W. Duane — Range of the a-Bays. 467 



by the secondary rays from the window, the total current 

 toward the electrode is 



The secondary rays s t and s 2 are stopped by the magnetic fields, 

 leaving only the current a. 



If the window is charged positively the electric field from it 

 toward the electrode stops part (if strong enough all) of the 

 current s„ and if the window is charged negatively some or all 

 of the current s l is stopped. 



To determine how strong the field must be in order to stop 

 all of the secondary rays, the radium was placed l"5 cm below 

 the window, and the currents toward the electrode measured 

 by the Wilson electroscope when the window was charged to 

 different positive potentials. The following values were 

 obtained : 



Potential 



+ 02 4 9-5 17 25 34 51 68 85 170 volts. 

 Current 



1-35 2-10 2-71 4-23 5'57 7*72 8'30 9'2 9*8 10-0 10'0 



It is evident from these figures that after a potential of some 

 70 volts is reached the current is not increased even if the 

 potential is doubled: i. e., 70 volts stops all of the second- 

 ary rays coming from the window. If the field is reversed 

 about the same potential, 70 volts stops all of the rays coming 

 from the electrode. 



The above readings were taken without a magnetic field. If 

 a magnetic field of 2800 gauss is produced the current is 1'6 on 

 the same scale, and is increased only a few per cent even by 170 

 volts. 



We now have a means of studying the secondary rays com- 

 ing from the surface of the electrode alone, s x , for by charging 

 the window to a potential of + 70 volts or more the current s 2 

 is suppressed. Then the difference between the currents with 

 and without the magnetic field is s 19 the secondary rays from 

 the electrode. 



To determine whether or not the power of the a-rays to 

 produce the secondary rays ceases at the point where the charge 

 and the ionization stop, the 2 rag of radium chloride were freed 

 from emanation and induced activity as before, and placed 

 at different distances below the window. The window was 

 charged to a potential of +85 volts, and the currents for each 

 distance were measured, first without and then with, the mag- 

 netic field. The difference between the two currents, repre- 

 senting the secondary rays from the electrode cut off by the 



