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:— Gold 103·05 Copper 100·00 Silver 81·26 Brass from 23·40 to 15·20 Platinum 22·50 Iron from 18·20 to 15·60 Cast Steel 14·75 Mercury 2·60 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:— 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.