Copper Iron Davy 100·00 14·60 Becquerel 100·00 15·80 Lenz 100·00 17·74 Ohm 100·00 17·40 Pouillet 100·00 16·90
Taking the average of these five statements, it will be found that the relative conductivity of copper to iron stands as 100 to 16½--that is, a little over six to one. The approximate correctness of this figure, being the result of all the investigations by the most eminent men who studied the subject, can therefore admit of no reasonable doubt.
The important researches as to the greatly varying degree in which given quantities of metals will act as conductors of the electric force, were made possible only by the discovery of the singular phenomena of electro-magnetism, due chiefly to the Danish philosopher and naturalist, Hans Christian Oersted. His career, in some respects, was not unlike that of Benjamin Franklin. The son of an apothecary, born in 1777, he set up in the same business, not despising trade, but devoting himself actively to it, as means to an honourable end, that of gaining independence. Fascinated by the study of the phenomena of electricity, Oersted devoted himself to it heart and soul, as Franklin had done; and the result achieved, if not fully as important as the invention of the lightning conductor, was one filling a prominent place in modern scientific discovery. It had been observed, long before Oersted, that there was a close connection between what was known as magnetism and lightning, or rather, to state it more directly, it was known that lightning exercised a strong influence upon the magnetic needle. One of the most notable reports, and one of the first on the subject, came from the captains of two English vessels, sailing in company from London to the West Indies in the year 1675. When near the Bermudas, a stroke of lightning fell upon the mast of one of the vessels, doing considerable damage, and, as the captain believed, swinging his ship round, the men at the helm seeing the compass violently disturbed. He continued steering in what he believed the old direction, but noticed, a few minutes afterwards, that the other vessel, his former companion on the route, and which had not been struck by lightning, was following an opposite course. He had the good sense to approach it, and explanations ensued, the result being the discovery that the lightning had completely reversed the polarity of the magnetic needle, it pointing now south instead of north. The story of this met with much doubt at the outset, but it was amply verified before long by the report of many similar occurrences. It became known, not only that the polarity of the magnetic needle might be reversed by a stroke of lightning, but that the effect of the latter frequently was to magnetise iron and steel. An instance of this kind, on a large scale, occurred at Wakefield, Yorkshire, in the month of June 1731, during a violent thunderstorm. The lightning here entered the warehouse of a merchant who had just packed a case of knives, forks, and other articles of steel and cutlery ware, for despatch to the colonies. The case was placed immediately under the chimney, which the lightning entered, breaking open the box, and scattering over the floor of the room its contents, which, when afterwards examined, were all found to be strongly magnetic. These, and many similar facts, were all clearly established; yet a considerable time elapsed before important conclusions were drawn therefrom. As in the case of Franklin, so in that of Oersted, it required not merely scientific acumen, but a thoroughly practical mind, to trace, in the one instance, the actual connection between electricity and lightning, and in the other that between magnetism and electricity.