﻿222 Mr. J. A. Brown on the Potential required 



potential to the potential required to maintain the current 

 would be very small even if nothing else entered to affect 

 the results. The curves found show, however, that there is such 

 a disturbing effect, and the cause is not difficult to see. 



The heat energy generated by the current must be con- 

 siderable, and the presumption is that it will in some way 

 show itself in the action of the discharge. The cause for 

 the noticeable rise of the potential with the current, must 

 then be sought in temperature changes. A direct calculation 

 of the amount of heat energy given to the ions by the passage 

 of the current, in itself is difficult to make on account of the 

 many disturbing influences which tend to lower the tempe- 

 rature. But the rise must be considerable, and, in the course 

 of the present experiments, must have had a modifying in- 

 fluence on the conditions of the gas. Some of the gas must 

 have been driven out through the openings to the spark- 

 chamber, so decreasing the amount of gas between the plates, 

 and consequently raising the potential required to maintain 

 the current. Owing to the small mass of the carriers, they 

 must have given up their heat immediately on striking the 

 massive metal electrodes, and the temperature of the whole 

 mass must have adjusted itself almost at once to the original 

 conditions, the change probably not taking more than a 

 fraction of a second. It will be interesting to test this 

 hypothesis further, and an experiment is being planned to 

 throw more light on the subject. 



On this view, then, the current should heat the gas, driving- 

 some of it out of the chamber — the greater the current, the 

 greater the amount of heating, and consequently the smaller 

 the amount of gas left between the plates. But below the 

 critical pressure the smaller the amount of gas, the higher is 

 the potential required to spark, and hence to maintain a 

 constant current. Therefore just as the curves (fig. 4) show 

 the actual state of affairs to be, the potential would rise as 

 the current was increased. 



At the higher piessures this effect would not be so 

 noticeable, the driving out of the gas in this case tending to 

 lower the sparking potential, and consequently also the 

 potential required to maintain the current, the effect being 

 in the same direction as that caused by the polarization or 

 charge in the gas spoken of before. 



Mention must be made of the explanation given by Stark * 

 in the new edition of Winkelm aim's Handbuck der Pliysik, of 

 similar results obtained by Riecke and others under much 

 * J, Stark, WinkeJmaim's Handbuch der Physik, iv. 1. p. 518. 



