282 Mr. A. S. Russell on some Variations observed in 



The normal leak is obtained also by weak ionization over a 

 correspondingly longer time, and indeed the natural ionization 

 of the electroscope's air is itself sufficient if the whole instru- 

 ment be left for 4 to 6 hours. It is not necessary that the 

 leaf be charged. The normal leak because of its constancy 

 for any one disposition, and for the ease and rapidity with 

 which it may be obtained, was used throughout in the work 

 on the absorption of the 7 rays of uranium X and radium. 

 The effects described have been regularly and clearly shown 

 though in varying degree by every system whether weighted 

 or not, tried in electroscopes of the ordinary pattern. It is 

 immaterial (1) what is the nature of the electroscope walls or 

 base ; (2) what is the nature of the ionizing source, whether 

 it be /? or 7 rays, or from radium or uranium ; (3) whether 

 the source be intense, or made weak by absorbing screens ; 

 (4) whether the potential be just great enough to ensure 

 saturation or much greater. If A : denotes the percentage of 

 maximum leak over normal leak, and A 2 that of normal over 

 minimum, a series of experiments has proved that : 



(1) A : is much greater for a weighted leaf than for an 

 ordinary gold leaf, varying from 4 per cent, with the first to 

 38 per cent, in the second for typical cases ; 



(2) for an unweighted gold leaf, A x increases as strength 

 of source decreases, for a given potential ; also A x is practi- 

 cally constant between a divergence of tip of leaf from its 

 discharged position from 7 mm. to 13 mm. 



For a weighted leaf : 



(1) Ai varies in same direction as potential of leaf if 

 strength of source be constant : thus, 



Source = 50 div. per min. 



Potential in volts 



300 



350 



400 



460 



530 



600 



Ai 



21-3 



20-o 



27-8 



28-0 



302 



35-8 





(2) A x decreases as strength of source, S, increases, if 

 potential be constant : 



Potential 460 volts. 



s 



S-40 



12-98 



1736 



469 



483 



80-0 







A x 



38-4 



32-2 



30-4 



280 



27-9 



28-0 







