SectionJII., 1908. [ 85 ] Trams. R.S. C. 



Vir. — Oti< Variations in the Conductivity of Air enclosed in Metallic 



Receivers. 



By C. S. Wright, B.A., 1851 Exhibition Scholar, University of Toronto. 

 (Communicated by Professor J. C. McLennan, and read May 26th, 1908.) 



I. — Introduction. 



In a paper in the Philosophical Magazine of December, 1907, 

 Professor McLennan records some observations made on the ionization 

 of air enclosed in cylindrical receivers of lead, zinc, and aluminium. 

 For " q," the number of ions generated per cc. per sec, in these receiv- 

 ers he obtained the value 15, when they were made of zinc and alu- 

 minium ; while with lead, values were found ranging all the way from 

 23 to 160 ions per cc. per sec, depending on the sample of lead from 

 which the cylinder was made. From these results. Prof. McLennan 

 drew the conclusion that ordinary commercial lead contained in general 

 varying amounts of some active impurity. 



From these and other experiments he pointed out also, that four 

 possible causes must be considered as contributing to the ionization in 

 the cylinders, viz.: — (1) penetrating radiation from the earth, (2) 

 secondary rays excited by this type in the metal of the receivers, 

 (3) radioactive impurities present in the metals, and (4) a possible 

 intrinsic radiation from the metals themselves. 



In view of the theoretical importance of ascertaining whether 

 metals generally possessed any specific activity it was thought highly 

 desirable to endeavour to obtain metals as free as possible from active 

 impurities, and also to take observations on the conductivity of air 

 enclosed in vessels made from them in localities and under conditions 

 in which the penetrating radiation from the earth and the accompanying 

 secondary radiation excited by it in the receiver, was very largely cut 

 off, or at least reduced to a minimum. 



Several attempts had been made during the last few years to find 

 some efficient screen for the earth's radiation. Cooke, wliile making 

 some measurements on the conductivity of the air enclosed in a brass 

 cylinder, found a decrease of about 30 per cent when the cylinder was 

 completely surrounded by large masses of lead. Shortly afterwards 

 Elster and GreiteP observed a fall of 28 per cent in the conductivity 



^Elster and Geitel, Phys. Zeit., Nov. 1, 1905, p. 753- 



