Associated cases of Current and Static Effects. 303 



of shell-lac upon a gold-leaf electrometer and a charged canier 

 (an insulated metal ball of two or three inches diameter) upon 

 it, the electrometer is diverged ; removing the carrier, this diver- 

 gence instantly falls, this is insulation and induction : if we replace 

 the shell-lac by metal, the carrier causes the leaves to diverge as 

 before ; but when removed, though after the shortest possible 

 contact, the electroscope is left diverged, — this is conduction. If 

 we employ a plate of spermaceti instead of the metal, and repeat 

 the experiment, we find the divergence partly falls and partly 

 remains, because the spermaceti insulates and also conducts, 

 doing both imperfectly : but the shell-lac also conducts, as is 

 shown if time be allowed ; and the metal also obstructs conduc- 

 tion, and therefore insulates, as is shown by simple arrangements. 

 For if a copper mre, 74 feet in length and y^gth of an inch in 

 diameter, be insulated in the aii', having its 

 end m a metal ball ; its end e connected with 

 the earth, and the parts near m and e brought 

 within half an inch of each other, as at « ; 

 then an ordinary Leyden jar being charged 

 sufficiently, its outside connected with e and 

 its inside with m, will give a charge to the 

 wire, which instead of travelling wholly 

 through it, though it be so excellent a con- 

 ductor, will pass in large proportion through 

 the air at s, as a bright spark ; for with such 

 a length of wire, the resistance in it is accu- 

 mulated until it becomes as much, or per- 

 haps even more, than that of the air, for 

 electricity of such high intensity. 



Admitting that such and similar experiments show that con- 

 duction through a wire is preceded by the act of induction (1338.), 

 then all the phsenomena presented by the submerged or subter- 

 ranean wires are explained ; and in their explanation confirm, as 

 I think, the principles given. After Mr. Wheatstone had, in 

 1834, measured the velocity of a wave of electricity through a 

 copper wire, and given it as 288,000 miles in a second, I said, 

 in 1838, upon the strength of these principles (1333.), " that 

 the velocity of discharge through the same wire may be greatly 

 varied, by attending to the circumstances which cause variations 

 of discharge through spermaceti or sulphur. Thus, for instance, 

 it must vary with the tension or intensity of the first urging 

 force, which tension is charge and induction. So if the two 

 ends of the wire in Professor "Wlieatstonc's experiment were 

 immediately connected with two large insulated metallic surfaces 

 exposed to the air, so that the primaiy act of induction, after 

 making the contact for discharge, might be in part removed 



P2 



