1903.] 



Electrical Conductivity of a Vacuum. 



415 



" The Electrical Conductivity imparted to a Vacuum 1 >y Hot 

 Conductors." By 0. W. RICHARDSON, B.A., B.Sc, Fellow of 

 Trinity College, Cambridge. Communicated by Professor 

 J. J. Thomson, F.R.S. Received February 28— Read March 

 26, 1903. 



(Abstract.) 



The experimental part of this paper is an investigation of the 

 electrical conductivity of the space surrounding hot surfaces of plati- 

 num, carbon, and sodium, at low pressures. In addition, the first 

 portion of the paper is occupied in deducing a theory by which the 

 experimental results are explained. Some of the results that have 

 been obtained with platinum were described in a preliminary note read 

 before the Cambridge Philosophical Society on November 25, 1901. 



The present paper is subdivided as follows : — 



A. Theoretical investigation. 



1. Calculation of the saturation current. 



2. Equilibrium of corpuscles near a hot conducting plane of 



infinite area. 



B. Experimental investigation. 



1. Experiments with platinum. 



2. Experiments with carbon. 



3. Experiments with sodium. 



■C. Conclusion. 



The experiments show that the negative leak from hot wires at low 

 pressures is a definite function of the temperature of the wire, and 

 increases very rapidly as the temperature is raised. Professor McClel- 

 land' 54 ' had previously found that this leak was independent of the 

 pressure at pressures less than - 04 mm., whilst Professor J. J. Thom- 

 son! had shown in addition that the current was carried by corpuscles 

 or electrons. 



The theory here put forward to explain these facts, and those to 

 be described later, is based on the corpuscular theory of conduction 

 in metals. On that view a metal contains a very great number of 

 free corpuscles whose mean free path is comparable with that of a 

 molecule in air at atmospheric pressure. The corpuscles must, there- 

 fore, be moving with a distribution of velocity given by the Boltz- 

 mann-Maxwell law. Since the corpuscles do not escape from the 

 metal at ordinary temperatures, it is evident that there must be a 

 discontinuity of potential at the surface which prevents their escape. 



* ' Camb. Phil. Soc. Proc.,' vol. 10, p. 241, and vol. 11, p. 296. 

 t ' Phil. Mag.,' vol. 43, p. 547. 



