'Discharge of Electricity from Points. 237 



agent. A small quantity of radium bromide, sealed up in a 

 small glass tube, was fixed to the support F' inside the glass 

 jar (the small tube is shown in fig. 1 marked Ra). The 

 ionization produced by this was very small, being insufficient 

 to discharge a gold-leaf electroscope placed near it. It had 

 no appreciable effect on the insulation o£ the gas at potentials 

 lower than the sparking potential. It was sufficient, however, 

 to diminish the lag very considerably, though without sensibly 

 affecting the sparking potential. Thus, after putting in the 

 radium, working with a pressure of 763 mm., the wire and 

 distance being the same as above, small sparks were obtained 

 successively at 3760 volts. 



The results obtained were quite regular apart from the 

 lag, sparking potentials obtained on different days, with and 

 without radium, being quite consistent. With the radium, 

 however, the sparking potentials could be obtained much 

 more easily and quickly, and so radium was used for all the 

 later determinations. 



Great care was exercised to exclude dust and to prevent 

 any large quantities of electricity passing from the point. 

 It is probably due to this that observations repeated after 

 various intervals of time showed good agreement. Small 

 particles of dust undoubtedly have a considerable influence 

 on the sparking potential; and Zeleny has shown that con- 

 siderable alterations in the shape of metal points may be 

 produced by the passage of currents for some time*. This, 

 and the deterioration of the gas, may account for the "ageing" 

 of points. 



4. If the sparking potential is determined by ionization by 

 collision, a discharge should occur whenever the values of the 

 electric force along a line of force satisfy a certain relation. 

 It is assumed, however, that there is already a certain amount 

 of ionization in the gas. In the case of a gas unsubjected to 

 any external ionizing agent, the ionization is very small, and 

 the spark will not pass at the lowest possible potential unless 

 there is a favourable arrangement of ions along the line in 

 which the electric force is greatest. For a discharge between 

 concentric cylinders all radii are equally probable lines of 

 discharge, but in the case of points there is a small region in 

 which the force is greater than in any other. Hence if the 

 ionization is small the probability of there being a favourable 

 distribution of ions is much greater with cylinders than with 

 points, and is greater with large points than with small. If 

 the distribution of ions is unfavourable there will be a lag. 



* J. Zeleny, Phys. Key. vol. xxvi. p. 137 (1008). 



