140 Dr. J. Moser on the Division of Induction in the 



On the two sides of the ebonite plate the sign of the dis- 

 charge is differently affected. 



1. The ebonite plate lying close to the metal plate, the 

 other, lifted, metal plate gives : — 



(a) The rubbed surface being above, a positive discharge; 



(b) The unrubbed surface being above, a negative discharge. 



2. The ebonite plate lying some height above the metal 

 plate, we get, on the contrary, quite indifferently, whether 

 (a) the rubbed surface is above, or (b) the unrubbed surface, 

 in both cases positive discharges. 



Thus we have the means of recognizing not only — 



(1) on which side an ebonite plate is charged positively, 



(2) on which side it is charged negatively, but also 



(3) if it had been charged originally negatively on the one 

 side or 



(4) positively on the other side. 



For this purpose we place the ebonite plate to be tested close 

 to an uninsulated metal plate. Then we put a second metal 

 plate on the ebonite disk, uninsulate it, and try the electricity 

 induced in it ; the opposite one exists in the upper surface 

 of the ebonite plate. 



The ebonite plate is now inverted, put again close to the 

 uninsulated metal plate, and the kind of electricity of the other 

 surface of the ebonite plate is determined in the same manner. 

 Thereby we learn which kind of electricity exists in each 

 surface. 



Now the ebonite plate is tested at some height over the 

 metal plate, at first in the original, then in the inverted posi- 

 tion ; it is, however, to be lifted so high that in both cases the 

 same kind of electricity is induced. Then the plate contains 

 a surplus of the opposite kind, and has been charged with it 

 originally on that side on which according to our trial it exists. 



Adding the two quantities of electricity which we have 

 found induced in the sole and in the cover, we get, in accord- 

 ance with our supposition, 



-E(l-a). 



The greater the quantity which is induced in the cover, the 

 less is induced in the sole, and vice versa. If we join the cover 

 with one electroscope and the sole with another, both electro- 

 scopes show a divergence on moving one of the plates, but in 

 opposite senses. 



Through a conducting wire from the cover to the sole there 

 flows a current on moving one of the plates; so that the quan- 

 tity which flows to the sole has left the cover; whilst in the 

 electrophorus itself between sole and cover (i. e. in the re- 



