HON. R. J. STRUTT ON THE LEAST POTENTIAL DIFFEUENCE 



more easily than does air. With a given E.M.F. ten times as long a spark was 

 obtained in helium as in air. This is a very striking result. It seemed, therefore, 

 well worth while to investigate the behaviour of helium as regards its spark potential 

 in the same way as the other gases. 



For the method of preparing this gas I must refer to a paper " On the Discharge 

 of Electricity through Argon and Helium," which I hope shortly to publish, where it 

 is described in detail. I merely state here that it was extracted from monazite by 

 means of strong sulphuric acid, and that nitrogen and other impurities were removed 

 from it by mixing with oxygen and exposing it to the action of electric sparks in 

 presence of caustic alkali. The oxygen was then removed by suitable absorbents. 



Although all the care I was able to give was spent on the purification of the gas, 

 some cause, the nature of which remains a mystery, made the spark potential 

 variable. It was found that if a specimen of helium remained in the apparatus, and 

 measurements of its spark potential were taken at intervals, the value of this 

 quantity went down. As an example of this, I will give a series of measurements 

 made on a sample of helium from which the surplus oxygen had been removed by the 

 copper-ammonia method of HEMPEL.* 



Diagram No. 4. Helium. 



700 



eo 



40 ao 



Pre&sure. mm. of Mercury. 



60 



Any gaseous ammonia which might remain in the helium was removed by means of 

 dilute sulphuric acid. The helium was dried by phosphoric anhydride, and examined 



' This method consists in bringing the gas into contact with metallic copper at the ordinary 

 temperature, the copper having just been washed free from oxide by a solution of ammonia to which 

 some ammonium carbonate has been added. See HEMPEL, ' Methods of Gas Analysis,' p. 126. 



