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, but stout thread will do. Wind 3 or 5
layers of No. 24 insulated copper wire upon the soft iron core. This is
useful for simple experiments, and this idea may be applied to magnets
to be used in pieces of apparatus. Hold the bundle of wires in a vise,
and file the ends smooth, before winding on the wire. Paraffine should
be used to hold the turns of insulated wire together.
[Illustration: Fig. 55.]
[Illustration: Fig. 56.]
[Illustration: Fig. 57.]
APPARATUS 83.
_112. Electro-Magnet._ Fig. 57. An electro-magnet with a removable core
may be made by winding the wire on a spool. The core is made, as in
App. 82, of soft iron wires, bound together with stout thread. A bolt
may be used instead of the wire, but the wire loses its magnetism much
quicker than a soft steel bolt would. (Study residual magnetism.) This
magnet is strong enough for many purposes, but the wire is too far from
the core, on account of the thickness of the wood, to make it efficient.
The wire may be wound on by hand, but a winder (App. 93) will do much
better and quicker work.
APPARATUS 84.
[Illustration: Fig. 58.]
[Illustration: Fig. 59.]
_113. Horseshoe Electro-Magnet._ Fig. 58. Bend soft iron wires, and make
a bundle of them. If you wish to wind the wire around spools, the bundle
cannot be very large. It will be found best to make the bundle about 3/8
in. in diameter, and not to use the spools. Strong paper should be
wrapped once or twice around the legs of the horseshoe, and the
insulated wire, say 4 layers, can then be wound directly upon this. (See
Sec. 115 for method of making connection between the coils.) It is a
little troublesome to wind wire upon a horseshoe like this, and for
App. 85. Spools are handier, because each can be wound separately, and
then be slipped in place. The ends of the horseshoe should be filed
smooth.
APPARATUS 85.
_114. Electro-Magnet._ Fig. 59. An ordinary iron staple is useful as the
core of a small magnet. One like this is shown also in Fig. 94, used as
a telegraph sounder. It takes some time to wind 4 layers of wire on to
each leg of the staple, so be sure to see Sec. 115 about the method of
winding. In Fig. 59 the half-hitches (Sec. 110) are not shown. Coat the
finished coils with paraffine.
115. Method of Joining Coils. Fig. 60. If A and B represent the two
cores of a horseshoe electro-magnet, the coils must be joined in such a
manner that the current will pass around them in opposite directions, in
order to make them unlike poles. The current is supposed to pass around
B, Fig. 60, in the direction taken by clock hands, while it passes
around A in an anti-clockwise direction. The inside ends, Sec. 123, of
the coils may be twisted together, or fastened under a screw-head. In
Fig. 60 one coil is shown to be a continuation of the other.
[Illustration: Fig. 60.]
APPARATUS 86.
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
_116. Electro-Magnet._ Fig. 61. Wind 6 layers of No. 24 or 25 insulated
copper wire around a 5/16 machine-bolt that is 2-1/2 in. long. Fig. 61
shows one method of holding the bolt solidly in an upright position, so
that magnetic figures can be easily made and the magnet studied. Two
nuts are used, the lower one being counter-sunk, so that the base will
stand flat upon the table. This bolt is shown without washers (Sec. 119),
and will do fairly well to show the action of electro-magnets. The ends
of the wire should always be left 5 or 6 in. long, and be led out to
binding-posts. The coil may be held in place, and its turns kept from
untwisting by coating it with paraffine. The base may be of any desired
size.
APPARATUS 87.
_117. Electro-Magnet Core._ Fig. 62. This shows another method of
fastening a bolt-core in an upright position. This is done without the
use of two nuts. A strip of tin, T, 1 in. wide, is punched and slipped
onto the 5/16 bolt before the nut is screwed on and the coil wound. This
is fastened to the base by screws, S. Washers, W, are here shown. (See
Sec. 119 for washers.)
APPARATUS 88.
_118. Bolt Electro-Magnets_ are easy to make, according to the
directions given, and they are, when finished, more like the regular
purchased magnets than any of the other forms described. With proper
batteries (App. 3, 4, etc.,) they can be used for a great variety of
purposes, as will be seen. There are many forms of bolts in the market,
but the ordinary "machine bolt," 5/16 in. in diameter, is best for our
purposes. The ones 2 and 2-1/2 in. long are used.
119. Washers or coil ends are used on the bolt magnets so that
considerable wire can be wound on closely and evenly. These are made out
of thick pasteboard, which cuts smoother if it has been soaked in melted
paraffine. Unless you know how, you will find it a hard job to make the
hole in the exact center of the washer. The method of easily making
washers is illustrated in Fig. 63.
First place a spool (the end of which is 7/8 or 1 in. in diameter) upon
the table, and lay the pasteboard upon this. Push a large round nail
through the pasteboard into the hole in the spool. The nail should be
nearly as large as the hole. Use the large nail as a handle, and with
the shears cut around the edge of the spool end. Cut the washer as round
as possible, and be careful not to cut into the spool.
The holes in the washers will be a little smaller than the 5/16 bolt.
This will make the washers hold tightly to the bolt when you force them
on. Fig. 64 shows the bolt-core, with the washers in place. If you
cannot get a large nail, a lead-pencil, or sharpened dowel, will do to
force through the pasteboard.
[Illustration: Fig. 63.]
120. Insulation of Cores. While the covering on the wire would probably
be all that is necessary to thoroughly insulate the coil from the core,
it is better to wind a layer or two of paraffine paper around the bolt
(Fig. 65) before winding.
[Illustration: Fig. 64.]
[Illustration: Fig. 65.]
[Illustration: Fig. 65-1/2.]
121. The Coils of wire to be used upon the bolt-cores should be put on
with the winder (App. 93). For all ordinary purposes No. 24 or 25 single
or double cotton covered copper wire will do. It is better to put on an
even number of layers. The winding (See Fig. 70) begins at the nut-end
of the bolt, and by using 6 or 8 layers of wire, instead of 5 or 7,
both coil ends will be at the same end of the bolt.
122. Method of Winding the Coils. The winders used for bolt magnets are
described in App. 91, etc. We shall suppose that the washer, W, Fig. 70,
and the insulation, I, are upon the bolt before screwing it into the
winder-nut, W N. Make a pinhole, P H, in the right-hand washer, as near
the bolt-nut, B N, as possible. Stick about 6 in. of the wire through P
H, and wind this end around W N, as shown, to hold the wire. The supply
of wire should be upon a spool slipped onto some stationary rod (App.
23), so that you can give your entire attention to winding. Begin to
turn the winder slowly at first. Turn the handle towards you when it is
at the bottom, as in Fig. 70; that is, if you look at it from the side,
turn the handle clockwise. Let the wire slip through your left hand as
the turns are made, and guide it so that the turns will be close
together. If they go on crooked, unwind at once, then rewind properly.
You can guide the wire best by holding your left hand about 8 or 10
inches from the bolt. As soon as you reach the left side or head end of
the bolt, feed the wire towards the right. If at any time the layers
become rough on account of one turn slipping down between turns of the
previous layer, fasten a piece of paraffine paper around the coil as
soon as the imperfect layer is completed. Wind on 8 layers, and count
the number of turns in one or two of them, so that you can tell about
how many turns in all you have around the core. Make a "half-hitch" (see
Sec. 110) with the wire when the last layer is finished, to keep it from
unwinding, and leave a 6 in. end.
The coil should be protected by fastening around it a piece of
dark-colored stiff paper. Paraffine paper is good for this purpose. With
a little practice you will be able to rapidly and neatly wind on the
wire. The winder-nut, W N, must hold the bolt solidly to keep it from
wobbling.
123. We shall call the starting end of the wire which passes through P
H, the inside end, and the end of the last layer the outside end. This
can pass out between the washer and the paper covering.
APPARATUS 89.
_124. Experimental Horseshoe Electro-Magnet._ Fig. 66. Among the most
useful pieces of apparatus for home use, is a good horseshoe
electro-magnet. Fig. 66 shows a very convenient and practical form. With
this, alone, can be shown all the principles of telegraph sounders,
electric bells, etc. They are excellent for making magnetic figures (See
text-book). You are supposed to be looking down on the App. in Fig. 66.
The bolts are 2 in. apart center to center.
[Illustration: Fig. 66.]
The bolt magnets are fully described in App. 88; the binding-posts, as
App. 46; the yoke, as App. 71; the method of fastening to the base, as
App. 90; the base is 5 x 4 x 7/8 in.; the magnets are made of 5/16
bolts, 2-1/2 in. long.
125. To Join the Coils, fasten the two inside ends (Sec.123) of the wire
to a middle binding-post, and carry the outside ends to the two outside
binding-posts. In this way you can use either magnet alone, if desired
(See experiments in text-book), or change the polarity at will by
changing the connections. (See Sec. 115 and 123.)
APPARATUS 90.
_126. Fastenings for Electro-Magnet._ Fig. 67. When both electro-magnets
are to be permanently fastened to a base, especially if tin yokes are to
be used, as in App. 89, it is best to use a nut on each side of the
yoke. It is important to have a perfectly tight connection between bolt
and yoke. Several ways of fastening the bolts and yokes are shown; but
it will be found best to cut holes in the base for the lower nuts, and
to screw the yoke directly to the base. This makes a solid and pleasing
arrangement. For the experimental magnets (App. 89) make the yoke 3-1/4
in. long, and place the magnets 2 in. apart center to center.
[Illustration: Fig. 67.]