Potential at the Electrodes in Vacuum-tube Discharge. 495 



The disk anode A of polished aluminium (diameter to fit the 

 tube) could be moved to any desired position by the action 

 of a magnet on the iron lug L. By measuring the difference 

 of potential between the inserted wire W and the anode at 

 definite distances, the anode drop and potential gradient in 

 the gas could both be obtained. In seeking the variation of 

 the drop with the force driving the ion into the electrode 

 field, it is obviously necessary that the conditions be so 

 chosen that the potential gradient in the gas possess if pos- 

 sible a constant value up to the boundary of the electrode 

 field. It has been found* that this is not the case at low 

 gas pressures, even though the positive column be unstriated, 

 but that to a certain short distance from the anode the 

 gradient may be very small. This condition is, however, 

 approached as the gas pressure increases, so that at the 

 pressure chosen, 2 mm., the gradient rises to a constant value 

 within a very short distance from the anode. The results 

 obtained, given in Table I. and plotted in fig. 3 (p. 496), show, as 

 expected, a marked increase in the anode drop with the 

 potential gradient. 



Table I. — Variation of Anode Drop with Potential Gradient 



in Positive Column. 



Gas Pressure constant, 2 mm. 





Distance from 



Corre- 





Pot. Grad. ! 





anode to ex- 



sponding 



Drop at 



in Gas 





ploring wire 



P.D. (volts). 



Anode. 



(volts/em.). 





(mm.). 









1st Tube (cliam. 18 mm.)... 







28-4 



28-4 







10 



95-3 





66-9 





20 



160-7 





654 



2nd Tube (diam. 12 mm.) . 



o 



299 



29-9 







20 



1682 





09-2 



3rd Tube (diam. 8 mm.) ... 







33-5 



33-5 







10 



108-9 





75-4 





20 



182-7 





73-8 



Anti-cathode Effects. 

 The phenomena attendant upon the crowding of the nega- 

 tive glow by the walls of the discharge-tube, or other ob- 

 struction, are on the whole complicated. Hittorff found 

 that when the negative glow is restricted in its natural ex- 

 pansion it is accompanied by an increased resistance to the 

 passage of a current through the tube. The idea suggested 

 itself to the writer that this increased resistance is caused by 



* C. A. Skinner, Wied, Ann. lxviii. p. 752 (1899). 



t See J.J. Thomson, Jtec. Res. in Elect, and Mag. p. 162. 



