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ROYAL SOCIETY OF CANADA 



or other equally steady sources of potential, which would be just suffi- 

 cient to produce discharge. 



The well known phenomenon of delay in the passage of such] 

 discharges, which has been investigated at length by Warburg^ and which 

 is especially marked in the neighbourhood of the critical pressures where 

 discharges are frequently not obtained until ten or fifteen minutes after 

 the application of the requisite voltage, renders the process of taking 

 readings an exceedingly laborious and tedious one. 



The write:r who had recently occasion to obtain such experimental 

 curves during an examination of the spark potentials in different media 

 found that they may be obtained with comparative case and with a great 

 reduction in time if the ordinary procedure be reversed and the spark- 

 ing potentials ascertained by lowering the pressure until the applied 

 potential will just cease to produce discharge instead of keeping the 

 pressure constant and increasing the applied potential until it is just 

 sufficient to break down the gas. 



The following paper contains a short description of the manner in 

 which the method was applied together with a few of the results ob- 

 tained with, discharges in air and hydrogen. 



II. — Apparatus and Method. 



The form of the discharge chamber used in the experiments )is 

 similar to that adopted in Carr's investigation, and is shewn in Fig. I. 

 The electrodes c,d, were supported by ebonite plugs and were kept 



separated by a disc of ebonite provided with small openings > which 

 permitted free communication between the air in the sparking space E 

 and the leading tubes A and B. The electrodes c and d were hollow 

 metallic boxes provided with inlet and egress tubes for the passage of 

 heated liquids and were designed for the purpose of studying the relation 

 borne by the sparking potentials to the temperature of the electrodes. 



The exhaustions were made by means of a Toepler-Hagan mercury 

 pump attached to the apparatus, and the pressures were determined from 

 the readings of a McLeod pressure gauge. 



1 Warburg, Ann. de Phys. Vol. 62, p. 385. 



