618 



Prof. Skinner on the Drop of Potential 



negative glow, all luminosity disappears from the face of the 

 anode, and does not reappear again to an appreciable extent 

 as it is advanced toward the cathode. When the anode is 

 brought very close to the cathode and the current passed, 

 the discharge changes to the form of a positive column con- 

 stricted to a diameter of two or three millimetres bursting 

 out at some point into the surrounding gas, and the negative- 

 glow spraying out into the space behind the cathode as if 

 reflected out by the anode. 



The drop at an anode of steel in these various positions is 

 given in Tables I. and II., and in the form of a curve in 

 fig. 2. In the figure the ordinates represent the drop in 

 volts ; the abscissas, the corresponding distance of the anode 

 from the cathode in millimetres. Along the positive column 

 the drop at the anode remains constant ; entering the Faraday 

 dark space it increases to a certain maximum value, then falls- 

 rapidly to a value approaching zero in the negative glow. 

 Throughout the negative glow this zero value holds ; but,, 

 entering the cathode dark space, the drop increases very 

 rapidly, as indicated by the curve changing quickly to a nearly 

 vertical direction. Two curves are given to show the effect 

 of gas-pressure — one at 0*5 mm., the other at 0*7 mm, — both 

 with a current of 0*5 milliampere. 



Table I. 



Variation in Drop of Potential at the Anode with Distance 

 from Cathode. 



Gas-pressure 0*7 mm. 



Current 0*5 m.a. 



Distance in 



Drop at Anode in 



Position of Anode 



Millimetres. 



Yolts. 



in Discharge. 



80 



29 



Positive column. 



60 



33 



Faraday dark space. 



53 



38 



>j 



45 



35-5 



» 



38 



31 





33 



29 





24 



26 



!,' 



19 



10 5 





15 



9-8 





12 



4-5 



Negative glow. 



10 



10 





6 



23 





45 



102 



Cathode dark space. 



3*5 



168 





