s attracted by and held against the two ends of the
horseshoe magnet. These blocks (C) or the bar (D) are called armatures.
Through them is represented the visible motion produced by the magnetic
field.

[Illustration: _Fig. 12._ ARMATURES FOR MAGNETS]

ACTION OF THE ELECTRO-MAGNET.--The electro-magnet exerts its force in
the same manner as a permanent magnet, so far as attraction and
repulsion are concerned, and it has a north and a south pole, as in the
case with the permanent magnet. An electro-magnet is simply a bar of
iron with a coil or coils of wire around it; when a current of
electricity flows through the wire, the bar is magnetized. The moment
the current is cut off, the bar is demagnetized. The question that now
arises is, why an electric current flowing through a wire, under those
conditions, magnetizes the bar, or _core_, as it is called.

[Illustration: _Fig. 13._ MAGNETIZED FIELD]

[Illustration: _Fig. 14._ MAGNETIZED BAR]

In Fig. 13 is shown a piece of wire (A). Let us assume that a current of
electricity is flowing through this wire in the direction of the darts.
What actually takes place is that the electricity extends out beyond the
surface of the wire in the form of the closed rings (B). If, now, this
wire (A) is wound around an iron core (C, Fig. 14), you will observe
that this electric field, as it is called, entirely surrounds the core,
or rather, that the core is within the magnetic field or influence of
the current flowing through the wire, and the core (C) thereby becomes
magnetized, but it is magnetized only when the current passes through
the wire coil (A).

[Illustration: _Fig. 15._ DIRECTION OF CURRENT]

From the foregoing, it will be understood that a wire carrying a current
of electricity not only is affected within its body, but that it also
has a sphere of influence exteriorly to the body of the wire, at all
points; and advantage is taken of this phenomenon in constructing
motors, dynamos, electrical measuring devices and almost every kind of
electrical mechanism in existence.

EXTERIOR MAGNETIC INFLUENCE AROUND A WIRE CARRYING A CURRENT.--Bear in
mind that the wire coil (A, Fig. 14) does not come into contact with the
core (C). It is insulated from the core, either by air or by rubber or
other insulating substance, and a current passing from A to C under
those conditions is a current of _induction_. On the other hand, the
current flowing through the wire (A) from end to end i