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LVII. Discharge of Electricity in an Imperfect Insulator. 

 By J. H. Poynting. M.A., Professor of Physics, Mason 

 College *. 



MAXWELL has shown that the phenomena known as 

 the Residual Discharge may be accounted for on the 

 supposition that the dielectric is an imperfect insulator in 

 which the conductivity varies in different parts. His theory 

 is really quite simple and straightforward and free from any 

 hypothesis beyond the fundamental one of electric displacement. 

 But its very generality makes it, I believe, difficult to grasp. 

 The idea of a yielding of displacement in the dielectric, accom- 

 panied by a conduction-current in the opposite direction, gives 

 us no help in forming a mental picture of the process actually 

 going on in the dielectric. A hypothesis as to the nature of 

 electric current, which will shortly be published in the ' Philo- 

 sophical Transactions/ seems to me to render the theory easier 

 to follow; and I propose in this paper to arrange Maxwell's 

 account of the Residual Discharge in accordance with it. 



I shall first give some account of the hypothesis referred to 

 in the special case of the discharge of a condenser. Let us 

 suppose that we have two conductors, A and B, which we may 

 suppose to be the two plates of a condenser charged with equal 

 and opposite amounts of electricity, that of A being positive. 

 Then the lines of force will run from A to B through the 

 medium, the condition of the medium being described by 

 saying that there is " electric displacement " from A to B. 

 Or we may describe it without introducing the confusing term 

 " displacement'''' by returning to Faraday's term " induction." 

 We may then say that tubes of electric induction pass through 

 the medium, each tube starting from + 1 of electricity on A 

 and ending in —1 on B. The total induction across any 

 section of a tube is then always equal to 1. If we draw the 

 level surfaces at unit differences of potential, the tubes will be 

 divided up into cells ; and if we suppose each cell to contain 

 half a unit of energy, then the whole energy of the electrified 

 system is accounted for. Maxwell has called these unit cells 

 (' Elementary Treatise on Electricity,' p. 47). According 

 to the views of Faraday and Maxwell, the charges on the 

 conductors bounding the dielectric are to be regarded as the 

 surface manifestations of the altered state of the dielectric 



* Communicated by the Author, having been read before the Birmingham 

 Philosophical Society, December 10, 1885. 



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