of Steel Needle-Points in Air. 



303 



The same principle of tilting was used in measuring the 

 attractions on the needle-points ; the needle in this case taking 

 the place of the suspended disk. The zero-position was deter- 

 mined by a hair in the eyepiece of a microscope through 

 which the point of the needle was observed ; the latter being 

 at the centre of curvature of C D (fig. 1) when it coincided 

 with the hair. The needle was illuminated by small windows 

 in T (not shown) . L was connected to the Wimshurst ; T to 



Fiff. 1. 





: ; .... .- ■• --•■'-■ „_ . _ "___/;., j 



T, brass tube forming body of instrument ; length 10 centim., diameter 



6 centim. 

 H, tube for suspension-wire G. F, ebonite plug. 

 M, mica disk suspended from G-, carrying light metal clip B holding 



needle N. 

 S, tin screen soldered into T. 

 K, metal cover. E, ebonite cover. 

 A A, ground air-tight j oints, greased. 

 C D, spherical metal cup to be electrified ; CD = 8"3o centim., radius 



of curvature = 1*76 centim. 

 P, short brass tube on S ; length = 0*5 centim., diameter = 1*2 centim. 



Point of N at centre of curvature of CD. 

 beyond P = 2 centim. 



Length of needle projecting 



