480 Effect of Screening on Ionisation in Closed Vessels. 



It will be seen that there are large differences, both in the 

 absolute amount of reduction effected and in the ratio of that 

 reduction to the normal ionisation. Similar experiments were 

 tried with an iron screen, the average thickness of which was 

 about 5*5 cm. The same vessels were used and the following 

 results obtained : 



From these figures it appears that the iron screen chosen gave 

 almost the same proportionate reduction as the lead one previously 

 used. The only marked exception is tin, the proportionate re- 

 duction in this case being only 12 °/ as against 22 °/ produced 

 by the lead screen. The observations were repeated but with the 

 same result. 



Incidentally the experiments here described confirm Strutt's* 

 results, as has been pointed out, but their main object was to 

 throw some light on the part played by the metal of the vessel in 

 ionising the gas and the nature of the radiation due to it. The 

 results obtained show that the proportionate reduction due to 

 a given screen is very far from being the same for vessels of 

 different materials. While not finally conclusive, this fact is strong 

 presumptive evidence that the ionisation in a closed vessel is in 

 part produced by a radiation from the walls which is independent 

 of the penetrating radiation from without. This intrinsic radia- 

 tion is great in lead and aluminium and small in iron, zinc, and 

 tin. The subject is being still further investigated by Campbell, 

 a short account of whose results appeared in Nature (March 31st, 

 1904). By varying the volume of his vessels he has been able to 

 determine the amount of the ionisation which is due to the walls 

 and the reduction which this suffers from a lead screen. From 

 unpublished results with which he has kindly furnished me, it 

 appears that in the case of lead and aluminium this radiation 

 from the walls is practically unaffected, while in the case of zinc 

 and tin the reduction is considerable. This confirms the con- 

 clusion arrived at in this paper that the radiation from lead and 

 aluminium is mainly intrinsic, while that from zinc and tin is 

 largely secondary. 



* Loc. cit. 



