Sir Humphrey Davy was followed in his researches on the conductivity of the different metals by the electric force, by a number of other scientific men. His immediate successor in entering upon this line of observations was a French naturalist of eminence, Antoine C. Becquerel. Perhaps no man after Benjamin Franklin studied the phenomena of electricity with such thorough insight, free from all misleading theoretical delusions, as Becquerel. He was educated at the Polytechnic School of Paris, and in 1810, at the age of twenty-two, entered the army as an officer of engineers, but quitted it five years afterwards with the rank of colonel, to devote himself entirely to scientific pursuits. Geology and mineralogy first engaged his attention, but he soon quitted these studies to devote himself, heart and soul, to the observation of the phenomena of electricity, which fascinated him as much as they had done Benjamin Franklin. The result was the discovery of a great many facts previously unknown, making Becquerel, amongst others, one of the founders of the science of electro-chemistry. The result of his researches concerning the conducting power of the electric force by different metals may be stated as follows:

Copper100·00
Gold93·60
Silver73·50
Zinc28·55
Platinum16·40
Iron15·80
Tin15·50
Lead8·30
Mercury3·45

It will be seen, in comparing this statement with the result of the investigations of Sir Humphrey Davy, that while the latter places silver before copper in conductivity, Becquerel puts copper at the head of the list. Probably, the explanation of this difference in the result of scientific research, by two men equally learned and equally able, may be found in the fact that the conductivity of copper varies greatly according to the purity of the metal. It has been ascertained that absolutely pure copper of the finest kind—such as that existing in the Isle of Cyprus, youngest of mother Britannia’s colonial children—has a conducting power of upwards of twenty per cent. more than the ordinary copper of commerce. While thus arriving at different estimates, Sir Humphrey Davy and Becquerel are singularly in agreement in one important respect: they both make the relative electrical conductivity of copper and iron about the same, placing it, the one a little under, and the other a little over 100 to 15. In other words, they both say that the value of copper as a lightning conductor to iron is as twenty to three, or between six and seven times as great.

Among a host of other investigators of the subject there stand forward, besides Sir Humphrey Davy and Antoine Becquerel, two Germans, Professors Lenz and Ohm, and another French savant, Claude Pouillet. In the opinion of many scientific authorities, especially in the United States, the experiments of Professor Lenz regarding the comparative electrical conductivity of different metals were more carefully made than any other, and are therefore deserving of the greatest credit. He had, indeed, ample means and great leisure at his disposal, making his scientific investigations under the patronage of the Grand Duke, afterwards Emperor, Nicholas of Russia, while acting as his private tutor at the university of St. Petersburg. The researches of Professor Lenz as to the comparative power of various metals to conduct the electric force were given in the following results—copper, as before, standing as the centesimal unit:—

Silver136·25
Copper100·00
Gold79·80
Tin30·84
Brass29·33
Iron17·74
Lead14·62
Platinum14·16

A comparison of the figures here given with those of Sir Humphrey Davy and of Becquerel shows that the results obtained by Professor Lenz differ from those of both the other investigators. Like Sir Humphrey Davy, Professor Lenz declared silver to be of greater electric conductivity than copper, but, on the other hand, he assigned lead a very low place, putting it under iron, instead of far above it. It is difficult to explain this wide divergence, even on the utmost allowance of purity, or impurity, of metals. As regards the most important question, from a practical point of view—that of the difference between copper and iron—Professor Lenz, it will be noticed, places iron higher in the scale than both Sir Humphrey Davy and Becquerel. Still, in his estimate also, copper was admitted to have about six times the conductive power of iron.

While, as just stated, the experiments of Professor Lenz on the electric conductivity of metals are held in the highest esteem in America, the same is the case in Germany as regards those of Professor Ohm. The latter is held to be there the highest authority on all subjects connected with the measurement of the electric force. The professor, born at Erlangen, 1787, and for many years teacher of natural history at Munich, where he died in 1854, devoted the utmost patience and an immense amount of time to the definite object of ascertaining the electric conductivity of all the metals, registering the result of his experiments in a special work, the most complete existing on the subject. According to Professor Ohm, the principal metals stand to each other in conductivity as follows:—

Copper100·00
Gold57·40
Silver35·60
Zinc33·30
Brass28·05
Iron17·40
Platinum17·10
Tin16·80
Lead9·70

Here again is a striking difference with the statements of other investigators. It seems absolutely inexplicable indeed, how it could happen that scientific men of eminence, and admitted authorities on the subject they are treating, came to vary on the electric conductivity of several of the metals. The difference is most astounding as regards silver, the conductivity of which, compared with the per cent. of copper, Professor Lenz places at 136·25, Sir Humphrey Davy at 109·10, Becquerel at 73·50, and Professor Ohm at only 35·60. The only conclusions that can be come to under the circumstances are, that the record of Professor Ohm’s results as regards silver is incorrect; or, that the relative degrees of purity of the samples of metal experimented upon by him and the other professors differed very widely. What is of more importance than this question, is the comparative rank of copper and iron. Here, it is satisfactory to find, the results ascertained by Ohm agree very nearly with the conclusions of the other investigators, it being laid down that copper has about six times the conductive power of iron.

The place filled in America by Lenz, and in Germany by Ohm, is generally assigned in France to Professor Claude Pouillet, a savant who devoted, perhaps, more time than any other in his own country to the study of the phenomena of electricity. Born in 1791, Professor Pouillet became, at a comparatively early age, the director of the celebrated scientific institution of Paris known as the ‘Conservatoire des arts et métiers,’ which led him to enter upon a course of experiments in electricity, and most particularly, at the request of the government, upon investigations as to the best material for lightning conductors. The result of these was published in a lengthened treatise, in which Professor Pouillet set down the electric conductivity of the principal metals, taking copper at a hundred, as follows:—