Electro-Magnetic Armatures. A description of this form of armature is given in the chapter on electric motors. (See Index.)
CHAPTER IX.
ELECTRO-MAGNETS.
107. Electro-Magnets are absolutely necessary in the construction of most pieces of electrical apparatus. There are several ways of making them at home. To quickly make a good-looking one, a winder ([App. 93]) is required. We shall divide our electro-magnets into four parts: Core, washers, insulation, and coil.
Of course, you know that when a current of electricity passes through a wire, a magnetic field is produced around the wire. A coil of wire, or helix, has a stronger field than a straight wire carrying the same current, because each turn or convolution adds its field to that of the other turns. By having the center of the helix made of iron, instead of air, wood, or other non-magnetic bodies, the strength of the magnet is greatly increased. This central core may be fixed permanently in the coil, or be removable. For our purposes fixed cores are just as good as movable ones, and the coils are easily wound upon them.
When wire is wound by hand from a spool into a coil, or around a core, it soon becomes twisted and tangled. Make a winder. This will keep the wire straight and save much time.