which may be wound around it. The finished compass-needle should
swing very freely, and should finally come to rest in an N and S line
after vibrating back and forth several times.

APPARATUS 67.

_90. Glass-Covered Compass._ A perspective view of this apparatus is
shown in the tangent galvanometer. (See Index.) The outside band, E, is
made of thick paper, 1 in. wide, and with such a diameter that it just
fits around the glass. In this model, the glass from an old alarm-clock
was used, it being 4 in. in diameter. Four pasteboard strips were sewed
to the inside of the paper band E. They were made 7/8 in. long, so that
the glass, when resting upon them, would be near the top of E.

The needle should be not over 1 in. long, if it is to be used in the
galvanometer. A long slender paper pointer should be stuck to the top of
the needle. Be careful to have the combined needle and pointer well
balanced, so that it will swing freely. A circle graduated into 5-degree
spaces should be fastened under the needle.

_91. Astatic Needles._ In the magnetic needles so far described, the
pointing-power has been quite strong. By pointing-power we mean the
tendency to swing around to the N and S. In App. 65 the 2 needle magnets
had considerable pointing-power, because they helped each other. For
some experiments in electricity a magnetic needle is required which has
but little pointing-power; in fact, to detect the presence of very
feeble currents by means of the needle, the less the pointing-power the
better. Can you think of any way to arrange App. 65 so that it shall
have very little pointing-power?

APPARATUS 68.

_92. Astatic Needle._ Fig. 43. Turn one of the needle magnets of App. 65
end for end, so that the N pole of one shall be at the same end of the
paper as the S pole of the other. You can see that by this arrangement
one needle pulls against the other. The magnetic field still remains
about the little magnets, otherwise this combination would be of no
value in the construction of galvanometers. The more nearly equal the
magnets are in strength, the less the pointing-power of the combination.

[Illustration: Fig. 43.]

[Illustration: Fig. 44.]

APPARATUS 69.

_93. Astatic Needle._ Fig. 44. Magnetize two sewing-needles as equally
as possible, by rubbing them over the pole of a magnet an equal number
of times. Remove the covering from a piece of fine copper wire, say No.
30, and use the bare wire to wind about th