Positive Emission Currents from Hot Platinum. 501 



♦could have access to consists of the rubber stoppers A and 

 A', and the glass tubes, and all of this is shielded from the 

 ions by metal screens. There is thus no possibility of trouble 

 from the insulation charging up. The joints were cemented 

 with sealing-wax and soft wax. There was no difficulty in 

 maintaining a vacuum of 0-0002 mm. or less, as indicated 

 by the McLeod gauge reading. 



Broadly speaking, the time-changes with the new ap- 

 paratus were similar to those observed with the glass tube. 

 A selection from a series of observations is exhibited in 

 fig. 3. The temperature in the different experiments was 



1-0 



o.S 



0.0 



80 !20 {60 200 24-0 280 320 



Potential (Volts) 



400 



equal to 379 + 1° C. The time which elapsed, after the 

 heating commenced, up to the beginning of each curve was 

 approximately as follows : — Curve 1, ; curve 2, 1^ hours ; 

 curve 3, 2 hours ; curve 4, 7 hours ; curve 5, 30 hours ; 

 curve 6, 48 hours. The initial curves are more curved to 

 the current axis than those observed with the glass tube 

 under similar conditions. In curve 1 the points shown 

 thus + were taken with rising, those shown thus ® with 

 falling potentials. When the potential is raised the current 

 is exceptionally large at first and vice versa. The final 

 approximately steady values of the current would lie 

 about half way between the sets of points, as is shown 

 by the curve. The current-E.M.F. curves for low potentials 



