358 Mr. 0. J. Lodo-e on a Mechanical Illustration 



r-i 



electricity detached from an equal quantity of negative than 

 we can get a north pole detached from a south: only in one 

 case steel is the polarized medium, in the other case air. 



§ 6. Instead of making momentary contacts as in § 4, we 

 might leave the screw S up so as to obtain a feeble continuous 

 current from the dielectric by reason of the gradual slipping 

 of some of its buttons, till it is completely discharged. The 

 strength of this current at any instant Maxwell calculates in 

 art, 330. 



Again, after having charged the dielectric, instead of re- 

 moving W, we might leave it on and not clamp the cord. 

 This would represent a jar permanently connected with an 

 electromotor maintaining a constant difference of potential. 

 If none of the buttons slipped on the cord, nothing would in 

 this case happen after the first charge. If all the buttons 

 slipped equally, we should get a feeble continuous conduction- 

 current through the dielectric after the first rush due to elec- 

 trical displacement had subsided. But if some of the buttons 

 remained firm on the cord while others slipped, we should have 

 a slight continuous current not purely of conduction, nor 

 purely of displacement, but of both combined. There would 

 be conduction through the slipping buttons, and there would 

 be displacement of the tight ones with increasing tension in 

 their elastics to make up for the relaxation of the others ; and 

 this might go on for a long time, the current continually 

 getting weaker. This is actually observed in the dielectric 

 of submarine cables (cf. Maxwell, art. 366). The insulating 

 material of a cable, in fact, consists of layers of different mate- 

 rials (gutta percha, Chatterton's compound, &c.) laid in strata 

 over the conductor, as if these residual-charge effects were 

 exactly what was wanted to make a good cable. I suppose 

 that they do not exert any detrimental influence on the use of 

 a cable, the method of signalling being what it now is ; but 

 if it were desired to lessen these effects, it might be done by 

 rendering the dielectric as homogeneous as possible, first of 

 all by making ip of only one material, and secondly by laying 

 this material on the conductor by some process which shall 

 not necessarily develop in it a stratified structure as the present 

 method does. 



Disruptive Discharge. 



§ 7. "When the electromotor is acting on the dielectric in 

 the way last described, the strain in some of the elastics may 

 become so great as to break some of them ; we should then 

 have partial or complete disruptive discharge, and the subse- 

 quent resistance of the dielectric would be much diminished. 



