56 ELEMENTARY LESSONS ON [CHAP. i. 



tricity upon the face nearest the earth-plate, the surface- 

 density on this face being therefore very great. Such 

 an arrangement is sometimes called a condenser, some- 

 times an accumulator. We shall call it an accumulator 

 when the purpose it serves is simply to accumulate a 

 charge. We shall call such an arrangement a condenser 

 when the object of the earth-connected plate is to increase 

 the surface-density of the charge upon one face of the 

 insulated conductor. 



The stratum of air between the two discs will suffice 

 to insulate the two charges one from the other. The 

 brass discs thus separated by a stratum of air constitute 

 an air-condenser. Such condensers were first devised 

 by Wilke and Aepinus. 



49. Dielectrics. In these experiments the sheet of 

 glass or layer of air plays an important part by permitting 

 the inductive electric influences to act across or through 

 them. On account of this property these substances 

 were termed by Faraday dielectrics. All dielectrics 

 are insulators, but equally good insulators are not neces- 

 sarily equally good dielectrics. Air and glass are far better 

 insulators than ebonite or paraffin in the sense of being 

 much worse conductors. But induction takes place better 

 across a slab of glass than across a slab of ebonite or 

 paraffin of equal thickness, and better still across these 

 than across a layer of air. In other words, glass is a 

 better dielectric than ebonite, or paraffin, or air. 

 Those substances which are good dielectrics are said -to 

 possess a high inductive capacity. 



50. Capacity of an Accumulator. It appears, 

 therefore, that the capacity of an accumulator will 

 depend upon 



(1) The size and form of the metal plates or 'coatings. 



(2) The thinness of the stratum of dielectric between 



them ; and 



(3) The inductive capacity of the dielectric. 



51. The Ley den Jar. The Ley den Jar, called after 



