Discharge of Electricity from Points, 239 



take place it is necessary that the charge in the gas should 

 come into effect, and there must be a balance between its 

 effect and the tendency to increase of ionization. Even 

 when this state of equilibrium is attained it may be unstable; 

 that is to say, a small increase or decrease of potential may 

 cause the discharge to increase to a discharging spark or to 

 cease altogether. In the present investigation it was usually 

 found that in cases where there were glow-discharges 

 at high pressures and sparks at low pressures, there was 

 some intermediate pressure where it was possible to obtain 

 both glow or spark discharges at the same potential. Or 

 a glow discharge would suddenly terminate with a spark. 

 Tins theory also gives an explanation of those cases, noted 

 by various experimenters*, where it was found possible to 

 obtain steady currents above a certain magnitude but not 

 below. 



It appears from the above considerations that it is not 

 possible to transform a discontinuous discharge into a con- 

 tinuous discharge by merely reducing the external capacity. 

 In a discontinuous discharge the amount of electricity 

 passing on each occasion is reduced by reducing the capacity; 

 but the tendency to become discontinuous is independent of 

 the capacity. If the system is continuously supplied with 

 electricity, the interval of time between successive discharges 

 may be extremely small. Thus Zelenyf, working with 

 liquid points, found that it was possible to have discharges 

 consisting of small impulses which follow each other so 

 rapidly that even a telephone fails to detect the dis- 

 continuity. 



6. If a be the radius of a wire and d the distance between 

 its point and the plane, the sparking potential for a pressure 



p is the same as that for a wire of radius y and distance T 



with a pressure hp. Thus if - is kept constant the sparking 



potentials for different values of a will be the same if the 

 pressure is adjusted to keep ap constant. In the present 

 investigation, three different silver wires were used for the 

 point : their diameters were 0*5, l'O, 1"5 millimetres respec- 

 tively, and the ends were rounded approximately to hemi- 

 spheres. Each of these wires was adjusted to give the three 



values 10, 20, 30 to - 9 and the sparking potentials were 



measured for positive and negative discharges for various 



* A. P. Chattock, Phil. Mag. [5] xxxii. p. 295 (1891). 

 t J. Zeleny, Phys. Rev. n. e. iii. p. 88, Feb. 1914. 



