Mr Vegard, An experwient on ionisation with y rays. 79 



inside the cylinder, we should get a different amount of ionisation 

 according to the different angle between the two beams. Another 

 way of putting it is to say, that the ionisation with such rays 

 with a continuous wave-front would not be an additive property if 

 the molecules had some axis of ionisation. 



For, as the wave-trains from one source in rapid succession 

 traverse the chamber, these wave-trains would ionise those mole- 

 cules that had a certain direction relative to the pulse, and if now 

 a second source was applied with waves mainly in the same 

 direction, a number of those molecules that otherwise would have 

 been ionised by the second source are already picked up by the 

 first one. 



2. It was from these considerations that I was led to undertake 

 the following experiment, the object of which was to find whether 

 the ionisation by 7 rays was strictly an additive property. 



Experiments on the additivity of ionisation have earlier been 

 made by T. Noda*. In his experiments, however, the two sources 

 gave rays of a different kind, and further one of his sources was 

 radium giving out a mixture of rays, so that these experiments 

 were of no use for deciding this question. 



Description of experiment. 



3. The ionising chamber had the form of a flat cylinder 

 (length 4*9 cm., diam. 15"5 cm.). The ends of the cylinder were 

 made of aluminium plates, the tube itself of a thin aluminium 

 sheet. A circular aluminium plate was fixed inside the cylinder 

 perpendicular to its axis by means of an aluminium rod connected 

 to an electroscope of the Wilson type. The rod was surrounded 

 by a guard-ring which was connected to earth, insulated from the 

 rod by means of sulphur, and fixed to the chamber by a plug of 

 ebonite. The wires leading to the electroscope were surrounded 

 in the usual way by conductors connected to earth. 



The ionising chamber was placed with its axis vertical upon 

 a small wooden bench fastened to the table. The 7 radiation came 

 from two sources of radium ; one of them could be placed in a 

 fixed position, the other on a moveable arm capable of rotating 

 about a vertical axis coinciding with the axis of the cylinder. 

 Thus the angle between the two bundles of rays could be altered. 

 Each piece of radium was placed behind two slits formed by large 

 lead blocks, in such a way that the radiation from each source was 

 mainly restricted to the space between two parallel horizontal 

 planes cutting the ionisation chamber at about equal distances 

 (1 cm.) above and below the inner plate. The distances from the 



* T. Noda, Proc. of Camb. Ph. Soc. Vol. xiii. 1906, p. 356. 



