﻿to maintain a Current in a Gas at Low Pressures. 223 



less definite or determined conditions. He explains the ob- 

 served phenomena by saying that the nature of the curve is 

 determined by the predominance in the tube of one or 

 another of the characteristic parts that are associated with 

 the discharge through gases at low pressures. In the '" dark 

 space," when the current increases, the potential rises, the 

 same effect being observed in the space of the "cathode fall 

 of potential," provided that this is no longer normal. /'. e. 

 that the glow covers the whole surface of the cathode, so 

 that it cannot expand any further. Before that, the cathode 

 fall is independent of the current as well as of the pressure. 

 In the "positive column/' on the other hand, the potential 

 decreases with a growing current. A perfectly general 

 carve, connecting the current and the potential required to 

 maintain it, would be given by fig. 8. Starting at small 



Eur. 8. 



^ z 



currents, the potential would at first rise with the current, 

 the dark space having the preponderating influence. Reach- 

 ing a maximum, the positive column would begin to encroach 

 upon the dark space until it would overbalance it, and the 

 potential would begin to fall. Hitherto the cathode fall has 

 been constant, but it now begins to rise, so that the potential 

 goes up again. Not all of these conditions, he declares, will 

 be observed at a given pressure. For high pressures the 

 dark space will predominate ; for smaller pressures, but 

 above one millimetre, the second characteristic part of the 

 curve will be observed, while finally, for very low pressures, 

 the cathode fall would alone determine the nature of the 

 curve. 



This representation, which is framed to account for the 

 observed tacts, cannot be said to be entirely satisfactory. 

 The cathode fall has been shown to equal the minimum 

 sparking potential, so that at the critical pressure the cathode 



