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V. Account of Experiments with a powerful Electro-mac/net. 

 By J. P. Joule, F.R.S. $c* 



SOME years ago I announced that if a particle of wire con- 

 ducting a voltaic current be made to act upon a very large 

 surface of iron, the intensity of the induced magnetism will not 

 be much diminished by an inci'ease in the distance of that par- 

 ticle from the surface of the iron. Guided by this principle, I 

 constructed a very powerful electro-magnet in 1843f, and soon 

 after prepared the iron of the electro-magnet employed in the 

 experiments related in the present paper. This was a plate of 

 the best wrought iron, 1 inch thick, 22 inches long, 12 inches 

 broad at the centre, but tapered thence to the breadth of 3 inches, 

 as represented in the adjoining sketch 

 (fig. 1). The plate was then bent into 

 a semicircular shape, so as to bring its 

 ends within 12 inches of one another. 

 Previously to fitting up this bar as an 

 electro-magnet, I made a few experi- 

 ments with a view to test the principle 

 above named more completely than I 

 had hitherto done. 



A length of about eight yards of insu- Yi&l 

 lated copper wire, ^yth °^ an mcn m 

 diameter, was divided into two exactly 

 equal portions, one of which was wound 

 four times round the broadest part of 

 the iron, and close to its surface ; the other was also wound four 

 times round the broadest part of the iron, but was kept at the 

 distance of one inch from its surface by means of interposed 

 pieces of wood. A constant current of electricity was alternately 

 passed through the wires ; and the deflections of a magnetic 

 needle half an inch long, placed at the distance of two feet from 

 the iron bar, were observed to be as follows : — 



6° 23' with the wire close to the surface of the iron. 

 6° 9' with the wire at the distance of one inch from the surface 

 of the iron ; 



showing only a trifling 'diminution of effect in consequence of 

 the removal of the wire to the distance of one inch from the 

 surface. 



Having been thus fortified in my previous conclusion as to 

 the propriety of enveloping broad electro-magnets with a very 

 large quantity of coils, even though the outer ones should be 



* Com rniin i rate il by the Author. 



t Philosophical Magazine, S. 3, vol. xxiii. p. 268. 



