556 Mr. A. Wood on Spontaneous Ionization of 



The following is a summary of another similar series of 

 e sets of readings : — 



Screened. Unscreened. Screened. Unscreened. Screened. 



0529 



0-692 



0-557 



0-727 



0-586 



0-588 



0666 



0-571 



0-666 



0-586 



0-589 



0-680 



0'586 



0-702 



0-612 



0-599 



0-640 



0-586 



0-656 



0-642 



0-603 



0-677 



0'628 



0661 



0-628 



0-573 



0-651 



0-571 



0-661 



0-612 



..0-576 



0-669 



0-588 



0679 



0-611 



Summary. 



Vessel Screened. Vessel Unscreened. 



0-59 0-67 



Reduction of Ionization due to screen, 12 per cent. 



It was thought that it might be interesting to extend these 

 experiments to screens of different materials. Wood and 

 paper were used, but the effect produced by them on the 

 ionization in the vessel, if any, was so small as to be within 

 the limits of experimental error. So far as one could judge, 

 however,, the wood gave a slight increase and the paper a. 

 slight decrease. 



The apparatus used in the above experiments had one or 

 two drawbacks. For instance, the paper screen referred to 

 above consisted of old volumes of the Phil. Trans., and was 

 about 45 cm. thick. If the ionization vessel was placed 

 inside and the electroscope outside, the long connecting-wire 

 which was necessary added considerably to the capacity of 

 the electrode system, and so diminished the sensitiveness of 

 the apparatus. On the other hand, if ionizing vessel, electro- 

 scope, and microscope were all enclosed within the screen, 

 then a large quantity of material was necessary in order to 

 get sufficient thickness all round. An attempt was there- 

 fore made to improve the apparatus previously used, so as to 

 combine the great sensitiveness of the Wilson electroscope with 

 the compactness of the self-contained apparatus commonly 

 used in experiments of the kind. It was impossible to use 

 the electroscope in its original form both as electroscope and 

 ionizing vessel. The volume of air enclosed was so small and 

 the capacity of the plate so large, that the leak was not 

 measurable. The new apparatus was of brass, cylindrical in 

 form, about 14 cm. high and 10 cm. in diameter. In the 

 centre of this cylinder, supported on a thin quartz pillar, 

 was a small rectangular brass plate set at an angle of 30° to 



