178 SUMMARY OF ELECTRON THEORY [CH. xvm. 



certain leakage was observed, but this was reduced 

 some thirty per cent, when the whole was plunged 

 into a tank of water so as to screen the interior 

 from outside radiation. 



Kutherford and Cooke surrounded the electroscope 

 with thick screens of various kinds, and once with 



K. 



FIG. 23. Righi form of electroscope or electrometer, of small capacity. 

 The vital parts DECB, with the gold leaf, are shown on a larger scale in 

 fig. 24. They are contained in a metal case, with a movable wire G, by 

 which they may be charged from outside. The instrument is read by a 

 reading microscope R; an ordinary millimetre scale being placed in a 

 conjugate focus and projected by the lens L on to the plane of observa- 

 tion where the gold leaf is. The radio-active substances are presented 

 to the box outside a window of very thin aluminium foil. 



as much as five tons of lead. A moderate amount 

 of screen, however, was found to produce the same 

 effect as a greater amount, showing that the influence 

 of the outside penetrating-radiation could be checked 

 without stopping more than thirty per cent, of the 



