176 SUMMARY OF ELECTRON THEORY [CH. xvm. 



immense initial velocity falls below a certain critical 

 speed at which ionisation ceases to be caused. A 

 charged electroscope of proper sensitiveness is able 

 to indicate by its leak the presence of this number of 

 ions, and accordingly is able to signal the presence 

 of only one a particle. But the number of such 

 particles expelled by a milligramme of fully active 

 radium is estimated as a hundred million every 

 second. 



Electric Production. 



The power of radium constantly to develop elec- 

 tricity was first demonstrated by W. Wien, by 

 suspending a tube in a vacuum. Dorn observed that 

 sometimes sparks were obtained on opening a tube 

 containing radium. Strutt contrived an ingenious 

 device for displaying this constant production of 

 electricity by reason of the very different penetrating 

 power of the + a and the /3 particles, so that one 

 set can be trapped while the other set escapes. 



Fig. 22 shows Strutt's apparatus and "perpetual" 

 more accurately, perennial electric generator and 

 source of mechanical energy. A small tube contain- 

 ing radium salt, with its outside made sufficiently 

 conducting, has a pair of gold or aluminium leaves 

 attached, to it, and is insulated in a very high 

 vacuum by a quartz rod ; a metallic strip, lining 

 the vacuum case, being connected to earth. The 

 radium fires off positive and negative particles in 

 equal quantities ; but the negative, being small and 

 penetrating, are many of them able to escape, while 

 the positive accumulate and thus keep on slowly 

 charging the gold leaves with positive electricity till 

 they periodically overflow. The high vacuum is 

 necessary to prevent the internal atmosphere from 



