234 J 



XXVII. The Discharge of Electricity from Points, 

 By P. J. Edmunds, B.A., Queen's College, Oxford*. 



1. rTlHE potential necessary to cause an electric discharge 

 X to pass through air between a pointed conductor 

 and a plane has been measured under various conditions, and 

 empirical formulae have been given which fit the results 

 more or less accurately for particular sizes and shapes of the 

 pointed conductor. 



On the other hand, most of the measurements were made 

 with the air at atmospheric pressure, and the results have 

 not been shown to satisfy any relation deduced from a theory 

 of the mechanism of the discharge. 



On the hypothesis that the discharge through a gas is 

 determined by ionization of the gas by collision, it has been 

 shown f that, if V be the potential difference necessary to 

 cause a discharge between two conductors A and B through 

 a gas at pressure p, then the same potential V will also pro- 

 duce a discharge between two conductors A' and B', obtained 

 from A and B by reducing the linear dimensions in the ratio 

 1/k, the gas being at pressure p' = kp. It has already been 

 found experimentally that this relation is true for the case 

 of cylindrical conductors, the gas being air. The present 

 investigation was primarily undertaken to test the theory 

 for discharges from the hemispherical ends of wires at right 

 angles to a conducting plate. This method of obtaining 

 " points " of a definite shape was first used by Zeleny J. 



2. The discharge apparatus is shown in fig. 1 drawn 



Fig. 1. 



fljMP t( GuAGE • 



DRYING /lf>Pfi*ATU6 



roughly to the scale of 1:6. EE are the walls of a glass 

 cylinder. The bottom was closed by the brass plate Z, which 



* Communicated by Prof. J. S. Townsend, F.R.S. 



t Phil. Mag. May 1914, p. 789. 



j J. Zeleny, Phys. Rev. vol. xxv. p. 317 (1907). 



