Gold103·05
Copper100·00
Silver81·26
Brassfrom 23·40 to 15·20
Platinum22·50
Iron from 18·20 to 15·60
Cast Steel14·75
Mercury2·60

It will be seen that Professor Pouillet, differing from other investigators, as they among themselves, regarding the relative conductivity of the precious metals, gold, silver, and platinum, agreed in the main with them as regards the relative proportions of copper and iron. Most painstaking and minute in his experiments, he found moreover that iron, as well as brass—the latter a mixed metal, and as such variable in composition—was not always the same in respect to conductivity, the changes being due to difference in temperature, as well as greater or lesser metallic purity. As set down by him, the variations in iron were between a maximum of 18·20 in regard to 100·00 of copper, and a minimum of 15·60, which gives a mean of 16·90. Taking this mean, the comparative list of the positions held by copper and iron in regard to electrical conductivity, according to the five investigators, may be set forth in the following summary:—

CopperIron
Davy100·0014·60
Becquerel100·0015·80
Lenz100·0017·74
Ohm100·0017·40
Pouillet100·0016·90

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.

It was in the year 1819 that Hans Oersted, now settled as a lecturer at Copenhagen, announced the result of a series of investigations which laid the foundation for the new science of electro-magnetism. He stated that he had found that if a magnetic needle, free to move like that of a compass, was brought parallel to a wire charged with electricity, it would leave its natural place and take up a new one, dependent on the position of the wire and the needle relative to each other. If the needle, he said, was placed horizontally under the wire, the pole of the needle nearest the negative end of the electric battery would move westward, but, on the other hand, if the needle was placed above the wire, the same pole would move eastward. Again, if the needle was placed on the same horizontal plane as the wire, no motion would be on that plane, but the inclination would be to a vertical movement. Finally, if the wire was laid to the west of the needle, the pole nearest the negative side of the battery would be depressed, but it would be raised if the wire was placed to the east of the needle. From these observations, verified in numerous experiments, Oersted concluded that the magnetic action of the electric force moved in a circular manner around the conducting object, which he expressed in the formula that ‘the pole above which the negative enters is turned to the west,’ and that ‘the pole under which it enters is turned to the east.’ The discoveries of Oersted resulted in the creation of that wonderful production of modern science—the electric telegraph. A minor result, highly important as regards the erection, and still more the maintenance, of lightning conductors, was the construction of galvanometers.

What the microscope is to the student of the inner secrets of animal and vegetable life, the galvanometer is to the investigator of the phenomena of electricity, in their practical applications. Until its invention, there existed no means of practically testing the strength of the electric force, or the ‘current,’ as it is usually called, and it was not possible, therefore, to ascertain, in any given case, whether lightning conductors, among others, were really efficient or not. Perhaps, had it been only for this purpose, the galvanometer would have waited long in being constructed, but what brought it into existence, and led it to its present perfection, was that greatest of practical uses of electricity, the telegraph. As it arose from small beginnings to gradually more extended employment, embracing ultimately some of the highest interests of civilised mankind, there came the necessity of having instruments for gauging accurately the effects of the mysterious force thus put in harness at the bidding of science. The galvanometer having been devised, the next step, indispensable for its use, was to frame a standard by which electrical energy might be measured, and to invent terms by which the amount of such energy could be expressed. It is well known that in order to be able to measure the dimensions of any material object, standard units are required. In this country the units adopted are: for length, the foot; for weight, the pound; for time, the second; and so on. To express mechanical force or power, the foot pound is the unit employed—that is, the mechanical energy necessary to raise a weight of one pound to a height of one foot. On the Continent, where the units of length and weight are the metre and the gramme, the unit of mechanical energy is the metre gramme. Apart from the fact that the latter units are very generally adopted by all the Continental States, the simplicity of the decimal method of multiplying and sub-multiplying them renders the system of particular usefulness for scientific purposes; and they are therefore very extensively employed even in England in scientific research. Thus experimental results obtained in one country are at once understood, and are directly comparable with results obtained in any other country, without the necessity of reducing the figures to terms of units of other kinds than those in which they are expressed.

Now electrical energy being merely a form of mechanical energy—the one being capable of conversion into the other—it follows that the units of the functions of either of the two powers can be expressed in units of the other; and this being the case, it is manifestly both convenient and desirable that in forming the dimensions of the standard electrical units, they should be constructed in terms of the metre gramme, second units.

The proposition to do this originated with Dr. Weber, and acting upon this proposition a committee of the British Association, comprising nearly all the leading electricians of Great Britain, was formed some years ago, which committee, with almost perfect experimental skill, determined an absolute measure for the values of the several units required for electrical measurement. Taking as the unit quantity of electricity that amount which would be generated by a gramme weight falling through a distance of one metre in one second, the value given to the unit of resistance was such as would allow this unit quantity to flow through it in one second. The means by which the values were experimentally arrived at cannot be described here. It suffices to say, that the unit of resistance being once determined, copies of it, formed of lengths of wire of a platinum-iridium alloy, were issued, from which copies the sets of resistances now so largely employed by electricians were adjusted. Out of compliment to the great German physicist who first proposed the fundamental law which governed the flow of the electrical current, the unit of resistance was called the ‘Ohm.’ It was a marked progress on the practical application of the electric force to be enabled to measure it, and, as it were, bring it under control.

Without its help the electric telegraph could not have become what it is; nor has it been without notable use in the art of protection against lightning. One of the greatest steps in advance in the application of the lightning conductor, from its discovery to the present day, has been the invention of the galvanometer. Franklin could not, but we can, test our lightning conductors.