294 
MR. joule’s introductory RESEARCH ON THE 
Table V. 
Attraction, at 9 inches, of steel electro-magnet, 7f inches long, ^ inch diameter, 
wound with 34 feet of wire. 
Number of 
experiments 
forming the 
mean result. 
Intensity of 
current. 
Total change of 
magnetism on 
reversing the 
current. 
Magnetic set. 
Total change 
minus magnetic 
set. 
Set divided by 
square of current. 
Total change 
minus set, divided 
by current. 
40 
•0045 
•00281 
•0000092 
•00280 
•454 
•622 
40 
•0089 
•00543 
•0000448 
•00539 
•566 
•606 
20 
•0263 
•01663 
•0002157 
•01641 
•312 
•624 
10 
•0489 
•03132 
•0008769 
•03044 
•367 
•622 
8 
•0921 
•06046 
•0032278 
•05723 
•381 
•621 
j 20 
•1594 
•22992 
•02356 
•20636 
•927 
1-294 
8 
•3201 
•65241 
•17791 
•47450 
1-736 
1-482 
6 
•4582 
1 09119 
•39722 
•69397 
1-892 
1-514 
6 
•5688 
1-45540 
•58421 
•87119 
1-806 
1-531 
6 ■ 
•8381 
2-22020 
1-03410 
1-18610 
1-472 
1-415 
2 
1-5108 
2-96510 
1-29880 
1-66630 
•569 
1-103 
1 
2 
3 
4 
6 
6 
7 
From the preceding Table it appears that the set of the steel bar increases almost 
exactly with the square of the current from the intensity ‘0045 to ‘0921 ; that thence 
to ‘1594 it increases more rapidly than the cube of the current; and that from that 
point it increases in a gradually diminishing ratio as the point of saturation is 
approached. It will be remarked that the first five numbers of column 7 are nearly 
equal to one another ; but that when the set begins to increase more rapidly than 
with the square of the current, the magnetism of the bar over and above the set 
increases more rapidly than the current. 
There is a striking and instructive analogy between the phenomena above pointed 
out and those relating to the set and elasticity of materials. Professor Hodgkinson 
has pointed out that the set or permanent change of figure in any beam is propor- 
tional to the square of the force which has been applied, a law which of course is 
transgressed near the breaking-point. May we not with propriety term the point at 
which, in the foregoing experiments, the set increases so abruptly, the magnetic hreak- 
ing-polnt} Mr. Thomson has propounded the view, that the elasticity of all bodies is 
perfect when abstraction is made of the effect of set. The foregoing Tables indicate 
approximately the same law respecting what might be termed the magnetic elasticity. 
The analogy thus established between magnetic and ordinary molecular actions, 
when viewed in connexion with those changes of dimension which take place in iron 
bars by magnetization, and which I propose to study more deeply, promises to afford 
a point of view whence a more perfect insight into the nature of magnetism than we 
at present possess, may ultimately be attained. 
Oak Field, Moss Side, Manchester, 
June 20, 1855. 
