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.

APPARATUS 81.

[Illustration: Fig. 54.]

_108. Electro-Magnet._ Fig. 54. Drive a nail into a board so that it
will project about 3/4 of an inch. A soft, or wrought-iron, nail is
best, but a short, thick wire-nail will do. If you do not have a thick
nail, use an iron screw. Wind 3 or 4 layers of insulated copper wire
around it, and fasten the bare ends of the wire down with bent pins.
Number 24 wire will be found a good size for experimental purposes.
Touch the wires leading from the battery to the ends of the coil, and
see if the nail will lift pieces of iron.

109. Note. Always leave at least 6 in. of wire at the ends of all coils
and windings. This is needed for connections and repairs, as the wire is
liable to get broken at any time around the binding-posts.

110. Note. After you have wound wire upon a core or spool, keep it from
untwisting by taking a loop or hitch around it with the wire. Fig. 55
shows how this is done. Pull the end of the wire enough to make the loop
stay in place.

APPARATUS 82.

_111. Electro-Magnet._ Fig. 56. Cut annealed iron wire into pieces, 3
inches long, straighten them (App. 28), and tie them with thread into a
bundle about 5/16 in. in diameter. Melted paraffine run in between the
wires will hold them in together, b