420 
Proceedings of Royal Society of Edinburgh. [sess. 
Glass 3 mms. thick did not entirely stop the deviation ; it reduced the 
deviation in the first minute, however, to J of the amount obtained 
with only air between the uranium and the outside wall of tinfoil. A 
copper screen, 0*24 mm. in thickness, reduced the rate to J ; two 
copper screens, total thickness 0*48 mm., reduced it to ; three 
copper screens reduced it to A mica screen did not reduce 
the rate at all. A zinc screen, 0*235 mm. thick, reduced it to 
Two zinc screens, total thickness 0*47 mm., reduced it to Paraffin^ 
3 mms. thick, when placed between the two mutually insulated 
metals, stopped the deviation to the conductance-zero. 
§ 28. Conductance-zero at different distances. 
In the experiments described in the preceding section, the dis- 
tances between the two mutually insulated metals was 2 crus.. 
To observe the conductance-zero at different distances an alumi- 
nium box connected to sheaths was substituted in place of the 
tinfoil one, and oxidised copper insulated on a glass stem inside 
it. As before, with the tinfoil box, the uranium was placed out- 
side the aluminium box, about 5 mms. from the end, to which the 
oxidised copper was kept parallel. The distance of the oxidised 
copper could be varied by moving the glass rod to which it was 
attached. The results obtained were as follows : — 
§ 29. Comparison of uranium-conductance-zero with water-arc- 
zero. 
In the first arrangement for measuring the conductance-zero in 
§25 we had discs of uranium and aluminium separated by air. 
We varied this by placing the uranium so close to the insulated 
aluminium that it could be brought into electric connection with 
it by a drop of water. The deviation obtained in the electrometer 
by this means was always in the same direction as the uranium- 
conductance-zero between the surfaces when dry, and was usually 
smaller in magnitude. For instance, with aluminium-air-uranium 
Distance in cms. 
Potential differences 
1*5 
4*0 
0*5 
8*0 
2-0 
in volts. 
+ 0*97 
+ 0*98 
+ 0*96 
+ 1*03 
+ 0-95 
