instant the
current ceases to pass. The combination, thus, of a coil of wire and a
magnetic needle, properly arranged, makes an instrument with which the
presence of electricity can be detected. When the strength of a current
is to be measured, or the strengths of two currents are to be compared,
the apparatus is called a galvanometer. The method of making these
pieces of apparatus will depend upon the strength of current to be
tested or measured.

APPARATUS 109.

_161. Current Detector._ Figs. 38 and 40 show magnetic needles. These
may be used to detect a current by holding the conducting wire near them
and parallel to the needle. This form is not sensitive to weak currents.
The delicacy of the apparatus is increased by allowing the wire to pass
above and below the needle several times as in the next apparatus.

APPARATUS 110.

_162. Current Detector._ Fig. 83 consists, like all detectors, of a coil
and a magnetic needle. The other parts are merely for convenience. Each
turn of the coil helps to move the needle when the current passes.

[Illustration: Fig. 83.]

163. The Coil is made by winding 10 feet of No. 30 insulated copper wire
around the end of a broom-handle or other cylinder that is about 1 inch
in diameter. This length of wire makes about 32 turns around such a
cylinder. The exact length of wire for this makes no difference. After
winding it, the coil should be slipped from the handle, being careful to
hold it in such a way that it cannot uncoil and spring away from you.
Tie the coil together with thread, in 3 or 4 places, to keep it in
shape, and leave 5 or 6 in. of wire free at each end, so that
connections can be made with other pieces of apparatus. After this is
done press the coil into the shape shown, Fig. 83. This brings the wire
near the needle and allows a longer needle to be used. The coil may be
fastened to a pasteboard base. To do this, prick 4 holes in the base
near the ends of the oval coil, and pass a strong thread through these
with the aid of a sewing-needle. Tie the thread on the underside of the
base at each end. If this is well done, the coil will be held firmly in
an upright position. Paraffine may be used instead of the thread.

The ends of the wire should be made bare, and these may be sewed to the
base to keep them in place.

164. The Needle may be supported upon a pin or needle-point. The piece
of needle should be stuck through a cork which has a slot cut into its
unders