Section III., 1909. [ lOl ] Trans. R. S. C. 



VII. — On an Improvement in the Method of Determining Minimum Spark 



Potentials. 



By H. S. FlERHELLER^ B.A.Sc. 



(Communicated by Professor J. C. McLennan, and read before the Royal 

 Society of Canada on May 26, 1909.) 



Spark discharge in gases lias long commanded the attention o£ 

 investigators and from the experimental data which has been accumu- 

 lated, especially through the researches of Paschen,i Peace,^ Strutt,^ 

 Carr/ and others, the general laws governing such discharge have come 

 to be fully established and widely known. 



The mechanism of such discharge, especially under certain definite 

 conditions, has also been the subject of a minute enquiry by Townsend ^ 

 and has been shewn by this investigator to be capable of a very simple 

 explanation on the basis of the theory of collisions by gaseous ions. 



Of the different phases of the spark discharge which have been 

 studied, that dealing with the potentials required to produce spark 

 discharges in gases at different pressures has been given the most careful 

 attention. 



It has been shewn when starting with a gas at atmospheric or 

 higher pressures and a constant spark length that the sparking potential 

 decreases as the pressure is lowered in such a way that the 

 changes in the sparking potential are almost directly proportional 

 to changes in the pressure of the gas, and further that as the 

 pressure in the gas is progressively diminished the sparking po- 

 tentials ultimately reach a minimum value at a certain critical 

 pressure which varies with the conditions of the discharge. Fur- 

 ther diminution in the pressure is always accompanied by a rapid in- 

 crease in the magnitude of the sparking potentials. This relationship 

 between the spark potentials and the pressure of a gas has been elabor- 

 ately worked out by Carr (loc. cit) for uniform electric fields in air and 

 other gases with different spa,rk lengths, and a curve which is 

 typical of his results and which illustrates the law of discharge under 

 those conditions is shewn in Fig. III. 



The results from which such curves are drawn have hitherto been 

 obtained by exhausting the discharge chamber to different pressures, and 

 then ascertaining by trial the voltage obtained from small storage cells 



1 Paschen, Ann. de Phys. Vol. 37, p. 69. i 



2 Peace, Proc. Roy. See. Vol. 52, p- ^9. 



3 Strutt, Phil. Trans. A. Vol. 193, p. 377. 



4 Carr, Phil. Trans. A. Vol. 211, p. 403. 



B Townsend, Phil, Phil. Mag. (6) 6, p. 598, 1903. 



