418 



Dr. Tyndall and Mr. Hughes on Cathode 



A number of similar virgin wires were taken and used 

 as cathodes for varying times. During discharge obser- 

 vations were made on current, cathode, and pressure, and the 

 following facts were very clearly brought out : — 



(1) When the discharge is " abnormal " — that is, when 

 the current is greater than that required to cover the cathode 

 with glow — the cathode-fall for constant curve and pressure 

 rises with time; this will be seen on reference to Table I.,. 

 which shows the results taken with three copper cathodes- 

 A, B, and C, at a pressure of 0*20 mm. and a current o>£ 

 6*15 milliamps. 



Table I. 







Cathode Fall. 





Duration of Discharge 

 in minutes. 

















A. 



B. 



C. 



01 



680 



700 



770 



10 



680 



800 



790 



2-0 



720 



780 



880 



30 



735 



910 



990 



4-0 



880 



990 



1070 



5-0 



920 



1050 



1100 



60 



980 



1100 



1150 



8-0 



1060 



1150 



1210 



10-0 



1120 



1190 



1210 



15-0 





1210 



1220 



20-0 





1280 



1330 



25-0 





1320 



1350 



30-0 





1370 



1390 



At high currents or cathode-falls, oxidation of cathodes- 

 may set in and vitiate the results ; but this was not the case 

 in the experiments cited above, the wire presenting a clean 

 surface of pure copper throughout. Moreover, the same 

 effect was observed to a smaller extent with platinum wires.. 

 The conclusion is that the cathode-fall, which is an important 

 factor in disintegration work, can only be maintained at a 

 constant value for a constant current by varying the pressure. 

 The problem of the influence of factors on disintegration 

 becomes therefore very complex unless the direct effect of 

 pressure is nil, or at any rate small. Granqvist showed that 

 the rate of disintegration increased rapidly with reduction 

 of pressure, but constant cathode-fall was not maintained in 

 his experiments. Both Kohlschutter and Holborn and Austin 

 have assumed that the direct effect of variation of pressure 



