230 Prof. G. C. Foster's Description of some 



electrical equilibrium which it exhibits. The dynamical pro- 

 cesses by which these statical conditions are brought about are 

 no doubt, in their main features, very easily traced, and are per- 

 fectly well known ; but, until quite recently, it has been a rare 

 exception for electricians to be in possession of the instrumental 

 means requisite for making them the subject of direct investi- 

 gation. Now, however, the form of reflecting galvanometer de- 

 vised by Professor Sir William Thomson is in the hands of a 

 great many experimenters ; and it accordingly seemed to me 

 that, with the view of calling attention to the ease with which 

 the transient electric currents accompanying the production and 

 disappearance of electrostatical charge in various familiar cases 

 can be observed, and even measured, by means of this instrument, 

 it might be worth while to describe the following experiments. 



An insulating table was made by laying a thin board across 

 two insulated cylindrical conductors, such as are to be found in 

 every collection of electrical apparatus. On this was placed a 

 piece of sheet zinc, to serve as the lower plate of an electropho- 

 rus, the " cake " of which consisted of a circular piece of vulca- 

 nized india-rubber, about 15 inches in diameter and J inch 

 thick, and the "-cover" of a circular brass plate 12 inches in 

 diameter, with a glass handle. The lower metal plate was con- 

 nected, by means of an insulated wire, with one terminal of a 

 Thomson's astatic galvanometer having copper-wire coils of up- 

 wards of 6900 B.A. units resistance, the other terminal of which 

 was connected with a gas-pipe in the laboratory, so as to make 

 a good earth-contact. On rubbing the india-rubber with the 

 hand, the cover having been removed, the galvanometer showed 

 a deflection which, as soon as it had become steady enough to 

 be read, amounted to 35 divisions of the scale on the side indi- 

 cating the passage of a positive current from the earth into the 

 electrophorus-plate. This deflection gradually diminished while 

 the rubbing was continued, the spot of light finally returning to 

 zero. The earth-wire was now removed from the galvanometer 

 and replaced by a wire connected with the cover : on laying the 

 cover upon the india-rubber, the galvanometer gave a deflection 

 of 250 divisions on the opposite side to that observed during 

 the rubbing. On lifting the cover again, there was a deflection 

 of 230 divisions in the original direction, followed by a deflection 

 of 200 to the other side on replacing it. On repeatedly lifting 

 and replacing the cover, deflections were obtained every time, 

 though gradually diminishing in amplitude in consequence of 

 the imperfect insulation of the india-rubber. 



In a second similar experiment, the maximum deflection during 

 the rubbing was 40 divisions ; the deflection on putting on the 

 cover, 260 divisions; on removing it, 240. 



