the Point of an Electrified Needle. 543 



If it be assumed that the density on the hemisphere is 

 sensibly uniform, a = l. The values o£ / were calculated on 

 this assumption in the paper referred to, and it was the object 

 of the present investigation to find how far that assumption 

 held good. 



Fiar. 1. 



Fig. 2. 



The investigation involved the experimental determination 

 of the distribution of the charge round the hemisphere. For 

 this purpose magnified models of the needle and plate were 

 made ; the sewing-needles employed in the above-mentioned 

 research were represented by two models about two metres 

 in length and with diameters of about 13 cms. and 8 cms. 

 respectively, which were fixed upright on the laboratory floor, 

 whilst the plate emplo}~ed was a screen about 3 metres square 

 suspended horizontally above. 



Of the two model needles employed the smaller was more 

 perfectly made and gave more consistent results than the 

 other. A section of it is shown in fig. 2. A is a metal tube 

 the end of which was turned to fit accurately a brass sphere B 

 of the same diameter, so that only a half of the sphere pro- 

 jected. In the sphere a hole 8 mm. in diameter was drilled 

 to receive a plug C 7 '5 mm. in diameter ; the plug was 



