ide, so that it will straddle the lower part of the coil. The
height of the needle-point should be fixed so that the horizontal ends
of the magnetic needle will be near the axis of the coil, that is, along
its central line.

165. To Use the Detector, turn its base around until the coil is in the
N and S line--that is, until the magnetic needle is parallel to the
length of the coil and wholly inside of it. Touch the ends of the coil
with the two ends of the wire, which is supposed to carry a current. The
needle will fly around until it is nearly perpendicular to its former
position, if the current is strong enough.

APPARATUS 111.

[Illustration: Fig. 84.]

_166. Current Detector._ Fig. 84. To make a more substantial detector
than App. 110, the coil should be fastened to a wooden base. The coil
may be made of 10 ft. No. 30 wire, as explained. (Sec. 163.) A hole
should be made in the base with a small awl or with a hot wire, and
into this should be set a pin, head down. The hole need not be larger
than the pin-head, and when you find out how high the pin-point should
be above the base, the pin may be fastened in place with a little
paraffine, which should be pressed into the hole around the pin. The
coil may be fastened in place with paraffine. The ends of the coil may
be connected with binding-posts, described in App. 46, as shown, or
with any other desired form.

The base should be 4 x 5 x 7/8 inches. The coil looks well when placed
about 1 in. from the edge of the base. The binding-posts may be about 1
in. from the edges.

APPARATUS 112.

_167. Current Detector._ Fig. 85. This is more troublesome to make than
App. 111, but perhaps it looks more scientific.

168. The Coil is wound around 2 ordinary spools which are glued to a
vertical piece, which, in turn, is screwed to a base. You should not use
iron nails or screws in the construction of electrical apparatus, when a
magnetic needle is to be used in connection with it, as these would
attract the needle. The spools may be pushed onto dowels which are
fastened into the vertical piece. Small brass screws are good for the
purpose also, if you haven't good glue or the dowels. This coil, etc.,
may be used in connection with an astatic needle. The coil may be wound
with App. 93 or 94, if you make the attachment of App. 95, and screw the
upright carrying the spools to the attachment.

[Illustration: Fig. 85.]

The binding-posts, shown in Fig. 85, are not to b