ELECTRO-MAGNETISM. 



the transverse bar of iron is small, it is 

 found that the iron retains its magnet- 

 ism for some time after the voltaic com- 

 munication is broken. If, instead of 

 merely breaking the connexion, the 

 electric poles are changed so as to re- 

 verse the direction of the current, then 

 the reversion of the magnetism takes 

 place with extraordinary rapidity. The 

 weight, indeed, falls ott', but is instantly 

 again attracted and sustained with the 

 same force as before. The rapidity of 

 this change is the more extraordinary 

 when it is compared with the slowness 

 and difficulty of changing the poles of a 

 magnet of equal force by the ordinary 

 method. If, instead of a heavy weight, 

 a light steel needle be in contact with 

 the poles of the electro-magnet, the 

 needle never falls off; the attractive 

 force being destroyed and re-established 

 before the weight of the needle has time 

 to effect its removal. 



(1.59.) An extraordinary sensation is 

 experienced when the piece of soft iron 

 connecting the poles is held in the hand 

 during this change. At first a power- 

 ful attraction is felt ; this on a sudden 

 fails, and the iron gives way ; but the 

 force is so instantaneously renewed, that 

 the hand is violently drawn up again by 

 an attraction as strong as before. The 

 moment the voltaic circuit is completed, 

 the iron is magnetized to a maximum, 

 and sustains its greatest weight. No 

 increase of magnetic power is obained 

 by augmenting the force of the voltaic 

 battery. 



(160.) With a larger horse-shoe mag- 

 net of soft iron, weighing twenty-six 

 pounds, and of which the diameter was 

 two inches and a half, the chord of the 

 arc being twelve inches and a half, and 

 the spiral wire being of brass, one-eighth 

 of an inch in diameter, and making 

 forty-four turns, and with the same 

 voltaic battery as in the former experi- 

 ment, the magnet supported 139 pounds. 

 When an iron wire was used, instead of 

 a brass one, this was increased to 154 

 pounds. 



~(161.) However great these effects 

 may appear, they are much increased 

 by augmenting the number of coils, 

 without extending the length of the 

 wire. Professor Henry, of the Albany 

 Academy, in the United States, and Dr. 

 Ten Eyck, employed for the construction 

 of the magnet a soft iron bar, two in- 

 ches square, and twenty inches long, 

 having the edges rounded, bent into the 



form of a horse-shoe, Five hundred 

 and forty feet of copper bell-wire was 

 wound round it, in nine coils of sixty 

 feet each. These coils were not con- 

 tinued from one end of the magnet to 

 the other, but each of them was wound 

 round a portion of the horse-shoe about 

 an inch in length, leaving the ends of 

 the wires projecting, and properly num- 

 bered. The alternate ends were soldered 

 to a copper cylinder, and the others to 

 a smaller cylinder of zinc, containing 

 only two-fifths of a square foot, and 

 forming a voltaic arrangement with 

 dilute ac-id. When the armature of soft 

 iron was placed across the ends of the 

 horse-shoe, it was found capable of sup- 

 porting 650 pounds; an astonishing 

 effect for so small a battery, which re- 

 quired a charge of only half a pint of 

 dilute acid. With a larger battery, the 

 weight sustained was 750 pounds, which 

 seemed to be the maximum of magnetic 

 power that could be developed in that 

 bar by voltaic electricity. It is remark- 

 able that when the ends of the wires were 

 united so as to form a continuous wire 

 of 540 feet, the weight raised was only 

 145 pounds. 



In a subsequent experiment, a mag- 

 net was wound with twenty-six strands 

 of copper bell-wire, covered with cotton 

 thread, thirty-one feet long ; about 

 eighteen inches of the ends were left 

 projecting, so that only twenty-eight 

 feet of each actually surrounded the 

 iron. The aggregate length of the coil 

 was, therefore, 728 feet. Each strand 

 was wound on a little less than an inch ; 

 in the middle of the horse- shoe it formed 

 three thicknesses of wire ; and on the 

 ends, or near the poles, it was wound so 

 as to form six thicknesses. With a 

 battery nearly five feet square, this 

 electro-magnet suspended 2063 pounds, 

 or nearly a ton weight. This appears 

 to be the most powerful single magnet 

 ever constructed, either by the ordinary 

 modes of magnetizing steel bars, or by 

 the voltaic current. 



(162.) Trials were also made to 

 procure a small temporary magnet, 

 which should raise the greatest weight, 

 compared with its own weight. A 

 small horse-shoe of round iron, slightly 

 flattened, one inch in length, and six 

 tenths of an inch in diameter, wound 

 round with three feet of brass wire, 

 raised, by means of a cylindrical battery, 

 420 times its own weight. Sir Isaac 

 Newton describes a magnet weighing 



