ELECTRO-MAGNETS. 



Fig. 57. 



direction of the convolutions shall be the same as if the coils had 

 been continued round the bend. 



So long as no electric current passes along the convolutions of 

 the wire the horse-shoe will be free from magnetism. But if the 

 ends of the wire, marked + and , be put in connection with the 

 poles of a voltaic battery, so that a current flow round its convo- 

 lutions, the horse-shoe will instantly become a magnet, and will 

 be so much the more powerful as the current is more intense, and 

 the coils more multiplied. 



If an armature loaded with a weight be presented to the ends 

 of the horse-shoe while the current passes on the wire, it will 

 adhere to them, and the weight, if not too great, will be sup- 

 ported. 



143. In 1830 an electro-magnet of extraordinary power was 

 constructed under the superintendence of M. Pouillet, at Paris. 

 This apparatus, represented in tig. 57, consists of two horse-shoes, 

 the legs of which are presented to 



each other, the bends being turned 

 in contrary directions. The superior 

 horse-shoe is fixed in the frame of the 

 apparatus, the inferior being attached 

 to a cross-piece which slides in ver- 

 tical grooves formed in the sides of 

 the frame. To this cross-piece a dish 

 or plateau is suspended in which 

 weights are placed, by the effect of 

 which the attraction which unites the 

 two horse-shoes is at length overcome. 

 Each of the horse-shoes is wrapped 

 with 10,000 feet of covered wire, and 

 they are so arranged that the poles 



of contrary names shall be in contact. With a current of 

 moderate intensity the apparatus is capable of supporting a weight 

 of several tons. 



144. It is found more convenient generally to construct 

 electro-magnets of two straight bars of soft iron, united at one 

 end by a straight bar transverse to them, and attached to them 

 by screws, so that the form of the magnet ceases to be that of 

 a horse-shoe, the end at which the legs are united being not 

 curved but square. The conductor of the heliacal current is 

 usually a copper wire of extreme tenuity. 



145. In whatever form these magnets are constructed, the 

 circumstance which in their telegraphic use is of most importance 

 to notice, is that if proper conditions be observed in their pre- 

 paration, their acquisition of the magnetic virtue upon the 



197 



