Differential Inductometer and in the Electropliorus. 139 



cue- being constant, changes its sign when d increases : viz. d 

 (that is, the distance of the cover) being small, negative elec- 

 tricity is induced in the sole, and we get, on lifting the sole, 

 a negative discharge. This distance d, however, is small, 

 almost nothing, if we invert the ebonite plate and lay it with 

 its rubbed surface close to the metal plate. Then we get from 

 the upper metal plate (the sole), on lifting it, a negative dis- 

 charge. Thus the two phenomena which we intended to ex- 

 plain, (1) positive discharge in the original position, (2) nega- 

 tive discharge in the inverted position, of the ebonite plate, are 

 deduced from our theory. 



The electropliorus contains, therefore, two inductric layers — 

 one negative, on the upper side, one positive, on the loiver side of 

 the ebonite plate. 



Experiments with the Electropliorus. 



If the theory developed by us is true, the consequence which 

 we are about to draw now must be confirmed by experiment. 

 According to the last formula for the numerator of the electric 

 quantity of the sole, positive electricity must be induced in the 

 sole when the distance d of the rubbed surface from the cover 

 is increased, so that we get 



d.(l— ct)>*c r . 



This happens indeed, as we see, if we lay the inverted ebonite 

 plate, not immediately on the cover, but place it insulated some 

 height above the cover, as Herr von Bezold has done already. 

 Now the other plate, the sole, being laid on the unrubbed sur- 

 face, uninsulated and lifted, gives a positive spark. Whilst 

 the sign of the electricity of the cover (the plate on the side of 

 the rubbed surface) is always positive, the sign of the elec- 

 tricity of the sole (the plate on the side of the unrubbed sur- 

 face) changes. It is positive when the distance of the other 

 metal plate (the cover) from the rubbed layer is small ; on 

 the contrary, it is negative when this distance is great. 



In other words, when the cover is on the side of the stronger 

 negative inductric layer, the induction of the latter always 

 prevails ; in it positive electricity is induced. 



In the sole, however, the nearer but weaker positive layer 

 induces negative electricity. To this there is to be added the 

 positive electricity induced by the stronger but more distant 

 negative inductric layer. The more distant the cover is, the 

 more positive electricity is induced in the sole ; so that, the 

 distance of the cover being large, more positive than negative 

 electricity can be induced. 



