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Prof. J. H. Poynting on the Discharge 



corresponding to the energy put into it, somewhat as the 

 pressure on a piston in the wall of a closed vessel of compressed 

 water might be regarded as the surface-manifestation of the 

 strained condition of the water. 



In order to follow out the process of discharge in the medium, 

 i. e., the mode in which it is relieved from its strained condition, 

 we will first take a simpler case in which we connect the two 

 plates, A and B (fig. 1) , of one condenser to the two plates, 



Fig. 1. 



WIRE 



WIRE 



C and D, of another condenser previously uncharged, and so 

 far from A and B that there is no appreciable direct inductive 

 action on C and D. When equilibrium is again restored, the 

 + charge is shared between A and C, the — charge between 

 B and D, while the difference of level has decreased. There 

 is the same total number of tubes of induction, but each con- 

 tains fewer unit cells than before, the energy corresponding 

 to the decrease having been transferred to the wires, where it 

 has been dissipated as heat. I shall use the term energy- 

 length to indicate the line-integral of the electric intensity 

 along its axis, this being the same as the difference of potential 

 when there is equilibrium. We may say then that the energy- 

 length of the tubes has decreased. During the change some 

 of the electric energy was converted into magnetic energy in 

 the medium. This might be observed if sufficiently delicate 

 means were used. 



If we confine our attention to the charges on the conductors 

 we must say that equal quantities of + and — have moved 

 respectively from A to C and B to D along the wires. 



But taking into account the condition of induction in the 

 medium described by the induction-tubes, we must say that 

 the induction-tubes move sideways out from the space between 

 A and B into the space between C and D, the motion of the 

 charges along the wires being really the motion of the ends of 



