of Electric Induction and Conduction. 359 



Such disruptive discharge as this represented by a snapping of 

 the elastics is analogous to the rupture of solid dielectrics like 

 glass. In fluid dielectrics like air, the disruptive discharge 

 may be better represented by a slipping of the pins to which 

 the elastics are fastened along a groove parallel with the cord : 

 at each slip the cord makes a bound forward; but no perma- 

 nent damage is done. (If at any one point a slip takes place, 

 the extra stress thrown on to the others is liable to make them 

 give way too.) A discharge of this sort is a kind of electro- 

 lysis, and may be supposed to go on slowly in glass at a high 

 temperature. It would he accompanied by a polarization of the 

 electrodes, as we shall see later (§ § 13 and 15). Discharge in a 

 vacuum-tube seems to be of this nature. If partial disruptive 

 discharge occurs at points inside a dielectric, it will in general 

 give rise to internal charge, even though the dielectric be of 

 uniform conductivity, because the buttons will not preserve 

 their original distances from one another on the cord. 



Maxwell's Theory of a Composite Dielectric. 



§ 8. Let us now see how the model must be made in order 

 to agree accurately with the dielectric, as investigated in art. 

 328, vol. i. of Maxwell's ' Electricity.' 



It is there supposed, " for the sake of simplicity, that the 

 dielectric consists of a number of plane strata of different ma- 

 terials and of area unity, and that the electric force acts in the 

 direction of the normal to the strata." The row of buttons in 

 our model (fig. 1) will represent this collection of strata; 

 and we shall, for simplicity, consider the normal distance be- 

 tween adjacent buttons in the row as unity. The thickness %, 

 a. 2r ... of any stratum is then equal to the number of buttons 

 (or molecules) which compose it; and the whole thickness of the 

 dielectric a x + a 2 + — + a n will equal n, the whole number of 

 buttons. Since all the buttons in any one stratum have exactly 

 the same properties, we may if we like consider each button 

 to represent an entire stratum, and take a 1 = a 2 = ... = l; and 

 this is what we shall virtually do in what follows. The other 

 symbols employed in the investigation are the following, with 

 their meanings affixed (1st, the electrical one after Maxwell, 

 and 2nd, the corresponding meaning expressed in terms of 

 the mechanical model): — 



X 1? X 2 , &c. [the resultant electrical force within each stra- 

 tum]. The tension in the elastic of each button if the 

 buttons are unit distance apart, or, more generally, the 

 difference between the pressures on the corresponding 

 faces of two consecutive buttons divided by the distance 

 between them. 



