tatic Needle is described for App. 69. The needles may be broken off,
if too long for the coil. They are supported by a fine thread hung from
a screw-eye, which may be turned to adjust the position of the needles.
This is not necessary, as the thread may be hung from a plain wire arm
that reaches out from the upright rod. This rod is a 6-in. piece of
dowel, 1/4 or 5/16 in. in diameter. It stands in an ordinary spool which
should be glued to the base. Do not nail it to the base. The wire arm
may be of iron, as it is some distance above the needle; but it is
better to use a stiff brass or copper one. In the figure one end of the
wire is twisted around the screw-eye, making a nut for the screw-eye to
turn in.

Hang the astatic needle so that the wire between the two parts will not
quite touch the coil. The needles should be parallel to the coil before
testing for currents. They will fly around very decidedly with even
fairly weak currents.

APPARATUS 114.

[Illustration: Fig. 87.]

_174. Astatic Current Detector._ Fig. 87. For a description of the
wood-work, coil, etc., see App. 112; for the astatic needle see App. 69;
for the method of supporting the needle see App. 113, Fig. 86. The top
part of the coil is spread apart a little to allow the lower needle to
be dropped through the opening thus made, and to allow the wire joining
the two needles to be free to turn. The needles may be broken off a
little, if necessary, or an opening may be cut into the vertical part of
the frame, so that they can swing more freely. This detector will
indicate quite feeble currents.

APPARATUS 115.

_175. Astatic Detector._ Fig. 88. As previously Stated, the
sensitiveness of a detector can be made greater by increasing the
strength of the coil-field for a given current. This may be done by
increasing the number of turns of wire in the coil. The most convenient
way will be to use two coils, one on each side of the astatic needle.

176. The Support, or framework, is a lamp chimney. By this the astatic
needle is suspended and protected from air currents. The chimney should
be at least 3 in. in diameter at the bottom, about 10 in. high, with a
plain round top. Upon the top of the chimney is placed the cover of a
wooden pill-box, 2 in. in diameter.

177. The Coils should be made separately, for convenience. Each should
be of 10 ft. No. 30 wire. (See details Sec. 163.) Cut out a round piece
of stiff pasteboard, just large enough to go