Physics. — "On the sparklng-jwtential of argon-nitrogen mixtures''. 

 Bj Dr. G. Hoi.sT and A. N. Koopmans. (Communicated bj Prof. 

 H. Kamertjngh Onnes). 



(Communicated in the meeting of September 29, 1917). 



1. Introduction. It is well known that the so-called halfwatt-lamps 

 are filled with a gas at a pressure of about an atmosphere in order 

 to obviate the evaporation of the tungsten wire. In lamps with a 

 relatively thick wire, in which the loss of heat to the gas is com- 

 paratively small as against the radiated heat, nitrogen is commonly 

 used, whereas argon is used in lamps with thin wires as its con- 

 ductivity for heat is very much smaller. The substitution of 

 argon for nitrogen produced a new difficulty : the occurrence of 

 electric discharges through the gas. By the addition of small quantities 

 of nitrogen it has been possible to raise the sparking-potential by a 

 considerable amount. ^) 



Our wish to learn more of this influence of nitrogen on the sparking- 

 potential was the origin of the present investigation. A few measu- 

 rements on this subject have been made by Bouty '); he also 

 discovered the great influence of traces of admixtures. 



2. Experimental method and apparatus. The method used by us 

 is based on the drop of potential at the terminals of a condensor 

 connected in parallel to the discharge tube at the moment of passage of 

 the spark (fig. 1). The discharge tube was a large bulb of 12 cms in 

 diameter, with two silver electrodes attached to stiff brass wires. 

 The electrodes had a diameter of 36 rams and their distance was 

 about 6,7 mm. ') The ends which were turned towards each other 

 were spherical with a radius of 10.8 cm. A side-tube was blown 

 to the bulb on which a quartz window was cemented. Through 

 this window a small spark between aluminium electrodes could be 



1) A. E. G. D. R. P. 289543. 



») E. Bouty, Journ. de Phys 4. série, 1904, p. 489. 593. 



^) Owing to a displacement of the electrodes by change of pressure it was 

 necessary to measure the distance of the electrodes each time with a kathelometer. 



