468 



W> Duane — Range of the a- Rays. 



magnetic field, is represented in fig. 3, curve 5, as a function of 

 the distance of the radium from the window. 



It is evident that the greater portion of the secondary rays 

 stop when the radium is removed further than about 2 cm from 

 the windoiu / and this is about the same distance as found 

 before for the charge of a-rays and the ionization. 



For better comparison the currents measured in the same 

 series with the magnetic field, representing the charge carried 

 by the a-rays, are plotted on curve 6, fig. 3. 



Owing to the form of the curves it is difficult to determine 

 the exact point where the effect of the a-rays from the radium 

 itself ceases. The effects due to the small amount of emanation 



Fig. 3. 



.n 



.10 



>09 

 i.08 



r <n 



i06 

 H Q5 



.0* 

 r 03 

 .02 

 .01 



\ 



























































V 







































.04 











^ 



















V 















Distance from radium to window in cm. 

 Charge of secondary rays. 



Distance from radium to window in cm. 

 Charge of a-rays. 



and induced activity are always appreciable. If there is any 

 difference, the curves seem to indicate that the power of pro- 

 ducing secondary rays is appreciable a little further away from 

 the radium than is the charge. I do not think, however, that 

 we can tell with certainty. 



This point is of great importance in connection with the 

 idea advanced by J. J. Thomson, that at a certain velocity the 

 a-particle attaches to itself an electron, which neutralizes its 

 charge and therefore changes its properties. 



I have tried the above experiments with polonium, but the 

 currents were too small to be satisfactory. I hope to be able 

 to repeat them with a more active preparation of polonium. 



