406 W. Duane — Range of the a-Rays. 



of mercury as measured by a McCleod gauge. This was to 

 prevent the charge being neutralized by the ionization of the 

 air in the box. I then measured the current flowing toward 

 the electrode by means of a Wilson gold leaf electroscope, 

 using much more radium than before (about 2 milligrams of 

 pure radium chloride). For these measurements the set of 

 fine glass tubes between the radium and the window was not 

 used, and the window was kept at zero potential. 



When the a-rays pass through the mica and when they 

 strike the metal electrode they produce slow-moving secondary 

 rays. In order to suppress these a magnetic field was produced 

 parallel to the surfaces of the electrode and window. That the 

 magnetic field stopped all the secondary rays was proved by 

 the fact that increasing its strength from 2400 gauss to 3600 

 gauss did not alter the current flowing to the electrode. 



That there was no appreciable ionization current in the 

 interior of the box is shown by the fact that with the mag- 

 netic field a difference of potential of several volts between 

 the window and the electrode did not alter the current from 

 one to the other. 



Curve 3, figure 2, represents the currents of electricity car- 

 ried to the electrode by the a-rays. It is evident that the 

 greater part of the charge carried by the rays does not pass 

 through the mica and reach the electrode, if the radium, is 

 more than 2 cm from the windoio. This is the same critical dis- 

 tance as found before for the ionization. By reason of the form 

 of the curves near the limit it is difficult to estimate the exact 

 length of the range, but we 'can say that approximately the 

 charge of the a-particles and the ionization produced by them 

 stop at the same point. 



Curve 4 represents the currents due to radium in which the 

 emanation and induced activity had been allowed to accumu- 

 late for over two days. The currents for distances greater 

 than 2 cm are due to the charges of electricity carried by the 

 rays from the emanation, and radium A, and C. 



In order to determine whether or not the a-rays lose their 

 power of producing secondary rays at the point where the 

 charge and ionization stop, the currents without the magnetic 

 field were measured with the radium at different distances from 

 the window. Under these conditions the current of electri- 

 city carried to the electrode is to some extent masked by the 

 secondary rays produced at the lower surface of the electrode 

 and the upper surface of the window. If a is the current car- 

 ried to the electrode by the a-rays, s 1 the negative charge car- 

 ried per second by the secondary rays away from the electrode, 

 and «9 2 the negative charge carried per second to the electrode 



