of Electric Induction and Conduction. 363 



of the force at that instant to something less, and at this 

 value it continues constant as long as the body continues to 

 move. This appears nearly what is wanted to imitate disrup- 

 tive discharge in a vacuum-tube (see Mr. Varley's experi- 

 ments, Maxwell, art, 369); but in the case of a metallic con- 

 ductor or an ordinary dielectric, the cord is required to slip 

 through the buttons, if it slip at all, even when the force is 

 slight ; and the rate of slip is always to be proportional to the 

 force. This might be roughly attained in the model by making- 

 each button of a short piece of india-rubber tube well greased 

 inside, and by attaching the ends of its elastics to a ring fixed 

 on the tube near one end, so that when it was displaced the 

 pull on its elastics should make it grip the cord more tightly, 

 and thus keep the ratio, between the force pulling the cord 

 through and the rate at which it came through, roughly con- 

 stant. But it would be probably unnecessary to do this ; for 

 by making very smooth and well-oiled buttons pinch the cord 

 tightly, one could arrange that the cord should be able to ooze 

 slowly through them without sticking; and then the residual- 

 charge phenomena would be imitated by the model with suffi- 

 cient closeness. Moreover I have reason to think that friction 

 between cord and metal is not independent of velocity. 



Metallic Conduction. 



§ 11. In order to make the model represent the flow of 

 electricity through a metallic conductor, we have only to make 

 the resistances r small and the elasticities k very great. We 

 thus approximate to a set of smooth buttons supported on 

 rigid fixed rods, as shown in fig. 3. In this case the displace- 

 ments are zero, the whole current is a conduction-current ; 

 and there is no tendency to a reversal of the current by reason 

 of an opposition electromotive force. In symbols, 



f=0j p = n, <r = 0, /c = yo , E = Ri£. 



To make it exhibit electrolytic conduction (or rather convec- 

 tion), we shall see in § 13 that the rods supporting the buttons 

 must still be rigid, but that the pins by which they are at- 

 tached to the beams must slide in a long groove parallel with 

 the cord. 



Submarine Cables. (See also § 6.) 



§ 12. If a circuit consists of a simple wire hung in space, 

 the passage of electricity through it will be represented in the 

 model by a single endless cord ; and the cord may be abso- 

 lutely inextensible, so that the propagation of potential takes 

 place instantaneously ; or possibly it may be more in ac- 

 cordance with Maxwell's theory to say that the cord is so very 



