504 
MR. H. A. WILSON ON THE ELECTRICAL CONDUCTIVITY 
at the centre, but it remained visibly hot even at 9 or 10 centims. above the lower 
electrode. It appeared probable that the cooling of the upper electrode might affect 
the observed current, and to test this point an electrode was constructed similar to 
those already described, but so arranged that the grating of platinum wires could be 
heated by passing an electric current through it. The grating was insulated by 
strips of mica, and the battery used to heat up the grating-was insulated, and one 
end of the grating wire connected to the brass disk supporting it. 
It was found that when the up23er electrode was kept at a bright red heat in this 
way the current, with 200 cells, was independent of the position of the upper elec¬ 
trode up to 8 centims. above the lower electrode. The following table gives some of 
the numbers obtained with 200 cells. 
Distance between the 
Electrodes. 
Current. 
Upper Electrode not 
heated. 
Upper Electrode heated. 
centims. 
1-3 
235 
235 
3-0 
236 
234 
4-2 
180 
230 
6-2 
18 
227 
8-0 
10 
235 
Diagram No. 3 shows the change in the relation of the current to the E.M.F. when 
the upper electrode is heated in this way. 
Thus it appears that keeping the upper electrode hot enables the current to attain 
its saturation value with a much smaller E.M.F. than is necessary when the electrode 
is not specially heated. 
Diagram No. 4 shows the relation between the current and E.M.F. for several 
positions of the upper electrode when this is negatively charged, the lower electrode 
being in the same position in each case as before. 
