Field at a Discharging Point. 643 



Supply of Ions from an external source. 



The effect on the critical field of supplying to the point 

 ions o£ a given sign from an external source has been 

 previously studied. 



The external source was another point of smaller radius 

 which sprayed ions on to the discharge-point. It was found 

 that as the distance between the two points was reduced to 

 small values, the critical field fell off rapidly to a small 

 fraction of its normal value. 



Now this would follow directly from the theory of collisions 

 if there was an accompanying increase in the density of the 

 ions supplied from without, and consequently already available 

 as a source of more by collisions. But apparently it was not 

 the case ; the current from the external point was measured 

 at the moment of excitation of glow at the discharge-point, 

 and was also found to fall to small values at short distances. 

 Explanations introducing other factors were therefore tenta- 

 tively advanced. 



The work has since been extended to lower pressures and 

 larger points ; these gave larger areas of glow for purposes 

 of observation. In a few cases, at short distances it was 

 noticed that there was a marked contraction in the area of the 

 glow excited on the discharge-point. Though the evidence 

 is not quite complete, it seems more than likely that the 

 reduction in exciting current at short distances is due to this 

 contraction; and that although the total current is actually 

 less, there may be at the centre of this region the necessary 

 increase in current density which the simple collision theory 

 would require. It is certain that considerable inaccuracy is 

 introduced in the calculation of the field when the two points 

 are close together. This is the simplest though not the only 

 explanation of the fact that in some cases values of field at a 

 negative point have apparently been obtained lower than 

 100 E.S. unit, which is the accepted minimum for appreciable 

 ionization by positive ions. 



Summary. 



I. It has been shown theoretically by Townsend that if X 

 is the critical field — namely, the field at the start or cessation 

 of discharge — at the surface of a point of radius a, at a 

 pressure^, then Xa is constant so long as ap is constant. 

 Direct determinations of the critical field by measurement 

 of the mechanical force on a hemispherical point are shown 

 to be in complete agreement with theorv. 



2 T 2 



