52 On Electromotive Forces of Contact. 



takes place with non-conductors. In order to test which 

 view is the more correct, I have made a few experiments on 

 the electrification of glass and ebonite when dipped into 

 mercury. This method of experimenting seemed to offer 

 several advantages. The mercury being permanently con- 

 nected to earth was always at a known potential (zero), and 

 it was only necessary to measure the charge of the ebonite 

 or glass. Then, again, contact was obtained over the whole 

 surface submerged, with very little mechanical violence when 

 desired ; and the conditions of contact could be considerably 

 varied by using more or less suddenness and violence and 

 longer or shorter duration of contact. 



Fig. 4. 



L& 



The ebonite or glass employed consisted of a small circular 

 slab S (fig. 4) with a convex surface A, which was attached 

 to one end of a rod BC of the same material. This rod was 

 encased in a brass tube DE, inside which it was fastened by 

 means of packing FG. The length BF of ebonite or glass 

 served to insulate the slab A. In the case of the glass the 

 length BF was varnished. The charge on A was measured 

 by means of the conductor L connected to a Thomson elec- 

 trometer. A was placed near to L, which was put to earth 

 and then insulated. On withdrawing A, a deflection was 

 produced proportional to the charge on A. The movement 

 of A was controlled by making the tube DE slide in metal 

 bearings between fixed stops. The tube DE being of metal, 

 no electrification was produced by friction in the bearings. 

 The following results were obtained : — 



(a) Ebonite and Mercury. deflection. 



Dipped once and quickly withdrawn . . . . + 10 to + 12 



„ very quickly withdrawn . . +15 



for 10" quickly withdrawn . . . . 4- 7 to -f 15 



„ 15" „ „ . . . . + 6to + 10 



„ 30" „ „ . . . . + 6to + 13 



„ 60" „ „ . . . . + 5to+ 9 



30 rapid splashes in the mercury .... — 6 



50 not very rapid splashes in the mercury . + 7 



100 rapid „ „ „ . — 11 



200 „ „ „ „ . -10 



J c 



