to produce Discharges in Gases at Low Pressures. 493 



thicker supporting disks of brass, and separated by a 

 ring of glass one centimetre thick, an arrangement which 

 eliminated discharge from the backs and edges of the plates. 

 Small holes in the centres of the disks enabled the gas to be 

 pumped through the apparatus. A battery of small storage- 

 cells provided the necessary difference of potential, and the 

 passage of the current was detected by a galvanometer of 

 the D'Arsonval type. The difference of potential was 

 measured by a Kelvin multicellular electrostatic voltmeter 

 reading to (500 volts, in place of the straight glass tubes 

 leading from the spark chamber in Brown's apparatus, capillary 

 tubes were inserted, with bends in them which could be tilled 

 with mercury and used as valves. The gas used was hydrogen. 

 It was allowed to stand for at least a day in a drying-tube 

 before being admitted to the apparatus, and there was another 

 drying-tube in the apparatus near the spark chamber. 



The sparking potential was found for a given pressure by 

 increasing the potential difference until the galvanometer 

 gave a deflexion. The current was then run for a few 

 minutes, with the mercury in the position shown in the figure, 

 so that gas heated between the electrodes would have an 

 opportunity to expand along the tubes into the rest of the 

 apparatus. The mercury was then raised far enough to 

 block the bends in the tubes, and the current instantly shut 

 off. After an interval a second determination of the 

 potential was made. And finally, the valves were opened, 

 the pressure permitted to adjust itself, and the sparking 

 potential again found, the mean of the first and third values 

 being taken as the value under normal conditions. 



The results of this method are shown in Table I. V 1? V 2 , 



Table I. 



V i- 



V,. 



V 8 . 



E. 



P. 



388 



403 



380 



19 



392 



487 



507 



487 



20 



608 



487 



508 



485 



22 (?) 



609 



306 



325 



308 



18 



•61 



333 



338 



334 



5 



•60 



325 



361 



325 



36(?) 



•oo 



41 I 



446 



410 



35 



4*55 



42.5 



453 



426 



27 



4-58 



269 



280 



271 



10 



103 



319 



342 



320 



22(?) 



■63 



320 



342 



321 



21 



•63 



321 



342 



323 



20 



•63 



354 



366 



350 



14 



•58 



350 



362 



348 



13 



'5S 



558 



582 



504 



21 



8"25 



