DISCOURSE OF W.B.TAYLOR: NOTES. 391 



that "it rendered powerfully magnetic an electro-magnet on which 

 were coiled 39 thick copper wires, each about 35 feet long." 



The only subsequent extension of Henry's results worthy of 

 note, is that made by the ingenious English physicist Joule. It 

 had been found that the maximum attractive force of the electro- 

 magnet is exhibited near its surface, and that an enlargement of 

 the iron does not correspondingly enhance its magnetic power.f If 

 we adopt the conception of Coulomb and of Weber that the con- 

 stituent molecules of the iron are each independent permanent mag- 

 nets, this variation of magnetic force in a large iron bar, receives 

 an easy explanation ; since the middle portion of the bar is not only 

 less coerced by the surrounding coil,J but is powerfully impressed 

 by the opposite induction of the outer belt of polarized molecules. 

 While therefore we should a priori expect the aggregate attractive 

 force to increase with the size of the bar, (i. e. the cross-section or 

 end -surface of the poles,) we find that this very extension occasions 

 a large amount of neutralization by the interior opposite magnet- 

 ism; such depolarization being obviously the condition of least 

 constraint. 



Acting on the theory that the power of the magnet would depend 

 on the extent of efficient polar surface, and at the same time on the 

 propinquity of the electric coil, Joule's highest magnetic triumph 

 consisted in giving a greatly increased depth to the horse-shoe, (as 

 though a vast number of small horse-shoes were laid side to side 

 and cemented together,) without an increase of its width; the former 

 dimension exceeding the latter many times : so that the two poles 

 presented a pair of long narrow parallel surfaces close together, 

 bounding a long trough or gutter. And the addition of the oblong 

 armature gave the whole the general appearance of a tube. The 

 author thus describes .its construction: "A piece of cylindrical 

 wrought-iron, eight inches long, had a hole one inch in diameter 

 bored the whole length of its axis; one side was then planed until 



* L. & E. Phil. Mag. Dec. 1836, vol. ix. p. 475. 



f Barlow had drawn the conclusion from his own experiments, that the mag- 

 netic power of iron resides entirely at the surface, and is irrespective of mass. 



t The direct action of the electric circuit in the coil would probably not be 

 sensibly less on the interior than on the exterior of a large iron core; but its 

 polarizing energy must necessarily be largely expended in coercing the homolo- 

 gous direction of the nearest outer layers of molecules, leaving the interior mass 

 more under the immediate inductive influence of its girdle of magnets. 



g Having this in view Joule (in imitation of Coulomb's faggot of thin magnets) 

 employed with success a bundle of wires for the electro-magnetic core. (Stur- 

 geon's Annals, etc. July, 1839, vol. iv. pp. 58-61.) It is evident also from the above, 

 that the removal of the central portion of the inner core, in other words the 

 employment of a tube of certain thickness, in place of the solid bar, would actu- 

 ally increase. the resultant power of the magnet, with a diminished mass of iron. 



