e advised. It will be
better to use those of App. 45. The magnetic needle is supported by a
sewing-needle stuck through a cork. This may be fastened to the base
with paraffine.

_169._ It is often troublesome to turn the apparatus around until the
needle becomes parallel to the length of the coil. To avoid this, a
small bar magnet, shown in the Fig. 85, may be laid on top of the coil.
A magnetized sewing-needle will do, and this will keep the magnetic
needle quiet and parallel to it when the current is not passing through
the coil. Of course, it takes a little more current to move the magnetic
needle when the bar magnet is in place, than it does without the magnet.

_170._ By allowing the current to enter the right-hand binding-post, as
you look at it from the front (Fig. 85), it will go around the coil in
the direction of the hands of a clock, that is, from left to right on
top. This, of course, is not necessary to merely detect the presence of
a current. In order, however, to determine the direction of currents by
means of a magnetic needle, study the effect with a single turn of wire
at first. (See text-book.)

171. Dimensions. The base is 5 x 4 x 5/8 in. The upright piece is 5 x
3-1/2 x 5/8 in. The spools are 2-1/2 in. apart center to center.

APPARATUS 113.

[Illustration: Fig. 86.]

_172. Astatic Current Detector._ Fig. 86. The ordinary magnetic needle
points to the north quite strongly. It is evident, then, that this
pointing-power must be overcome by the magnetic field around the coil of
wire, before the needle can be forced from the N and S line. Very weak
currents will not visibly move the magnetic needle in the detectors so
far described. You should remember that no action will take place unless
the magnetic field around the magnetic needle is acted upon by that
around the coil. In order to make an instrument that will be very
sensitive, we must have strong fields about the needle and coil, and we
must, at the same time, decrease the pointing-power of the needle. We
can increase the strength of the field about the needle, and at the same
time decrease its pointing-power by using an astatic needle. (See App.
69.) The arrangement shown in Fig. 86 is a very simple one, and it is
quite sensitive.

173. Details of Construction. The base is 4 x 5 x 7/8 in. The coil is
made from 10 ft. of No. 30 insulated copper wire. (See Sec. 163 for
details about coil making.) The binding-posts are like App. 41. The
As