416 Dr. Tyndall and Mr. Hughes on Cathode 



ground-glass joint J. The cathode C was hung by a hook F 

 from wire enclosed in a glass sheath H, and the cathode-fall 

 of potential was measured by an exploring wire K consisting 

 of fine platinum wire protruding 1 mm. from the end of a 

 glass sheath. The potential difference between K and C 

 was measured by a Braun electrometer. The distance 

 between K and C could be varied by turning the ground- 

 joint W to which was attached a suitable screw 



glass 

 arrangement 



Fisr. 1. 



Tb Earth. 



Tump 



The anode was in most cases a brass cylinder A, and was- 

 connected to the high potential pole of a battery of small 

 accumulators. In some cases, however, a small aluminium 

 plate was used as anode, in which case the brass cylinder A 

 was replaced by one of glass, to collect most of the deposit 

 and to prevent cathode rays from striking the grease of the 

 ground joint. It was found, however, that under these 

 conditions the deposit collected very readily on the glass- 

 sheath of the cathode, forming a film of metal which in time 

 joined with the cathode in discharging; it was then necessary 

 of course to stop the discharge. It is probable that this 

 deposit on the sheath was due to a repulsion between the 

 negatively-charged particles of disintegration and the charge 

 acquired by the glass surroundings. The cylindrical form of 

 anode greatly diminished this effect and had the additional 

 advantage of ensuring a symmetrical form of discharge. The 

 cathode was earthed through a milliammeter (M.A.), and the 



