r 17G ] 



XX. On the Point-Discharge in Mixtures of Gases. 

 By Kakl Przibram, PIl.D. (Graz)'*. 



THE observations which led to the following investioration 

 were made whilst experimenting on the point-discharge 

 in different gases. The apparatus used in these experiments 

 consisted of a thin platinum wire whose distance from a 

 fixed circular metal plate could be altered at will, the whole 

 being inclosed in a glass vessel, which could be tilled with the 

 gas to be investigated. A Wimshurst machine was used and 

 the potential-difference measured with a Braun's electro- 

 meter. It was found, in accordance with the results of 

 Warburg t, PrechtJ, and others, that the potential-difference 

 of the point-discharge was greater when the point was 

 positive than when it was negative. This holds for air, CO2 

 and Ho- J^n CO2 the potential-difference when the point is 

 positive is considerably greater than in the same case for air, 

 being for instance 4400 volts for air, 5200 for CO2, the dis- 

 tance between the electrodes being in both cases 5 mm. If 

 the point is negative, the potential-difference in COg was 

 found to be slightly less § than in air, 3700 volts for air, 3500 

 for CO2. 



When the apparatus had been filled with CO2, and the gas 

 afterwards replaced by air, then, the point being positive, the 

 electrometer gave readings quite different to those previously 

 obtained in air ; the values for the potential-difference were 

 in these cases always too small. This was found to be due to 

 traces of CO 2 that had not been removed. As one would 

 expect an admixture of CO2 rather to increase the potential- 

 difference, the potential-difference in pure CO2 being higher 

 than that in pure air, the point-discharge in a mixture of air 

 and CO2 in varying proportions was investigated. The vessel 

 used contained a platinum wire of 0*09 mm. diameter fused 

 into a glass tube, so that only a few mm. were left free, inside 

 a platinum cylinder of 54 mm. height and 15 mm. diameter, 

 with a rectangular opening to allow the luminosity at the 

 point to be observed. When the vessel contained air (dried 

 over CaCl2 and freed from dust by a plug of cotton-wool) 

 and the point was positive, the luminosity was restricted to a 

 small star just at the point. But if one now introduced ^ 



* Communicated by Prof. J. J. Thomson, F.E.S. 

 t Ann. der Phys. Neiie Folge, vol. ii. p. 295 (1900). 

 X Ann. der Phys. vol xlix. p. 150 (1893). 



§ Precht (Joe. cit.) found the potential-difference for the negative 

 point-discharge in CO2 slightly greater than in air. 



