emitted by an Electrical Discharge at Low Pressures. 413 



lower part. Any desired pressure could be obtained by 

 exhausting the apparatus with a mercury pump and then 

 adjusting the level of the mercury in the reservoir. The 

 pressure was read on a McLeod gauge. A drying-tube was 

 also inserted. 



The electrodes were circular disks of aluminium about 

 1*5 cm. in diameter arid distant apart 7*5 cm.; the connecting- 

 wires being fused through the quartz and the seal covered 

 with mercury to prevent any air-leak. The electrodes were 

 first of all subjected to a discharge for a considerable time 

 to drive off all traces of hydrogen and other occluded 

 gases. 



The apparatus is depicted diagram matically in fig. 1. To 



Fur. 1. 



.,<-»<■** 



B_L_B 



A - *JhM |i|j — s 





get a steady discharge a very large number of small storage- 

 cells were connected in series and joined through an ammeter 

 to the terminals of the tube, a large resistance E, being in the 

 circuit. The current was very small (about 4 milliamperes), 

 the storage-cells being able to give this without running 

 down, and the current could be conveniently measured 

 on the sensitive part of a Nalder combined ammeter and 

 voltmeter. 



The drop of potential down the tube was measured by a 

 Kelvin electrostatic voltmeter V. 



The tube was contained in a lead box (shown shaded) 

 having a vertical slit 1 cm. wide cut in the side ; and the 

 tube was mounted in such a way that any portion of its 

 length could be placed in front of this slit.. 



