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XI. On the Conductivity of Gases under the Becquerel Rays. 
By Hox. R. J. Strutt, Fellow of Trinity College, Cambridge. 
Communicated, by Lord Rayleigh, F.R.S. 
Eeceived December 15, 1900,—Read February 21 , 1901. 
§ 1. Introduction. 
It has been known almost from the first discovery of the Becquerel Rays that these 
mysterious emanations caused gases through which they passed to conduct electricity. 
But though careful measurements have been made of the relative conductivity* of 
gases under the Rdntgen rays (J. J. Thomson, ‘ Proc. Camh. Phil. Soc.,’ vol. 10, p. 9) 
and under cathode rays (MacLennan, ‘Proc. Roy. Soc,,’ vol. 66, p. 375), little seems 
to have been done in this direction for the Becquerel rays. This paper deals with 
measurements of the kind in questiou, and with the conclusions which may be drawn 
from them. It will be first desirable to state clearly what conditions must be 
complied with in order that the quantities measured may have an intelligible 
meanino^. 
In the first place, the E.M.F. applied to the conducting gas must suffice to 
“ saturate” the current. In other words, it must be so great that a further increase 
will not increase the current a})preciably (see J. J. Thomson, ‘ Phil. Mag.,’ November, 
1896). In the second place, it is essential to make certain that the layer of gas 
enqiloyed is so far rarefied tliat the absorption of the radiation by it is inajipreciable. 
If this condition is not complied with, the layers of gas more remote from source of 
radiation are less powerfully affected by it than the nearer ones. The effective 
strength of the radiation will thus depend on the absorbing power of tlie gas at the 
particular pressure, and the observed ratio of conductivities of two gases at the same 
pressure will not represent the ratio of their conductivities under radiation of a given 
strength. 
It is universally recognised that the conductivity of gases under Rdntgen and 
Becquerel rays is due to the jiroduction in them of positive and negatively charged 
It is hardly necessary to state that the word “ conductivity ” applied to an ionised gas is used in 
quite a different sense from the same word when applied to a metal. The expression “ conductivity ” of 
a gaseous conductor is used to denote the current which it will carry under a saturating electromotive 
force. 
3 T 2 
8.6.1901. 
