350 Steady Currents in continuous Media [CH. x 



so that we may suppose that the plates of the condenser are reduced to the 

 same potential before the charges imprisoned in the dielectric have begun to 

 move. For simplicity, let us suppose that the plates of the condenser are 

 both reduced to potential zero. Then the surface of the dielectric may, 

 with fair accuracy, be regarded an an equipotential surface, the potential 

 being zero all over it. It follows that there can be no lines of force outside 

 this equipotential : all lines of force which originate on the charges im- 

 prisoned in the dielectric, and which do not terminate on similar charges, 

 must terminate on the surface of the dielectric. Thus we shall have a 

 system of charges on the surface of the dielectric, these charges being equal 

 in magnitude but opposite in sign to those of the Green's "equivalent 

 stratum" corresponding to the system of charges imprisoned in the dielectric. 

 This system of charges on the surface of the dielectric is of the kind which 

 Faraday would call a "bound" charge (cf. 141). 



Suppose the plates of the condenser to be again insulated. The system 

 of charges inside the dielectric and at its surface is not an equilibrium dis- 

 tribution, so that currents will be set up in the dielectric, and a general 

 rearrangement of electricity will take place. The potentials throughout the 

 dielectric will change, and in particular the potentials of the condenser-plates 

 at the surface of the dielectric will change. In other words, the charge on 

 these plates is no longer a "bound" charge, but becomes, at least partially, a 

 "free" charge. On joining the two plates by a wire, a new discharge will 

 take place. 



This is Maxwell's explanation of the phenomenon of " residual discharge." 

 It is found that, some time after a condenser has been discharged and 

 insulated, a second and smaller discharge can be obtained on joining the 

 plates, after this a third, and so on, almost indefinitely. It should be 

 noticed that, on the explanation which has been given, no residual discharge 

 ought to take place if the dielectric is perfectly homogeneous. Maxwell's 

 theory accordingly receives confirmation from the experiments of Rowland 

 and Nichols* and others, who shewed that the residual discharge disappeared 

 when homogeneous dielectrics were employed. 



REFERENCES. 



Flow in Conductors : 



MAXWELL. Electricity and Magnetism. Vol. i. Part u. Chaps, vn, vm, ix. 

 Flow in Dielectrics, Residual Charges, etc. : 



MAXWELL. Electricity and Magnetism. Vol. I. Part n. Chaps, x, XII. 



WINKELMANN'S Handbuch der Physik. Vol. iv. 1, pp. 157 et seq. 



HOPKINSON. Original Papers (Camb. Univ. Press, 1901). Vol. n. 



* Phil. Mag. [5] vol. n. p. 414 (1881). 



