nail placed across the poles will greatly aid in
keeping in the strength. The little magnets should not be dropped or
jarred. (Study the theory of magnetism in text-book.)

CHAPTER VII.

MAGNETIC NEEDLES AND COMPASSES.

_81. Magnetic Needles and Compasses_ consist chiefly of a short
bar-magnet. When used to tell the directions, north, east, etc., the
apparatus is generally called a compass. When we speak of the "needle,"
we really mean the compass-needle. The little magnet may be almost any
piece of magnetized steel, provided it is arranged so that it can easily
swing around. There are several ways of supporting the compass-needle.
It may rest upon a pivot, it may be hung from a fine thread, or it may
be floated upon water with the aid of a cork, etc.

82. Uses. We all know that compasses are used to point to the north and
south, and we speak of the "points of the compass." This, of course, is
the most important use of the compass, and it has been known for
centuries. In the laboratory it is used to show or detect the presence
of currents of electricity, and, in connection with coils of wire, it
may show the relative strengths of two currents, etc. When used for such
purposes it generally has special forms and sizes. (See Galvanometers
and Detectors.)

APPARATUS 62.

_83. Compass._ An oily sewing-needle will float upon the surface of
water, when it is carefully let down to the water. A little butter may
be rubbed upon the previously-magnetized needle to make it float better.

[Illustration: Fig. 38.]

APPARATUS 63.

_84. Compass._ Fig. 38 shows a magnetized sewing-needle floated upon a
cork. The needle may be permanently fastened to the cork with a few
drops of melted paraffine.

APPARATUS 64.

_85. Compass._ Fig. 39. With a sharp knife make a cut part way through a
flat cork. Into the cut push a short length of magnetized watch-spring.
In the illustration the spring is shown partly removed from the cut.
Float the cork.

APPARATUS 65.

_86. Compass._ Fig. 40. Stick a pin, P, into a pasteboard, cork, or
wooden base, B. Bend a piece of stiff paper double, as shown, and then
stick through it, on each side, a magnetized sewing-needle, S N. The
north poles of the needles should be at the same end of the paper. Why?
Balance the paper upon the pin-pivot, and see it fly around to the north
and south.

[Illustration: Fig. 39.]

[Illustration: Fig. 40.]

[Illustration: Fig. 41.]

APPARA