78 Mr Vegard, An experiment on ionisation with 7 o^ays. 



An experiment on ionisation with 7 rays. By L. Vegard, 

 Cand. real, Universitetsstipendiat of tlie University of Christiania. 

 (Communicated by Professor Sir J. J. Thomson.) 



[Bead 8 March 1909.] 



1. When a gas is ionised by Rontgeu or 7 rays, we are led to 

 consider the action of ether waves or pulses upon the molecules of 

 a gas. In order to explain the small amount of ionisation it is 

 necessary to assume that the various molecules are in a different 

 position relative to the pulse. Several possibilities are discussed 

 by Sir J. J. Thomson in his book. Conduction of Electricity through 

 Gases. As a way in which the molecules can have a different 

 position relative to the pulse, he mentions the possibility that the 

 wave-front has a structure. There is, however, another manner 

 in which a different relative position could arise even when we 

 assume a continuous wave-front, namely if the molecule had 

 certain directions of ionisation. 



The directions of the electric forces in a set of pulses travelling 

 in the same direction must lie within narrow limits, e.g., all nearly 

 parallel or perpendicular to a certain line. Now for a molecule to 

 become ionised it might be required that the electric force should 

 act in a certain direction relatively to some axis in the molecule, or 

 it might require a series of pulses along this same direction. 



If such conditions were necessary for ionisation, it would 

 naturally cut down to a great extent the number of molecules 

 ionised by the rays. It might possibly not be the only condition, 

 but if in general n conditions, 1, 2, 3, . . ., n, were necessary for ionisa- 

 tion, and if the probability for each of them were ^^i, p^, ... , J^n, the 

 ionisation would be proportional to the product pi,po,ps, ...,pn- 

 We see from this, that if the probabiHty for the occurrence of one 

 of the conditions is altered a certain number of per cent., the total 

 amount of ionisation would be altered in the same proportion. 

 Thus if the action of the pulse within a certain angle relative to 

 some axis of the molecule were a necessary condition, the change 

 in the probability of the occurrence of this condition would affect 

 the total amount of ionisation proportionally. 



If now the rays had a continuous wave-front, we might be able 

 to effect a detectable change in the probability of this condition. 

 Suppose a cylindrical ionisation chamber is traversed by a beam 

 of parallel rays perpendicular to the axis of the cylinder. If now 

 we add another beam of rays traversing the same space of air 



