278 Prof. A. P. Chattock and Mr. A. M. Tyndall on the 



In what follows /+ stands for the field at an electrified 

 point in which positive ions are able to ionize, and/— for 

 the corresponding field for negative ions ; /+ having different 

 values according as the positive ions produce others in the 

 gas or at the metal surface. 



Suppose now that to a charged point ions of opposite sign 

 to itself are supplied in considerable quantities from some 

 source in its neighbourhood. We may call such ions 

 external ions. 



With a positively charged point there are three distinct 

 cases that may occur : — 



{a) If the external ions find a field at the point which 

 is less than /'— they will simply pass to the point and 

 give rise to a current ft om it equal to that which they 

 themselves carry. 



(l>) If the field lies between /— and /+ , each external 

 ion will produce several more before reaching the 

 point, and the current resulting may be a considerable 

 multiple of that carried by the external ions. 



(c) If the field exceeds /+ the double ionization by both 

 positive and negative ions will accompany the ion- 

 ization by the external ions, and a current due to 

 ordinary positive point discharge will be added to that 

 of the external ions. 



With a negatively charged point these cases reduce 

 themselves to a and c, the signs 4- and — being interchanged : 

 b does not occur because /+ is greater than /— and the 

 external ions are positively charged. 



If the appearance of light at the point is to be taken as 

 indicating ionization there, it will follow that for a the point 

 will be dark, while for b and c it will glow. 



It is to be understood in the above, that the initially 

 ionized molecules are too few in number to be taken account 

 of in comparison with the external ions supplied. 



These principles are illustrated in a general way by experi- 

 ments on discharge between two points, made some years 

 ago, which we have lately repeated and extended, and of 

 which the following is an account : — 



A horizontal platinum w^ire P (fig. 1) with its end rounded 

 to a hemisphere in the blowpipe was suspended so that it 

 protruded through a hole in a vertical metal plate Q. 



