— Conduction of Galvanic Electricity through Moist Air, 209 
two reasons why such wire would not answer as well as before. 
First, its conducting power would be impaired, and secondly, it 
is extremely difficult to straighten such wire, so as to wind the sev- 
eral turns as closely as before. In regard to the tables which 
have heretofore been made for the relative conducting powers of 
different metals, it must be observed, that they cannot be regard- 
ed as any thing more than approximations to the truth. What 
is true of many practical applications of science, is particularly 
_ $0 of galvanism, viz. that laws, rules, or principles deduced from 
miniature experiments, have entirely failed when applied to ope- 
rations upon a large scale. As for instance, the law of Pouillet, 
that wires conducted directly as their cross sections and inversely 
as their length, was admitted until the experiments in telegraphic 
Operations upon hundreds of miles of wire, disproved this law and 
confirmed the law of Ohm. I have some interesting facts here- 
after to be communicated, showing the importance of operations 
. upon grand ‘scales, in establishing general principles. Most of 
your readers will remember the signal discovery of Liebig of 
the existence and economy .of ammonia in the atmosphere, 
which he owed to his departure from the limited eudiometrical 
experiments of other philosophers and investigating large quanti- 
ties of air instead of small. ‘The effect before alluded to of 
bending and twisting wires is much more apparent in some spe- 
“ies of wires than others. There has been a spurious copper 
Wire now for some time in the market, and it has been my mis- 
fortune to purchase about ten thousand feet of it.* Its conduct- 
ing capacity as measured by the arial galvanometer, in lengths 
of 1000 feet, is about one-third less than pure copper awite. 
It is well known that a wire may soon be broken by bending it 
back and forth for a few times. Each bend of course approaches 
to this solution of continuity, and must interrupt that integ- 
tity of molecular arrangement which is essential to good mnt 
‘uction. Now a few such short bends may not be appreciable 
by ordinary tests, but when multiplied several hundred times, the 
resistance to the current becomes of sufficient moment to be a 
subject of attention. ‘ G. G. P 
3 Washington, D. C,, July 3d, 1846. 
ecg ee OTe Cl ne 
“Tt is the Same wire that has so perplexed the telegraphic operations between 
; New Y AE iad Philadelphia, breaking almost daily merely by its own weight. 
