Neighbourhood of Electrodes in Point -plane Discharge. 617 



in the potential of the discharge point necessary to maintain 

 the same current as before: this difference gives the original 

 potential of the equipotential surface. From curves of dis- 

 charge-potential plotted with current, previously obtained 

 for different distances between point and plate, Hovda con- 

 structed potential gradient curves, on the assumption that 

 each equipotential surface may, in this case, be replaced by 

 a plane. (Table I., in the present paper, gives a rough in- 

 dication of how near to the point this assumption holds good.) 

 This method throws no light on the nature of the drop at the 

 plate. At present, therefore, the cause of this drop must be 

 left uncertain, though probably the evidence is somewhat in 

 favour of a disturbance effect. 



Whatever may be its cause, if the effect is constant 

 throughout the region near the plate, the field at the plate 

 may be obtained from the potential gradient curves, by sub- 

 tracting the observed sudden potential drop from the other 

 readings in this region. Although the assumption is an 

 arbitrary one, values of the field (/), of at any rate the right 

 order of magnitude, were obtained in this way and tabulated 

 for use in future work on point discharge (Table I.). 



Table I. 



P positive. 



P negative. 



X. 



| 



(mm.) 



(voi.ts/cm.) 



7-0 



4060 



15-5 



2670 



25-5 



1830 



35-4 



1640 



47-5 



1370 



69-0 



1250 



X. 



y: 



(mm.) 



(volts/cm.) 



73 



3670 



141 



2410 



25-5 



1690 



353 



1480 



680 



1000 



It is seen that the field increases as the distance between 

 the electrodes decreases ; this is due to the fact that, the 

 field at the point being constant, the spread of the lines of 

 force, and of the current, decreases as the distance decreases, 

 and that therefore the field at the centre of the plate is 

 increased. 



Curves were obtained for different currents also (for 



