144 



Note on Gravitation. 



The sensibility was found directly. A number of small 

 weights were cut from the aluminium foil mentioned above, 

 the mass of each being 2*18 x 10 ~ 5 gm. These could be 

 picked up on a small brush and pushed through the holes (h) 

 directly over the centre of each vane. Independent deter- 

 minations gave the sensibility to be 1*42 x 10 ~ 6 gm. per 

 millimetre deflexion on the scale, and as this was con- 

 siderably magnified by the telescope a motion of one-tenth 

 of a division could easily be observed. 



Firi 1. 



fed 



Lo^ 



/Ocm 



~m 



The method of experimenting was as follows. The lid of 

 the box was first adjusted so that it was approximately 

 parallel to the bottom and at a distance of about 2*6 cm. 

 from it. One vane and the box were kept earthed and the 

 other vane charged to, say, 100 volts ; generally there was 

 a motion of the balance, but by means of the flag the vane 

 to be charged could be adjusted so that it lay practically 

 midway between the top and bottom of the box. On re- 

 charging, it was now almost indifferent as to which way it 

 •was attracted, up or down. The final adjustment was made 



