Neighbourhood of Electrodes in Point-plane Discharge. 611 



the plate, for a constant current, increased rapidly as E 

 approached P. There was a further effect which unfortunately 

 limited the possibility of obtaining quantitative results, viz., 

 the passage of sparks between P and E at short distances. 

 The critical distance for this effect decreased with decrease 

 in size of P, and by making P very small, readings could 

 have been taken within a few tenths of a millimetre of its 

 surface. In this case, however, the disturbing influence of 

 E would have been so great that even comparative results of 

 positive and negative discharge would not have been reliable. 



The main work was therefore done with P = 0'36 mm. 

 diameter, and E fixed at 2 mm. from P. 



The results may be divided into two groups (a) and (/>). 



(a) No external ions supplied. — For this N was withdrawn, 

 leaving a plate opposite P. 



By taking the potential of E with various currents for a 

 given value of x, and extrapolating to zero current, the 

 potential drop (V ) close to the point, when the latter was 

 about to discharge, was obtained. Various sets of these 

 readings were taken, corresponding to different values of x, 

 ranging from x~0 m 6 cm. to x = 6'"6 cm. 



Curves I. (open circles) show V plotted with x. The curve 

 for P— is drawn to the same scale as that for P + , but is 

 displaced along the V axis, for convenience. The mean 

 values of V + and V — are 1980 and 2270 volts respectively. 



Curves I. 





•, * 







. 







o 



rHeya.tlirt_ 





■ 







• 





m m 







o 







_•_ 



P fr'ai-rl*^ 



^^ 



*S=*Z=Z- 



It will be seen that, for both positive and negative dis- 

 charge, Y is constant for all values of x. This result is 

 consistent with the measurements, by the pull method, of 



