630 



Prof. Skinner on the Drop of Potential 



possessing discharge-areas of 2*5 and 5*0 sq. cm. respectively, 

 was performed and the variation in the cathode drop with 

 current noted. A second similar experiment was carried out in 

 which electrodes of areas 2*5 and 1*25 sq. cm., respectively, 

 were used. The observations are given in Table VI., plotted in 

 fig. 4. In the second experiment for the same current-density 

 the drop is the same, the drop for the smaller electrode increas- 

 ing with current twice as rapidly as that for the larger. But in 

 the first experiment the larger cathode gives with the same 

 current-density a drop five per cent, greater than the smaller. 

 This difference may be due to the area of the larger electrode 

 being comparable in dimensions with the cross-sectional area 

 of the discharge-tube. The difference is not so large but that 

 the drop at the cathode may be designated as a linear function 

 of the current density. 



Table VI. 



Drop of Potential at Cathode with Discharge Surface of 

 Different Magnitudes. 



(Cathode of Steel.) 



Current 



(Milli- 



anips). 



Gas-pressure . 



1*0 mm. 



Gas-pressure . 



0*5 mm. 



Dis. Surface 

 2'5 sq. cm. 



Dis. Surface 

 5 - sq. cm. 



Dis. Surface 

 2*5 sq. cm. 



Dis. Surface 

 1*25 sq. cm. 



Current- 

 density. 



Drop 



(Volts). 



Current-! Drop 

 density. (Volts). 



! 



1 



Current- 

 density. 



Drop 



(Volts). 



Current- 

 density. 



Drop 



(Volts). 



0-5 









... | 







0-4 



320 



1-0 



0-4 



280 



0-2 



260 



0-4 



310 



0-8 



360 



1-5 



0-6 



295 



0-3 



275 ! 



06 



330 



1-2 



410 



2-0 



0-8 



315 



0-4 



280 



0-8 



355 



1-6 



465 



2-5 







... 





10 



365 







3-0 



1-2 



345 



0-6 



304 



1-2 



405 







4-0 



... 





0-8 ' 325 











Effect of Kind of Metal on Drop at Cathode. — To study 

 this, one electrode was of steel (area 2*5), the other of 

 aluminium (area 3 sq. cm.). In the same gas observations 

 were made, first with one as cathode, then the other. (The 



