RADIOACTIVITY 



19 



This effect may be examined much more closely than in the 

 original experiment. Consider an improved form of the 

 apparatus as sketched in fig. 2. The chamber is far shallower 

 than Mine. Curie's and for its lower plate has been substituted a 

 sheet of gauze, through the meshes of which the a particle can 

 stream if the radium be close enough. In order to simplify the 

 interpretation of the experiment, let a set of little tubes be 

 placed over the radium so that only those rays which are 

 streaming almost vertically upwards can emerge from the top 



lorvisauorx CkarrvDer 



"To EL\ectronr\eter^ 



9 au 5 e 



A A 4v 



CXf* 



a 11s 



I cm. 





Radiurw 



Fig. 2. 



of the bundle. The radium is spread in a very thin layer over 

 the surface of the plate ; in fact, a drop or two of water con- 

 taining radium in solution has been placed on the plate and 

 evaporated down until the remainder seems merely a stain on 

 the plate. The extreme thinness of the layer is also a factor 

 making for simplicity, as will appear presently. 



When the distance of the radium and its tubes from the 

 ionisation chamber is varied as in Mme. Curie's experiment, the 

 effect in the chamber is practically zero when the radium is 

 more than 7 cm. from the gauze : a very slight degree of further 

 approach causes a large increase in the current, which now 

 grows rapidly until the distance is reduced to 6*5 cm. nearly. 



