716 CONSTRUCTION OF ELECTRO-MAGNETIC ENGINE. 



takes place between a and d, d also being urged in the 

 direction in which it began to move ; and then, after a 

 complete revolution has been accomplished, the poles 

 are again reversed, and the motion is continued. 



The electro-magnet, fig. 361 A, is prepared of iron wire, 5 mm 

 thick, which is made red hot and hammered into shape with the 

 mallet, the hot wire being placed upon a small round piece of iron 

 which is clamped in the vice (fig. 48, page 57), or one of the beaks 

 of the parallel vice (fig. 49) may be used for the purpose. First the 

 poles are filed flat, paper is pasted round the horse-shoe as far as 

 the coil will touch the iron, and then the wire (O mm> 6 thick) is 

 wound on, beginning at the middle of the bent portion. From the 

 middle the wire is wound round until one pole is reached, then a 

 second coil is wound round on the top of that portion, going back 

 to the middle and proceeding to the other pole, where again the 

 wire is taken back to the middle. The free end of the wire left at 

 starting is held fast by the superposed coil, but the second end 

 should be tied with thread if it appears loose ; it frequently becomes 

 squeezed between the adjoining turns, and remains firm without 

 further* fastening. The keeper is filed into the shape shown in the 

 figure out of a small piece of bar iron ; a hole is drilled through it 

 and provided with a wire hook for suspending weights. 



The thin wire of this small electro-magnet is sensibly heated by 

 the current of a strong cell ; by that of two cells the heating effect 

 is generally so great that it is impossible to hold the magnet in the 

 hand. With one Meidinger element it will have a * lifting power ' 

 of about fifty grammes. If four coils are wound round it instead 

 of two it could support nearly 200 grammes. 



The horse-shoe required for the larger electro-magnet, fig. 361 B, 

 and a b in fig. 362, will have to be forged out of round bar iron by a 

 blacksmith ; the keeper for the electro-magnet, fig. 361 .5, should also 

 be hammered into shape on a smith's anvil, so as to diminish the 

 work of filing as much as possible. The wire for the spiral should 

 be l mm thick (without covering) or a little more ; one coil is suffi- 

 cient for obtaining a considerable lifting power when two strong 

 cells are used. A Meidinger cell is too weak in this case, as its 

 effect upon a large piece of iron with so few turns in the coil is 

 scarcely perceptible. So thick a wire needs no special fastening ; 

 if the turns are close together, and each of them is drawn as tight 

 as possible in winding on the wire, it will not unwind itself after- 

 wards. 



