THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



7 X «$> 



JUNE 19 



LX. Ihe Variation of the Positive Emission Currents from 

 Hot Platinum ivith the Applied Potential Difference. By 

 0. W. Richardson, F.R.S., Wheatstone Professor of 

 Physics, University of London, King's College, and 

 Charles Shearp, Professor of Applied Optics, Ohio 

 State University *. 



I^HE incentive to the present investigation was an obser- 

 vation recorded by one o£ the authors f to the effect 

 that when a fresh metal wire is positively charged and 

 heated in a vacuum, the relation between current and 

 electromotive force, when due allowance is made for the 

 decay of the current with time, is approximately linear, 

 although it is known that no measurable quantity of nega- 

 tive ions is emitted by the wire at the temperature of the 

 experiment. Since one would expect saturation to occur 

 with a quite small potential under these circumstances, this 

 phenomenon seemed to call for further examination. 



The experiments have been made with two forms of testing- 

 vessel : — (1) A glass tube in which the electrodes are a loop 

 of heated platinum wire and a platinum plate. This tube is 

 identical with the one figured in Phil. Trans. A. vol. ccvii. 

 p. 4, fig. 1 (1906). (2) A brass cylinder with an axial 

 platinum wire. The wires were heated electrically, the 

 temperature being controlled and 'measured by placing 

 the wires in one arm of a WheatstoneV bridge, which was 



* Communicated by the Authors. 



t 0. W. Richardson, Phil. Mag. vol. vi. p. 80 (1903). 



Phil. Mag. S. 6. Vol. 31. No. 186. June 1916. 2 L C 



