494 Royal Society, 



experiment, toward which the increments in the force continually 

 approach, as if the opposed bodies were only susceptible of a given 

 amount of induction under the existing circumstances. 



Taking the force toward the limit of action, the amount of induc- 

 tion is in some inverse ratio greater than that of the simple distance ; 

 it was not however in any case found to exceed the inverse sesqui- 

 plicate ratio or f power of the distance ; as the distance is diminished 

 the induction is as the distance inversely, but may in the mean time 

 be as the f or |^ powers of the distances inversely, or near those 

 powers. On further diminishing the distance, the induction was 

 found in certain cases to be as the f and \ powers of the distances, 

 thus causing a series of changes in the law of magnetic attraction as 

 commonly observed, which have hitherto greatly embarrassed the 

 views of philosophers in their inquiries into this species of force. 

 "When the convergence is slow the induced force may not for a long 

 series of terms appear to change, but when from any circumstance 

 the convergence is accelerated, then the changes become more 

 marked and successive. As a general result, however, the author is 

 led to conclude, that magnetic induction is as the magnetic intensity 

 directly, and from the \ to the f power of the distance inversely. 



In the course of these inquiries, it was found that the induc- 

 tive action depended, not on the mass, but on the surface of the 

 magnetic substance, and that magnetism, like electricity, exhibits a 

 decrease of intensity when the surface of the iron upon which it is 

 disposed is extended. A hollow cylinder of soft iron was carefully 

 prepared in a lathe, and fitted with a solid interior core capable of 

 being drawn out from within the cylinder ; this compound body was 

 exposed to the inductive action of a poM'erful magnetic bar, and the 

 induced force estimated by the reciprocal force of attraction ex- 

 erted between the mass and a cylinder of soft iron suspended from 

 the author's magnetic balance, or from one arm of a light beam, set 

 up in the way of a common balance. The degree of force being 

 observed, the solid core was drawn out so as to extend the surface 

 of the mass under induction. The intensity immediately declined, 

 and again increased on replacing the solid within the hollow cylin- 

 der, being a result of exactly the same character as that produced 

 by the extension of an electrified surface. When the interior solid 

 core was removed altogether, then the induced force remained un- 

 changed, it being precisely the same whether the body were taken 

 hollow or solid. In accordance with this result, hollow cylindri- 

 cal magnets were found as susceptible of magnetic power as solid 

 masses of the same temperament and dimensions; an unmagnetized 

 solid and tempered steel cylinder, placed within a hollow tempered 

 steel cylinder, does not become magnetic on touching the external 

 cylinder in the usual way. The magnetism, however, of a hollow 

 cylindrical magnet is partially destroyed by placing within it a cy- 

 linder of soft iron, or the reverse poles of another magnet ; nor can a 

 hollow cylinder of tempered steel having a solid core of soft iron 

 be rendered magnetic by the usual methods of touch. These re- 

 sults, it is considered, supply the experiments thought by Mr. Barlow 



