204 Prof. Faraday on Electric Induction — 



from the internal portion of the wire at the first instant, and 

 disposed for the moment on its surface jointly with the air and 

 surrounding conductors, then I venture to anticipate that the 

 middle spark would be more retarded than before ; and if these 

 two plates were the inner and outer coating of a large jar, or a 

 Leyden battery, then the retardation of that spark would be still 

 greater." Now this is precisely the case of the submerged or 

 subterraneous wires, except that instead of carrying their surfaces 

 towards the iuducteous coatings (1483.), the latter are brought 

 near the former ; in both cases the induction consequent upon 

 charge, instead of being exerted almost entirely at the moment 

 within the wire, is to a very large extent determined externally ; 

 and so the discharge or conduction being caused by a lower ten- 

 sion, therefore requires a longer time. Hence the reason why, 

 wdth 1500 miles of subterraneous wire, the wave was two seconds 

 in passing from end to end ; whilst with the same length of air 

 wire, the time was almost inappreciable. 



With these lights it is interesting to look at the measured 

 velocities of electricity in wires of metal, as given by different 

 experimenters. 



Miles per second. 



* Wheatstone, in 1834, with copper wire made it 288,000 



* Walker, in America, with telegraph iron wire . 18,780 



* O'Mitchell, ditto ditto . 28,524 



* Fizeau and Gonnelle (copper wire) .... 112,680 



* Ditto (iron wire) 62,600 



.t A.B.G. (copper) London and Brussels Telegraph 2,700 



t Ditto (copper) London andEdinburgh Telegraph 7,600 



Here the difference in copper is seen by the first and sixth 

 result to be above a hundred fold. It is further remarked in 

 Liebig's report of Fizeau and Gonnelle^s experiments, that the 

 velocity is not proportional to the condu.ctive capacity, and is 

 independent of the thickness of the wire. All these circum- 

 stances and incompatibilities appear rapidly to vanish, as we 

 recognize and take into consideration the lateral induction of 

 the wire carrying the current. If the velocity of a brief electric 

 discharge is to be ascertained in a given length of wire, the 

 simple circumstances of the latter being twined round a frame 

 in small space, or spread through the air through a large space, 

 or adhering to walls, or lying on the ground, will make a dif- 

 ference in the results. And in regard to long circuits, such as 

 those described, their conducting power cannot be understood 

 whilst no reference is made to their lateral static induction, or 



* Liebig and Kopp's Report, 1850 (translated), p. 168. 

 t Athenseum, January 14, 1854, p. 54. 



