d used from time to time for later editions, and the original type
can be put back into the cases and used again.

CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS

296. An Electric Current acts like a Magnet. In order to understand
the action of the electric bell, we must consider a third effect which
an electric current can cause. Connect some cells as shown in Figure
200 and close the circuit through a stout heavy copper wire, dipping a
portion of the wire into fine iron filings. A thick cluster of filings
will adhere to the wire (Fig. 210), and will continue to cling to it
so long as the current flows. If the current is broken, the filings
fall from the wire, and only so long as the current flows through the
wire does the wire have power to attract iron filings. An electric
current makes a wire equivalent to a magnet, giving it the power to
attract iron filings.

[Illustration: FIG. 210.--A wire carrying current attracts iron
filings.]

[Illustration: FIG. 211.--A loosely wound coil of wire.]

Although such a straight current bearing wire attracts iron filings,
its power of attraction is very small; but its magnetic strength can
be increased by coiling as in Figure 211. Such an arrangement of wire
is known as a helix or solenoid, and is capable of lifting or pulling
larger and more numerous filings and even good-sized pieces of iron,
such as tacks. Filings do not adhere to the sides of the helix, but
they cling in clusters to the ends of the coil. This shows that the
ends of the helix have magnetic power but not the sides.

If a soft iron nail (Fig. 212) or its equivalent is slipped within the
coil, the lifting and attractive power of the coil is increased, and
comparatively heavy weights can be lifted.

[Illustration: FIG. 212.--Coil and soft iron rod.]

A coil of wire traversed by an electric current and containing a core
of soft iron has the power of attracting and moving heavy iron
objects; that is, it acts like a magnet. Such an arrangement is called
an electromagnet. As soon as the current ceases to flow, the
electromagnet loses its magnetic power and becomes merely iron and
wire without magnetic attraction.

If many cells are used, the strength of the electromagnet is
increased, and if the coil is wound closely, as in Figure 213, instead
of loosely, as in Figure 211, the magnetic strength is still further
increased. The strength of any electromagnet depends upon the number
of coils wound on th