422 Prof. J. H. Poynting on the Discharge 



We know from experiment that if a galvanometer be inserted 

 on the connecting wire then the same magnetic impulse is 

 observed wherever in the circuit the galvanometer be placed, 

 the impulse depending on the galvanometer-constant and on 

 the total discharge. The same experimental result may be 

 stated in an equivalent form, viz. that the line-integral of the 

 magnetic intensity round a closed curve encircling the wire, 

 if integrated for the time of discharge, is the same for all 

 positions of the curve. On the hypothesis here described all 

 the electric induction-tubes of the system finally pass sideways 

 from the medium into the wire. They must, therefore, on 

 their way pass inwards across any curve encircling the wire, 

 so that the total number of induction-tubes cutting such a 

 curve is the same for all positions of the curve. In the paper 

 above referred to, I have sought to connect these two constants 

 by supposing that the magnetic effect is due to, or, more cor- 

 rectly, accompanies, the motion inwards of the condition of 

 electric induction. As soon as motion commences some of 

 the electric energy is converted into magnetic, and the mag- 

 netic induction may be represented by ring-shaped closed 

 tubes surrounding the wire. The two inductions, electric and 

 magnetic, coexisting, will propagate the energy onwards till 

 it finally arrives in the wire and is dissipated as heat, the 

 induction there losing its directed condition. 



The flowing of electric charges along the wire, which is 

 usually considered as the essential part of the phenomena, or 

 at least that to which attention is to be chiefly directed, be- 

 comes on this hypothesis merely the last stage of the process, 

 which consists of a propagation from the surrounding dielectric 

 towards the wire of electric and magnetic induction, which we 

 may symbolize by the motion inwards of two sets of tubes, the 

 electric tubes being, on the whole, more or less in the direction 

 of the wire, the magnetic tubes being closed rings surrounding 

 it. The wire plays the part of the refrigerator in a heat-engine, 

 turning the energy it receives into heat — a necessary condition 

 for the working of the machinery. 



Let us now take the case of a condenser in which the 

 dielectric, though homogeneous, is imperfectly insulating, so 

 that the charge gradually disappears. According to Maxwell, 

 in this case " induction and conduction are going on at the 

 same time." Though Maxwell gave no precise account of 

 the process of discharge, his theory and the mechanical illus- 

 tration accompanying it are based on the supposition that two 

 processes are going on at the same time in every part of the 

 medium., viz. : — (1) a yielding of the electric strain or "dis- 

 placement"''' in the dielectric equivalent to a displacement 



