510 
HON. R. J. STRUTT ON THE CONDUCTIVITY 
Uranium, as well as radium, gives off two types of radiation, one much more 
penetrating than the other. (See Rutherford, ‘Phil. Mag.,’ January, 1899.) 
Becquerel has shown (‘ Comptes Bendus,’ vol. 130, p. 1583) that some at least of 
the uranium rays are deflected by the magnet, hut he does not seem to have made 
any experiments to decide whether both types are so, or only one. In fact, the 
radiation is so feeble as to make such experiments very difficult. But it seems 
probable that, as in the case of radium, the penetrating type of rays are deflected, 
while tlie others are not so. 
My experiments were exclusively concerned with the rays from the bare compound, 
which consist for the most part of the easily absorbed type. Only a small fraction of 
the total conductivity is due to the other kind, so small, in fact, as to make any 
determination of the conductivities difficidt, unless some more powerful preparation 
than mine were available. The experiments were accordingly confined to the easily 
absorbed type of radiation. 
§ 3. Method of Experimenting. 
The gas of which tlie conductivity was to be determined occupied the space 
between two parallel plate electrodes. Oiie of these was maintained at a high 
potential, the other connected to the one pair of cpiadrants of an electrometer, the 
other pair being to earth. The rate of movement of the needle then gave a measui-e 
of the current through the gas. 
When the rays which had penetrated through a ccmsiderable tlnckness of metal 
were to be investigated, the arrangement was as follows :— 
Tlie gas of which the conductivity was to be determined was contained in the 
apparatus represented in section by fig. 1. It consists of an air-tight cylindrical 
brass box «, aliout 1 inch long, 3 inches diameter, provided with a bottom of thin 
copper sheet 6, soldered on, through which the rays penetrated into it. The bottom 
was ’007 centim. thick. Inside this cylinder, and parallel to its ends, was the 
insulated metal plate/i carried on tlie rod g. This plate was used as the lov^-jiotential 
electrode. 
The arrangements for insulating the plate f from the outside cylindrical brass box, 
which served as the high potential electrode, were somewhat special. 
At e e is seen the brass neck of the main vessel. Inside it is fixed an ebonite 
collar /;, and inside this again a second Iwass tnbe h. This inside brass tube carries 
tlie ebonite stopper /, up the middle of which the brass wire g passes. This latter 
cari'ies the electrode /’ and is connected outside to the electrometer. The intermediate 
brass tulie h is put to eartli. Finally, the outside vessel is put to the + terminal of 
a. battery of 100 storage cells, the other terminal of which is to earth. 
The ebonite collar h lias thus to sustain an E.M.F. of 200 volts; but the ebonite 
stopper I has only to sustain 3 or 4 volts, since the jiotential to which f was allowed 
