﻿to maintain a Current in a Gas at Low Pressures. 227 



(\>-jj ampere) had been running for some time, it was found 

 that the sparking potential was considerably changed. It 

 was at first supposed that the gas was being heated up and 

 driven out from between the electrodes. A thermometer, 

 however, placed in close contact with the plates showed not 

 the slightest increase in temperature, and further, all the gas 

 being pumped out to -^ of a millimetre, and fresh gas ad- 

 mitted and adjusted to the same pressure as before, seemed 

 to have no effect on bringing back the sparking potential to 

 its original value. If the plates were left to themselves the 

 sparking potential would gradually return, after about twenty- 

 four hours, to what it was before the current passed. But 

 the most peculiar thing was that while for zinc, running the 

 current raised the sparking potential, for aluminium it lowered 

 it. The following Table shows this effect: — 



Table II. 



Effect of Current on Sparking Potential. 



p = 45/80 mm. Dec. 19, 1905. 



Sp. V. . . Al=436. Zu=380. Ag=458. 



Current (1/4000 amp.) run for 15 min. between Al electrodes. 



p = 47/80, adjusted to 45/80. 



Sp. V. .. Al=392. Zn=362. Ag = 442. 



Allowed to stand o hrs. p then = 45/80. 



Sp. V. . . Al=422. Zn=358. Ag=438. 



Current run for 15 min. between Zn electrodes. 

 p = 47/80 adj usted to 45/80. 



Sp. V. .. Al=446. Zn = 472. Ag=448. 



Allowed to stand 20 min. 



Sp. V. . . Al = 448. Zn = 458. Ag=454. 



Allowed to stand 20 min. 



Sp. V. . . Al = 446. Zn=448. Ag=454. 



It will be noticed that the values of the sparking potentials 

 for all the spark-gaps seems to change in the same direction, 

 an effect undoubtedly due to a change in pressure in all the 

 gaps ; as it is difficult to readjust the pressure instantly, and 

 small variations make large changes in the sparking potentials, 

 it cannot be expected that the other spark potentials should 

 return exactly to their previous values. It will be seen, 

 however, that after running the current between the aluminium 

 plates, the decrease in the sparking potential for these is much 

 greater than for the other two sets, when the current had not 

 been flowing ; while the same is true for the marked increase 



Q2 



