LMPAETED TO A VACUUM BY HOT CONDUCTORS. 
549 
temperature of the hot conductor to a sufficient extent. The experiments also seem 
to show that as far as electrical conductivity is concerned, the boundary of a hot 
conductor is an indefinite term; since so many of the corpuscles pass freely to the 
outside of the metal it is evident that at high enough temperatures quite an 
appreciable fraction of the current along a wire must be carried by the ions in the 
surrounding space. 
In conclusion, I wish to thank Professor Thomson for his never-ftiiling advice and 
encouragement during the course of these experiments, which were carried out in the 
Cavendish Laboratory. 
[Note, added June 30, 1903.—Since the present paper was written Mr. H, A. Wilson 
has made some experiments on the conductivity produced Ijy hot platinum at low 
pressures, in which he finds that by carefully treating the wire the current can be 
reduced to about one two hundred thousandth of the value found by the author at 
the same temperature. Mr. Wilson also shows that the current is greatly increased 
by admitting hydrogen into the apparatus, and concludes that the high values found 
in this paper are due to hydrogen absorbed by the wire, which is only given off very 
slowly, if at all, by mere heating. 
These results are not, however, inconsistent with the view that the effects are due 
to electrons shot out of the metal. To obtain the observed facts we have only to 
suppose that the occlusion of hydrogen diminishes the work which a corpuscle has to 
do in escaping from the surface. Mr. Wilson’s own results are in agreement with 
this theory, for he finds that raising the pressure of hydrogen from 0 to 133 millims. 
reduces the value of the work in question in the ratio of 155 to 36. It might be 
thought that on this view the constant A which deteimines the number of ions per 
cub. centim. of platinum should be independent of the pressure of the hydrogen 
outside. The numbers found by Mr. Wilson do not support this supposition, but the 
numerous practical and theoretical difficulties demand that little weight should be 
attached to the difterence. 
It is possible that Mr. Wilson’s process of removing hydrogen from a wire by 
oxidation may, as it were, overshoot the mark by leaving an electrical double layer 
with negatively charged oxygen on the outside. Such a double layer would increase 
the work for the coi'puscles to get out and so would reduce the leak in the manner 
observed.] 
