of Electricity in an Imperfect Insulator. 423 



current from the negative towards the positive plate, and (2) 

 a conduction-current from the positive plate to the negative 

 equal to (1) in amount. This latter is accompanied by dissi- 

 pation of energy. The two equal and opposite currents, being 

 superposed, have no external magnetic effect. 



But it seems to me that we may equally well and more 

 simply represent the facts by considering the first process 

 alone, viz. the yielding of the electric strain, the medium 

 being incapable of bearing it permanently. The electric 

 energy is gradually converted into heat in the same part of 

 the dielectric where it was previously electric, i. e. there is 

 here no transfer of energy. The decrease of induction in the 

 medium is accompanied by a corresponding decrease of charge 

 on the plates, not by conduction of + or — electricity either 

 way through the medium, but simply because there is a de- 

 crease of the induction in the medium of which the charges 

 on the plates are the surface manifestations. The induction 

 decreases equally through the whole length of a tube, so thai 

 the tube " weakens " at the same rate throughout its length. 



There will be no magnetic effect in the surrounding space, 

 for there is no movement inwards of electric induction-tubes 

 to supply the place of those which decay. 



Perhaps we may take the following as illustrating the two 

 modes of regarding the process. Suppose that a solid is sub- 

 mitted to some strain and kept in the strained position, but 

 that the energy of the strain gradually dissipates ; then we 

 may confine ourselves simply to the statement that, owing to 

 some rearrangement of the molecules, they cease to have 

 molecular- strain energy, the energy in each portion of the 

 mass being transformed to heat in that portion ; or we may 

 imagine that there is a continual return from the strained 

 towards the original position, accompanied by an equal reverse 

 flow of the matter towards the strained position, this latter 

 not storing up energy, but dissipating the energy given up 

 by the yielding of the strain. The ultimate result according 

 to each is the same, but the latter account is purely hypothe- 

 tical. We may at once obtain the equation giving the value 

 of the charge at any time in terms of the initial charge when 

 the condenser is left insulated. 



Let a be the charge per unit area, this being equal to the 

 electric induction across unit area in the dielectric ; 

 a be the thickness of the dielectric ; 

 K be the specific induction T capacity ; 

 c be the capacity per unit area ; 



X be the electric intensity in the dielectric, i. e. force 

 per unit electricity on a small electrified body. 



