Effect of Screening on Ionisation in Closed Vessels. -4 77 



Effect of Screening on Ionisation in Closed Vessels. By 

 A. Wood, Emmanuel College. 



[Received 20 May 1904.] 



" Spontaneous " ionisation in closed vessels has recently been 

 the object of much investigation. C. T. R. Wilson*, as the result 

 of experiments on the ionisation of hydrogen, air and carbon 

 dioxide in small vessels, came to the conclusion that at least some 

 of the ionisation was due to a radiation coming from the walls of 

 the vessel and of only slight penetrating power. This conclusion 

 was further supported by some experiments of Struttf, who 

 obtained very different values for the ionisation in vessels of the 

 same size but of different materials. On the other hand M c Lennan^ 

 and Rutherford and Cooke § have shown that the ionisation is also 

 to some extent due to a very penetrating radiation from without 

 which passes through the walls of the vessels. The question at 

 once arises whether this radiation from the walls of the vessel is 

 primary or secondary in character — whether it is due to a radio- 

 activity of the material or whether it is a secondary radiation 

 excited by the penetrating one. In the latter case one would 

 expect the radiation from the walls of the vessel to be cut down in 

 the same ratio as the penetrating radiation by a lead or other 

 screen, and so one would expect the same screen to produce the 

 same proportionate reduction of the ionisation for all vessels of 

 the same dimensions, whatever the material of which they are 

 constructed. If, on the other hand, the radiation from the walls 

 or any considerable fraction of it be due to a radio-activity of the 

 material it will be independent of the penetrating radiation, and 

 so unaffected by a screen. The proportionate reduction of ionisa- 

 tion due to any screen would thus vary greatly for different 

 materials, being least in the case of vessels of comparatively great 

 radio-activity. It was with the intention of obtaining information 

 on this point that the later experiments described in this paper 

 were undertaken. 



The apparatus consisted of a cylindrical metal vessel, 6*25 cm. 

 in diameter and 13 cm. high, placed on an insulating stand and 

 kept at a potential of about 300 volts by means of a battery of 

 small storage cells. A stout brass wire fixed in a sulphur plug 

 was used as an electrode and was connected to an electroscope of 

 the type described by C. T. R. Wilson||. Surrounding the sulphur 

 and electrode was a brass tube connected to earth in order to 

 obviate all possibility of leakage from the vessel to the electrode 



* Proc. Roy. Soc. Vol. lxix. p. 278. 



t Phil. Mag. June, 1903. 



X Phys. Rev. xvi. p. 184 ; Phil. Mag. v. p. 419. 



§ Amer. Phys. Soc. Dec. 1902. 



|| Proc. Camb. Phil. Soc. Vol. xn. Pt. ii. p. 135. 



