

HON. K. J. STRUTT ON THE LEAST POTENTIAL DIFFliKKNCl. 



It will thus be seen that, after the gas has been sparked through, its sparking 

 potential is lowered. In this case the minimum was lowered from 326 to 288, and 

 then to 274. 



To see what the effect of further sparking would be, a torrent of sparks was passed 

 through the gas by means of the machine for an hour. The readings then taken 



were : 



4. 



Minimum 26 L 



It will be seen that the minimum has gone down still further. Another three 

 hours' sparking did not make any further difference. The minimum spark potential 

 after this treatment was 262 volts, practically the same as before. Other specimens 

 of helium from which the surplus oxygen had been removed by means of phosphorus 

 (at the ordinary temperature) behaved very similarly. 



It may be remarked that this behaviour of helium is very like that observed by 

 WARBURG in the case of nitrogen which had not been put through any exceptional 

 treatment for the removal of its surplus oxygen : with this difference, however, that 

 he found the cathode faU go up (from 315 to 410 volts) with the sparking, whereas, in 

 my measurements on helium, the spark potential went down. 



It seems not unlikely that if the last traces of oxygen were removed from the 

 helium by means of the sodium-potassium alloy, normal and constant results might be 

 obtained. I hope to examine this point on some future occasion. 



General Conclusions. 



It remains to discuss the results obtained, and to inquire how far they bear out the 

 conclusion that the minimum spark potential is equal to the cathode fall measured 

 over the whole extent of the negative glow in a vacuum tube. 



For this purpose it will be convenient to tabulate the values of these quantities for 

 the various gases side by side. 



