Discharge from an Electrified Point. 429 



of its value at atmospheric pressure taken arbitrarily ns TOO. 

 These are shown in Table II. for positive discharge. Similar 

 effects were obtained with a negative point. 





Table II. 





V. 





vp. 



755 





1-00 



55 





•98 



40 





-90 



30 





S2 



20 





•74 



10 





•64 



It will be seen that the deviation from vp = l'00 increases 

 with fall of pressure, but whether this is due to an extension 

 of the region of modified discharge or to a marked setting in 

 of back discharge cannot be stated. At any rate the method 

 as one for comparing specific velocities under varying 

 physical conditions is not satisfactory, since in the case of 

 varying pressure it breaks down entirely. 



Effect of Discharge from the Plate. 



It was shown by the author"* that in certain cases there 

 was a marked change in the character of the electric wind 

 as the distance between point and plate was increased, the 

 pressure of the wind in each case falling short of that 

 anticipated from theory. The effect, since it was accompanied 

 by a speck of light upon the plate* was no doubt due to a 

 back discharge from this speck which contributed to the 

 current but which gave a wind effect opposed to that of the 

 main discharge. 



With the above apparatus some idea of the magnitude of 

 this effect could be obtained by observations' on the current 

 arriving at each zone of the plate. 



In order that the speck of light on the plate could be 

 obtained under controllable conditions it was made to occur 

 at a fine point N, which could be projected at will through 

 the zone A of the plate to the extent of a millimetre or so. 



* Phil. Mag. vol. xxi. p. 585 (1911). 



