common horse-shoe magnets about six inches long, all of
the same size; these retail in the market at about a dollar apiece. They
should be strong enough to hold up several times their own weight each.
Next, have turned out of good hard wood,--such as maple or boxwood,--two
spools not over half an inch long and an inch and a half broad, the
sides cut square both inside and out, as shown at S, Fig. 15; a hole
the third of an inch in diameter is to be made through the spool. Into
this hole is to be fitted a short rod of soft iron, I, about an inch
long, which should be a little rounded at the outer end. The bobbins may
be wound with as much insulated copper wire as they will hold. The wire
may be from the one-fortieth to the one-fiftieth of an inch in diameter,
as is most convenient to obtain, the latter size being preferable. The
resistance of such bobbins will probably be from two to three ohms each.
The soft-iron core I must project backwards far enough to be clamped
between the two outer magnets 1 and 3, while the inner one, 2, is drawn
back. When the bobbins are in their places, and are clamped between the
upper and lower magnets, they will stand as shown in Fig. 16, where the
view is from above; the magnets being buttoned down to the block they
rest on (see Fig. 17), which at the same time holds the soft-iron rods
with the bobbins upon them. The wires on these coils must be connected
in the same way they would be in order to make opposite poles of their
outer ends, if a current of electricity were to be sent through the
coils. An upright board B (Fig. 17) six or seven inches square, having a
round hole four inches in diameter cut out from the middle of it, must
be fixed near the end of the base-board; and over this hole is to be
screwed _tightly_ a piece of thin sheet iron or steel; it may be from
the one-twentieth to the one-fiftieth of an inch in thickness. It does
not seem to make much difference about the thickness of this plate. I
have generally got the best results from a plate one-fiftieth of an inch
thick. The upright board carrying this plate must be very rigid,
otherwise the plate will be kept tight to the magnets all the time; and
one of the conditions of success in working is, that this plate shall be
as close as possible to the magnet-ends, but not to touch: therefore fix
the board tight, and adjust the magnets by means of the button shown on
top of them in the perspective figure.

[Illustration: FIG. 17