2S2 Prof. A. P. Chattock and Mr. A. M. Tyndall on the 



point, but in this case (y = 1'0) it reached right across from 

 N to P in the ibrm of faintly luminous streamers, which 

 occasionally passed into sparks for currents above about 

 12 microamperes. It is true that 1*0 does not lie between 

 0*3 and 0*15, but the current was evidently on the verge 

 of sparking all along ?/ = l'0, and a very small change in 

 the conditions was probably enough to cause streamers to 

 pass into sparks or sparks into streamers for ?/ = l*Q, 0*5, 

 and 03. Streamers were, in fact, once or twice obtained for 

 y-0-8. 



Values of Ionizing Fields. 



There is no indication in Curves I., as the supply of 

 external ions increases, of any limit to the lowering of the 

 H portions beyond the accidental one of sparking. 



This was at first sight disappointing ; for according to the 

 theory when the field at the point is below/—, it ought not 

 to be possible to obtain luminous discharge, and it seemed 

 unlikely that the positive and negative ions should require 

 such very different fields to ionize in as corresponded with 

 the highest and lowest H curves obtained. 



Now, provided there are enough external ions present to 

 produce a detectable amount of light, the beginnings of glow 

 should occur when, and not before, the field reaches the 

 value /— . 



To test this we measured f at the moment the glow first 

 became visible for a wide range of distances between P and 

 N. Under these conditions it was to be expected that the 

 field in which the glow was first seen would be constant and 

 equal to /— so long as the supply of external ions was 

 sufficient ; but that when the supply fell short the glow 

 produced in this field, though still present, would not be 

 detectable, and the field corresponding to the first visible 

 glow would therefore be higher than/—. 



Further, the field would continue to rise as the external 

 ions became fewer until it reached the value at which 

 ordinary positive point discharge sets in. 



The supply of external ions may be reduced by increasing 

 either x or y. As, however, a sufficient increase of y made 

 it impossible for our wimshurst to produce the highest field 

 at P, we kept y constant at 1'5 cm. and varied x. 



To limit the spreading of the ions from N this needle was 

 made to project from the flat plate R (fig. 1) and x was 

 altered by moving N and R together. In this way we were 

 able to reach values of f which were practically identical 

 with those obtained when N was removed from R. 



