[ 287 ] 
XIII. Introductory Research on the Induction of Magnetism by Electrical Currents. 
By J. P. Joule, F.R.S., Corr. Mem. R.A., Turin, Hon. Mem. of the Philosophical 
Society, Carnhridge, 8^c. 
Received June 21, — Read June 21, 1855. 
The researches of Jacobi and Lenz led them some years ago to the announcement 
as a law, that when two bars of iron of different diameters but equal to one another 
in length and surrounded with coils of wire of the same length carry equal streams 
of electricity, the magnetism developed in the bars is proportional to their respective 
diameters. Experiments which I made about the same time threw doubts on my 
mind as to the general accuracy of the above proposition, for I found that the mag- 
netism induced in straight bars of a variety of dimensions varying from ^ to 1 inch 
in diameter, and from 7 inches to one yard in length, was nearly proportional to the 
length of the wire and the intensity of the current it conveyed, irrespectively of the 
shape or magnitude of the bars. The valuable experimental researches which have 
recently been made by Weber, Robinson, Muller, Dub and others, refer chiefly to .the 
attraction of the keeper or submagnet, and are not calculated to confirm or disprove 
either of the above propositions ; and the correct view is probably that of Professor 
Thomson, who considers both of them as corollaries (applying to the particular con- 
ditions under which the experiments were made) of the general law, that “similar 
bars of different dimensions, similarly rolled with lengths of wire proportional to the 
squares of their linear dimensions and carrying equal currents, cause equal forces at 
points similarly situated with reference to them*.” I have been induced to under- 
take some further experiments with an endeavour to elucidate the subject, and also 
to open the way to the investigation of the molecular changes which occur during 
magnetization. 
I procured four iron bars one yard long and of the respective diameters \ and 
1 inch, their weights being 1736, 3802, 14560, and 55060 grs. Each bar was wound 
with 56 feet of copper wire -^th of an inch in diameter covered with silk, the number 
of convolutions being 1020, 712, 388, and 207 respectively. The smallest bar was 
closely covered throughout its entire length, but on account of the larger surface of 
the other bars the coils had to be distributed upon them as evenly as possible. Four 
other bars were also procured of the same diameters as the above. They were 
however twice as long, weighing 3500, 7624, 29944, and 108574 grs., and were wrapped 
with double the length of wire, forming 2060, 1435, 768, and 418 convolutions 
respectively. 
* Letter to the author. 
2 p 2 
