_ action upon soft iron, as proportional to the surface i ndent: 
of the mass, [had long since suspected the fallacy of the posi- 
_ tion; and this mainly from the fact, that when the magnetism of 
-» _ soft iron was made to react upon the helix in the development of 
» __—s secondary currents, the intensity of such currents always appear- 
ed to be augmented in proportion to the increase of the mass of 
‘Soft iron—that is, with the same helix. For instance, a tube of 
soft iron gave much less reaction, as judged of from the spark and 
shock, than when a solid bar of the same diameter and length 
Was used; and from the well known law of action and reaetion, 
____- nothing else could have been inferred than that the development 
Of magnetism was in proportion to the mass and not to the sur- 
face. The azial galvanometer clearly establishes this point. 
_, The apparatus used in the experiments consisted of a small 
Grove’s battery, of six pairs—three helices, formed each of the 
same length of wire, but each having central openings or bores of 
different diameter—several bars and tubes of soft iron,—and a deli- 
i 2 tate spring balance. The bore of helix No. 1 was. 32 inch di- 
___- ameter ; helix No. 2, 43, and helix No. 3, 42. Soft-iron bar No. 
1 was four inches longer than the helices, and ; inch*diameter, 
_ . and-when suspended within the helix No. 1, was distant from it 
__ #y inch; having therefore freedom of motion within the helix. 
Tt is not important in these experiments, that the axis of the bar 
: Should ‘exactly coincide with the axis of the helix; for the sum 
of all the actions is the same for every position of the axis, as 
7 found by careful experiment. Soft-iron bar No. 2 was hollow, 
Ps of the same length and diameter as bar No. 1, and half its weight. 
ga Tt is important that the lengths should correspond, for reasons 
to be hereafter given. Soft-iron bar No. 3 was of the same di- 
_. Ameter as bars 1 and 2, but shorter by two inches. ae 
| ee When bar No. 1 was suspended to the hook of the spring bal- 
_ Alice, and inserted about half way through helix 1, and the helix 
_ Sennected with the battery, the bar was drawn down into the 
helix with a force of six pounds, as indicated by the spring bal- 
_ nee.’ This balance is sensitive to the ;'; of an ounce, and when 
_ dealing: with pounds, may be considered as sufficiently accurate. 
: The’ hollow bar No. 2 was then substituted for bar No. 1, and 
Was drawn down by a force of three pounds and a fraction over, 
“the fraction doubtless arising from some difference in the textures 
3 of the two bars. We are thus furnished, by this satisfactory and 
_ Practical test, with a definite law of inductive action, viz. 
mas 
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