Differential InduclQmeter and in the Electropliorus, 137 



over, the upper metal covering, on being lifted off, gives nega- 

 tive electricity, while formerly it gave positive. 



The differential inductometer, which gave positive in both 

 cases, is therefore not a sufficiently accurate scheme to explain 

 why we get, 



(1) when the rubbed surface is above, a + spark, 



(2) when the rubbed surface is below, a — spark, from 

 the lifted metal. 



The simplest completion of the differential inductometer 

 having only one inductor plate, to make a scheme of the elec- 

 trophorus, is effected by adding a second inductor plate (a posi- 

 tively charged one), which corresponds with the underside of 

 the ebonite plate. In fact, after beating, the ebonite plate 

 attaches itself so closely to the metal sole, that I could hold the 

 sole vertical without the ebonite plate sliding down. At first 

 the negative layer of the rubbed surface induced positive elec- 

 tricity in the metallic sole. The induction is followed by con- 

 duction and discharge; a part of the positive electricity goes 

 over from the sole to the lower side of the ebonite plate. On 

 the other side the negative electricity penetrates deeper from 

 the upper surface into the ebonite; so that there are two layers 

 on the ebonite plate — a negative one on the upper side, a posi- 

 tive one on the lower side. For the understanding of the ques- 

 tion, it is here sufficient to regard these layers as plane, 

 without discussing the question of the thickness. 



With the help of these two layers we have to explain the two 

 observations : — 



(1) The + spark when the metal plate is lifted from the 

 rubbed surface; 



(2) The — spark when it is lifted from the unrubbed one.* 

 For the proof of this theory we shall deduce further conse- 

 quences from it ; and we shall have to confirm these by experi- 

 ment. The complete scheme of the electrophorus consists of 

 four plates : — 



(1) One negatively charged; 



(2) One positively charged. 

 These two are as inductric bodies. 



(3 and 4) Both metal coverings as inducteous bodies (indu- 

 cendus). 



To be brief, we will call the covering on the rubbed negative 

 surface of the ebonite plate "cover;" that on the other, positive, 

 unrubbed side, "sole;" so that we have in succession 

 cover, 



negative layer, 

 positive layer, 

 sole. 



