ELECTRICAL CONDUCTIVITY OF AIR AND SALT VAPOURS. 
421 
the positive electrode the ciuTeiit continued to increase rapidly witli the E.M.F., and 
was much greater than the current in tire reverse direction, except with very small 
E.M.F.’s. 
Diagram No. 1. 
When no air current was passed tlirough the tid^e the inner electrode was much 
colder than the outside tube, especially near tlie flange, owing to its losing heat l)y 
conduction along the tube supporting it. This diflerence of temperature Ijetween 
the two electrodes enables the great diflerence between tlie two currents to be 
explained in the same way as the similar difterences observed in flames can be 
explained. 
It was shown by Guthrie (‘ Phil. Mag.,’ vol. 46, p. 257, 1873), that a red hot 
metal can retain a negative hut not a positive charge .in air. This is the reverse 
of what occurs in a flame where a negative charge leaks away much more rapidly 
than a positive one. Consequently when tlie hotter tube is positive the current is 
greater than when it is neg-ative. 
The electric intensity is of course greater at the inner tube than it would be 
if both electrodes had the same area, and it is possilile that this circumstance helps 
to exaggerate the diflerence between the two currents. 
An experiment was therefore tried, using a wire 1 millim. in diameter instead 
of the inner tube. The results are shown graphically in diagram No. 2. 
The general character of the curves is the same as before, but the two curi'ents 
at any E.M.F. are much smaller. 
