Discharge of Electricity through Gases, 



693 



The potential- differences between the fixed electrodes 

 A, B, D, E, and K when the cathode is at a temperature o£ 



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400 



300 



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1-0 1-5 £-0mm 



Pressure, — ruin, of mercury. 



about 1500° C. are shown in fig. 3 (p. 694) and Table VIII. The 

 fall of potential close to the anode (B— A) decreases steadily 

 with diminishing pressure. The potential-gradient in the 

 positive column (D — B) is approximately proportional to 

 the pressure above 0*5 mm. The gradient at the negative 

 end of the positive column, and including the Faraday dark 

 space (E— D), is very much less than anywhere else in the 

 discharge, and seems almost to vanish at a pressure of about 

 0*65 mm. 



In Tables IX. to XIV. we pass on to the results obtained by 

 pumping down to any required exhaustion and then gradually 

 heating up the cathode, the actual pressure being measured 

 and recorded at intervals. As the volume of the discharge- 

 tube bore but a small ratio to that of the pump cylinder, 

 McLeod gauge, and P 2 5 bulb, the heating of the cathode 

 only produced trifling changes of pressure, and it has been 

 shown already that the cathode fall is hardly affected by 



