320 NiKU JLlcSlrical Experiments 



■j^ of an inch in diameter and 36 feet in length, was reduced 

 to a (late of ignition by a charge from 98 turns of the ma- 

 chine, Dr. Van Alarum made a chanre of the fame kind to 

 pafs through a copper one T \- inch in diameter, and faw, 

 with aftonifhment, that tills wire was fufed into fmall glo- 

 bule?. Another copper wire, -^ inch in diameter, was 

 broken in two places from a like charge ; a third, of T r T inch, 

 remained whole. The caufe of this difference, after a clofe 

 examination, was found to be in the difference of the pu- 

 rity of the copper. In the laft experiments the wire was of 

 common copper, but in the former they had been drawn 

 from an ingot of purified copper, like that prepared to be 

 mixed with gold. Now, as conductors are made of common 

 copper, in determining their ftrength attention mult be paid 

 to the laft experiments, and their diameter mull amount to 

 half that of iron conductors, in order to prefent an equal re- 

 finance to lightning. It is here fuppofed that the length is 

 always the fame. As it has been found that fquare iron rods 

 of half an inch in thicknefs can withftand the ftrongeft light- 

 ning, in regard to copper ones, it will be fuffieient if each 

 fide he about four or five lines. 



Dr. Van Marum fattened the wire, through which he 

 made the fhock to pais, on a piece of baked fir-lath, and 

 found tliLst it was burnt in (bine places where it had touched 

 the bends of the wire. Another lime he bound agaric of the 

 oak round the wire, fo that it lay faft ; and the confequence,* 

 was, that it took fire alone; its whole length. Con- 

 ductors therefore which pafs along fir, or wooden work, 

 mult be made fomewhat thicker than would otherwife be 

 neceffa.v. 



Patterfon has recommended plumbasro for the fummits of 

 condu&ors, becaufe it cannot be fuftd bv lightning; but a 

 charge of the large battery always reduced the ftrongeft 

 plumbago to powder. If conductors are to be made pointed, 

 they ought to be furnifhed with feveral points, that, in cafe 

 one of them fhould be fufed, the reft may be in a ftale 10 

 conduct the lightning. But it appears from former experi- 

 ments that pouitcd conductors do not deferve that preference 

 over blunt ones which fome have given to them. 



21. Con- 



