inside of the bottom of
the chimney. Fasten the coils to this by sewing (Sec. 163), or with
paraffine, so that they shall be symmetrically located and 3/8 in.
apart. The pasteboard circle may be fastened to the base with small
brass screws. Do not use any iron nails or tacks. In this, all four
ends of wire are brought out under the edge of the chimney (Fig. 88).
Cut little grooves in the base for the wire to sink into, so that the
chimney will rest firmly upon the base all around. The ends of the
wires are fastened to three binding-posts.

[Illustration: Fig. 88.]

178. Joining the Coils. The end of one coil must be joined to the
beginning of the other properly, or the action of one will destroy that
of the other. Fig. 89 shows the two coils, A and B. If the current
enters at the binding-post, X, it will pass through the turns of coil A,
in the direction of clock-hands, then out to Y, where B begins, around B
in the same way, and then to Z. Y may be simply a screw-eye
binding-post (App. 41). By this arrangement one or both coils can be
used at a time. If the current is very weak, use both coils; that is,
connect the ends of wires to be tested with the two outside
binding-posts. If they are joined to the middle and one outside post,
one coil only will be in the circuit.

179. The Base should be about 7 x 5 x 7/8 in. Fasten three bent brass or
copper strips to the base with brass screws to hold the chimney steady.
By bending them in more or less you can make a snug fit around the
chimney.

180. Adjusting the Needle. In the center of the box-cover is a small
hole. The thread from the needle passes through this. The upper end of
the thread is wound around a screw-eye, which is screwed into the cover
near one edge. By turning the cover around, the needle can be made to
hang parallel to the coils, and by turning the screw-eye, the needle can
be raised or lowered. A small hole should be made in the cover before
putting in the screw-eye, or you will be liable to split the wood.

[Illustration: Fig. 89.]

181. Use. This apparatus will indicate very slight currents; in fact, as
feeble ones as the student will have occasion to experiment with, such
as induced currents, currents of thermo-electricity, and currents
produced by exceedingly weak batteries. (See text-book.)

APPARATUS 116.

_182. Tangent Galvanometer._ Fig. 90. For the uses of this form of
galvanometer see text-book. Do not use any iron in making this
app