460 



Mr. J. B. B. Burke on the 



small holes each 1 mm. in diameter drilled through the plug. 

 These holes were covered with a thin sheet of aluminium such 

 as is used in the production of Lenard rays, about O002 mm. 

 thickness. The aluminium lay flat upon the surface of the 

 brass, which, however, was sunk about 1 mm. or so into the 



Fig-. 5. 



t BRASS PLAT£ 

 / ALUMINUM SH££7 



m 



pluo* or plate, and a brass ring with a narrow rim fitted 

 accurately into this, so that there was perfectly close contact 

 between the aluminium and the brass. The ring was screwed 

 on to the plug. 



"When the pressure on both sides of the plug was the same, 

 and within the limits at which the glow can be produced, it 

 was found that the glow travelled down the tube apparently 

 through the aluminium as Lenard rays do ; and with pro- 

 longed sparking assumed the same intensity throughout the 

 whole length of the tube. 



When the pressure between the two sides of the tube was 

 altered so that the pressure in the tube on the far side of the 

 aluminium was greater than that in the bulb by more than 

 half a millimetre, the glow did not get through. 



This accounts for the negative result obtained in trying to 

 o-et the glow out into the air of the room through aluminium 

 windows placed at two different parts of the bulb itself, one in 

 a line with the axis of the tube, the other in the immediate 

 neighbourhood of the ring-discharge. ^ 



These experiments led to the substitution of a solid brass 

 plate in the place of the sheet of aluminium ; and, strange to 

 state, it was found that precisely similar effects took place. 



On carefully watching the tubes on the far side of the bulb 

 it was found that a very slight and feeble discharge which 

 extended down the whole length of the tube into the pump 

 sometimes accompanied the ring-discharge ; but by covering 

 the tube from the bulb to the brass plug with tinfoil put to 

 earth it was apparently stopped. 



Nevertheless it was by no means certain that the brass 

 plug, though earthed, did not act somewhat as a secondary 

 electrode whenever the ring-discharge passed through the 

 bulb, in consequence of the powerful discharge in the bulb, 

 just as it is possible to get a discharge in a tube with an 

 electrode put to earth if the rest of the tube can be raised to a 



