462 Prof. F. Horton on the Application of Thermionic 



electrodes, and must be large compared with the mean free 

 paths of the ions. It was found by experiment that when 

 the gas pressure p and the electric intensity X between the 

 plates are kept constant, the currents obtained by the action 

 of a constant source of ultra-violet light on the negative 

 electrode are in agreement with the above formula. The 

 values of a and /3 depend upon those of X and p. They were 

 determined by measuring the currents with different dis- 

 tances between the electrodes, keeping X and p constant. 

 It follows from the theory of ionization by collision that 



- and — are functions of — , and this result was verified 

 p p p' 



experimentally by obtaining values of <x and j3 for a wide 



X 



range of values of X and p. The values of — were then 



plotted as abscissae against the corresponding values of 



- ( or -J as ordinates, and it was found that the resulting 



p\ pj 



points all lie on a continuous curve, showing that the 

 functional relation is satisfied. 



The experiments described in the present paper were 

 made two years ago, and were the commencement of an 

 investigation of the ionizing properties of the ions emitted 

 by glowing solids. The completion of these experiments 

 has been suspended while investigations in another direction 

 are being made, but the fact that the new problem is closely 

 related to the present subject has led to the collection of the 

 earlier results. The method of studying ionization by col- 

 lision is, in principle, that of Townsend, but a glowing solid 

 is used as the initial source of negative ions, instead of a zinc 

 plate illuminated by ultra-violet light. The apparatus is 

 represented in fig. 1. It is contained in a glass vessel made 

 in two pieces which fit together by a ground joint rendered 

 air-tight by a mercury seal. The two parallel plate elec- 

 trodes, A and B, are circular in shape and each 1*9 cm. in 

 diameter. The upper plate A is of aluminium 3 mm. thick. 

 It has an aluminium rod 2 mm. in diameter and 4 cm. long 

 screwed into the centre of its upper face, and by means of 

 this rod the plate is suspended. The upper end of the rod 

 is turned down to a small diameter and has a hole drilled 

 transversely through it. A small platinum hook goes 

 through this hole and the hook is attached to a fine platinum 

 wire *025 mm. in diameter. The wire passes through a small 

 platinum tube E, making contact with the right-hand side 

 of it, as seen in the figure. A few centimetres above E the 



