Page:Scientific Memoirs, Vol. 2 (1841).djvu/20

soft iron is subjected. I shall take the liberty of making some remarks on this subject. In examining the phænomena presented by the conjunctive wire, we see that the beautiful theory of M. Ohm completely accounts for them. Considerably enlarged by the ingenious researches of M. Lenz, and conjointly with the electro-chemical views of Mr. Faraday, this theory has become capable of connecting under one sole point of view a multitude of facts. But, nevertheless, the elements serving as basis to this theory are not placed beyond objection. The resistance opposed by any conductor to the passage of the electric current is there admitted as a permanent force, and enters as such into the general expression of the force of the current. Let $E$ be the electro-motive force, $R$ the resistance of a pair, and $R'$ the resistance of the conjunctive wire, the force of the current, measured in any way whatsoever, will be expressed by ${\frac {mnE}{mR+nR'}}$ and this force will increase indefinitely by multiplying at the same time the surface $n$ and the number $m$ of the pairs. But it would not be necessary to excite to a great degree the energy of the battery, in order to destroy the conducting wire by the development of heat, or rather by the heat which the wire itself developes, in opposing the passage of the voltaic current. Certain powerful effects, which do not take place suddenly like other physical phænomena, (for instance, the solidifying of water when its temperature has sunk below zero), but which accompany all electrical actions even from their most feeble indications, and which are always directed towards the weakening of the conducting power, must not be neglected when the nature of the conducting wire has to be taken into account. M. Lenz, in his valuable memoir on the conducting power of metals at various temperatures, has drawn the attention of philosophers to the complication of effects caused by the influence of the temperature of the conducting wire; the power of the current, the temperature, and the resistance being in an intimate and reciprocal relation. In another memoir, this philosopher has announced some important facts relative to the conducting power, which is changed by the least difference in the chemical or physical conditions of the metals; so that this power, measured with precision, the type of which has been given by this able physicist, may serve as the most delicate test of the purity of the metals. I cannot, moreover, pass unnoticed the re-