight traces of radium emauation from being in contact 

 with the carbide. Hence the values of n obtained on the 

 second day are considerably greater than those obtained on 

 the first day. However, the difference between the two 

 readings taken on either day, which is the important thing 

 here, is almost the same for both days. 



First day. 



n. 



Second day. 



». 



Brass cup in place 



87 

 10-6 



Carbide and cup in place ... 

 Both removed 



122 

 104 



Carbide and cup in place ... 

 Difference 



Difference 



1-9 



1*8 







