Experimental researches in electro-magnetism

This communication consists of three parts. In the first part the author shows that the common deflecting galvanometer, in which the deflecting forces are assumed to be as the tangents of deflection, is founded on false principles, and consequently leads to erroneous re­sults. The wire forming the coil is of considerable thickness, and therefore there is no fixed zero from which the deflections can be reckoned. The length of the coil, also, being generally short, occasions another serious error, us the theoretical investigation is founded on the supposition of an indefinite length. In proof of the inaccuracy of the indications of the common deflecting galvanometer, the author took two elementary batteries, the plates of one being one inch square, and those of the other two inches. The tangents of the deflections of the needle (proper precautions having been taken for the equally free passage of all the electricity evolved in either case,) were very nearly as 1 to 2, though it is obvious that the real quantities of voltaic electricity were as 1 to 4. The author’s torsion galvanometer gave the degrees of torsion nearly as 1 to 4. Other experiments led to similar conclusions. The author then examines the laws which were supposed to connect the conducting power of a wire for electricity, with its length and diameter, and which, according to Professors Cumming and Barlow, varies directly as the diameter, and inversely as the square root of the length; but, according to MM. Becquerel and Pouillet, directly as the square of the diameter, and inversely as the length. He points out the false conclusions of M. Becquerel, and that he has, in fact, deduced the value oftwo unknown quantities from one equation j and that M. Pouillet having arrived at his through the fallacious indica­tions of the common deflecting galvanometer, they are equally erroneous. The author then hows that the law pointed out by Cumming and Barlow is, in ordinary cases, nearest the truth; though, under certain circumstances, the limits f even that law may be passed. Hence, and from a series of experiments with the torsion galvanome­ter, he arrives at the unexpected conclusion, hat there is no deter­minate law of conduction, either for the length or diameter of the wire, but that it must vary, in every case, with the size of he plates, and the energy of the acid solution used in exciting them. his con­clusion the author shows to be in accordance with the views of conduction which he had previously published; namely, that there is no actual transfer of electricity, but that all the phenomena result from the definite arrangement of the electric fluid essentially belonging to the conducting wire.