W. Duane — Range of the a-Rays. 



465 



in succession at an interval of several hours, in order to free 

 it from most of its emanation and induced activity. Finally 

 it was dried upon a flat sheet of platinum. The platinum was 

 then held horizontally below the mica window at different 

 distances from it, and the ionization currents between the win- 

 dow and the electrode were measured by a quadrant electro- 

 meter, the window being at a potential of 88 volts above the 

 electrode. To make the rays that entered the box parallel to 

 each other a set of line glass tubes (not shown in the figure) 

 was fastened between the radium and the window with their 

 axes vertical. 



Curve 1, figure 2, represents the ionization current as a 

 function of the distance from the radium to the bottom of the 

 box. It is evident that most of the ionization in the interior 

 of the oox disappears if the radium is removed to a distance 

 from the window greater than about 2 cm . 



Fig. 2. 



o 



o _ 



— <u 

 © w 



*. OjM 



s 



5 



g 



S 



\ 



K 



Distance from radium to window. 



Distance from radium to window. 



The currents for distances greater than two centimeters are 

 due to a small amount of emanation and induced activity 

 remaining after or having accumulated since the final crystalli- 

 zation. The a-rays from these, as is well known, have greater 

 ranges than have the a-rays from radium itself. The "ioniza- 

 tion due to these a-rays of longer range is well shown by curve 

 2, which represents the currents due to radium (a smaller 

 amount than before) that had been left two days in a dry state, 

 and which therefore contained considerable amounts of emana- 

 tion and induced activity. 



In order to measure the positive charge of electricity car- 

 ried by the a-rays, I exhausted the air from the box by means of 

 a mercury pump, producing a high vacuum of less than *0001 mm 



