466 Mr. J. P. Joule on the Magnetizabikty of Iron, 



^i'.Cl.i/i ./ On Electro-Magnetic Forces^. 



•^'h ^' ••+■*-• ***** 

 • Suspecting that the extreme power of the large electro-magnet, 

 No. 1, had not been attained in my last experiments, on account 

 of the imperfect insulation of its coils, I detennined to try it 

 again, using every precaution which was calculated to develope 

 its magnetism to the full extent. 



The old \Tire was removed, and a bundle consisting of twenty- 

 one copper wires, each 23 feet long and jj'jth of an inch in dia- 

 meter, was formed by binding the wires together with cotton 

 tape. This bundle of wires was coiled on the iron of the electro- 

 magnet, which had been previously insulated by a fold of calico. 



Sixteen cast-iron cells, of the same size as those described in 

 my last, were arranged in a series of four elements, and con- 

 nected by good conductors with the electro -magnet. The attrac- 

 tive force developed was then found to be so great, that a weight 

 of 2775 lbs. had to be applied to the armature in order to sepa- 

 rate it from the electro-magnet. 



Now by the formula ,r=280« given in my last paper, we ob- 

 tain 2800 lbs. for the greatest lifting power of this electro- 

 magnet, or only 25 lbs. more than that actually found, which 

 coincidence cannot but be considered as a striking proof of the 

 accuracy of the general principles I have before advanced. That 

 the saturation of the iron must have been veiy nearly effected in 

 the above experiment appears from the fact, that the quantity of 

 electrical cun-ent employed was four times as great as that which 

 was competent to make the same electro-magnet sustain 2128 lbs. 



Although the battery used above for obtaining maximum effects 

 was very powerful, each cell exposing an active surface of two 

 square feet of cast iron, I have found that a very good lifting 

 power may be obtained mth this electro- magnet by means of a 

 very small voltaic arrangement. For instance, it can lift 8 cwt. 

 when the current generated by a single pair of 4-inch plates of 

 iron and amalgamated zinc is passed through its coils; and with 

 a single pair of platinized silver and amalgamated zinc plates 

 exposing only two inches of surface, the attraction is such as to 

 require the utmost force I can exert, even to slide the armature 

 upon its poles. 



Broom Hill, near Manchester, 

 November 23, 1840. 



♦ Annals of Electricity, vol. v. p. 470. 



