Ionizing Processes at a Point discharging in Air. 279 



P and Q were both earthed, Q directly and P through a 

 galvanometer. Opposite P and in the same vertical plane 

 was a sharp sewing-needle N connected to a wimshurst, 



Fisr. 1. 







i=* 



i*y 



IN 





x. 







3C 





- — • 



-K 



i 

 i 



K 



To 



Wimsli 



and so arranged that the vertical component x and the 

 horizontal component y of its distance from P could be 

 varied. The radius of P (0*031 cm.) was about 7 or 8 

 times greater than that o£ N, so that N discharged more 

 readily than P ; and the tendency of N to start first was 

 farther increased by surrounding P with a wire ring about 

 11 mm. in diameter, with its plane about 7 mm. behind the 

 point P. By varying x and y it was thus possible to supply 

 P with varying numbers of ions from N both before and 

 after P itself began to discharge on its own account. 



The end of P was viewed through a reading microscope, 

 and the resultant pull, P, upon its surface was measured 

 by tilting the whole apparatus so as to keep P always upon 

 the cross -wire. 



It has been shown * that under these conditions, if the 

 current from the point is not greater than 15 microamperes, 

 the disturbing effect of the discharge upon the pull of the 

 field on the point is probably negligible compared with the 

 pull itself, and that if r is the radius of the point, and f 

 the field at the centre of its surface due to the lines of force 

 in the ionizing layer, 



/o= — x constant, 



where the constant is 2*83 for positive, and approximately 

 3*07 for negative discharge. 



Jn most of the earlier observations N was about 2 cm. 

 long and projected from a flat plate R parallel to Q ; y was 

 kept at 1*6 cm. and x was varied. In the second set which 

 was made with entirely new apparatus, y was varied, and 



* Cliattock, Phil. &Ta< 



pp. 272-274 of present number. 



