Electro-magnet

(Elect.) A bar of soft iron which, under the influence of a voltaic current, becomes magnet; but this magnetism is only temporary, for the coercive force of perfectly soft iron is null, and the two magnetic fluids neutralize each other as soo nas the current ceases to pa through the wire. An E. has the horse-shoe form (Fig. 299), and a copper-wire, covered with silk or cotton, is rolled several times round them on the two branches, so as to form two bobbins, AandB. In order that the two ends of the horse-shoe maybe of the opposite polarity, the winding on the two limbs A and B must be such that if the horse-shoe were straightened out, it would be in the same direction. A B Fig. 299.-ELECTRO-MAGNET. The power of the E. is enormously greater than that of any permanent magnet. A permanent magnet, weighing 1 pound, has been made to carry 27; but Dr. Joule wa sable to construct a small E., by arranging the coils to advantage, and proportioning the wire of the core, and the thickne and length of the wire, which would carry3500 times its own weight. E., instead of being made in one piece, are frequently constructed of two cylinders, jointly screwed to a stout piece of the same metal. Such is the E. in Morse's telegraph. [s. 322] IMG:p0351_0.jpg:Gravür