to produce Discharges in Gases at Low Pressures. 481 



for subsequent sparks at the same pressure, and some experi- 

 ments were made to determine the effect of previous dis- 

 charge on the sparking potential. Different lengths o£ time 

 were allowed to elapse, varying from a few seconds to two 

 hours, and slight variations in the sparking potential were 

 observed ; but within the limits of time actually used in the 

 main experiments, no regular variation of the potential was 

 observed. 



The results of the experiments showed that the sparking 

 potential assumed quite different values according as the 

 inner cylinder was positive or negative. In every case two 

 distinct curves were obtained which, in all but the largest 

 diameters, crossed near the critical pressure. For air the 

 critical pressure varied from "1 to '7 mm , according to the 

 diameter of the inner cylinder, and for hydrogen it varied 

 from '3 to 1*1 mm. ; while the point of crossing of the 

 curves varied only from '1 to '2 mm. and from *3 to *5 mm. 

 for air and hydrogen respectively. As the diameter of the 

 inner cylinder increased, the position of the positive and 

 negative curves shifted so that the point of crossing of the 

 two would be relatively nearer to that part of the curve 

 which corresponded to the lowest values of the pressure, the 

 curves gradually approached along their whole length, and, 

 for the largest diameters employed, the two curves seemed 

 to practically coincide as the pressure receded from the 

 critical value. It is of course to be expected that the two 

 curves would approach as the diameter increased, since for 

 parallel plates they must completely coincide. 



The minimum sparking potential was always lower when 

 the inner cylinder was negative than when it was positive, 

 though the difference was less for the large diameters. 

 When the inner cylinder was positive, the minimum sparking 

 potential was higher than for parallel plates ; and when the 

 inner cylinder was negative, the minimum sparking poten- 

 tial was in most cases lower than for parallel plates. The 

 highest values obtained were 463 volts for air and 335 volts 

 for hydrogen ; the lowest were 311 volts for air and 240 volts 

 for hydrogen. 



Fig. 2 shows two typical pairs of curves in detail, together 

 with curves calculated for parallel plate electrodes at the 

 given distances apart in air and in hydrogen. The curves 

 AA' are for air, with a brass rod 3*23 mm. in diameter as 

 the inner electrode. For curves BB' the gas was hydrogen, 

 and the inner electrode was a brass tube 8 mm. in diameter. 

 The inner electrode was positive for A and B, negative for 



