Electricity for Boys | Page 8

magnet (B) above it, so that it is within the magnetic field of the
large magnet. This may be done by means of a short pin (C), which is
located in the middle of the magnet (B), the upper end of this pin
having thereon a loop to which a thread (D) is attached. The pin also
carries thereon a pointer (E), which is directed toward the north pole of
the bar (B).
[Illustration: Fig. 9. EARTH'S MAGNETIC LINES]
You will now take note of the interior magnetic lines (X), and the
exterior magnetic lines (Z) of the large magnet (A), and compare the
direction of their flow with the similar lines in the small magnet (B).
The small magnet has both its exterior and its interior lines within the
exterior lines (Z) of the large magnet (A), so that as the small magnet
(B) is capable of swinging around, the N pole of the bar (B) will point
toward the S pole of the larger bar (A). The small bar, therefore, is
influenced by the exterior magnetic field (Z).
[Illustration: Fig. 10. TWO PERMANENT MAGNETS]
[Illustration: Fig. 11. MAGNETS IN THE EARTH'S MAGNETIC
FIELD]
Let us now take the outline represented by the earth's surface (Fig. 11),
and suspend a magnet (A) at any point, like the needle of a compass,
and it will be seen that the needle will arrange itself north and south,
within the magnetic field which flows from the north to the south pole.
PECULIARITY OF A MAGNET.--One characteristic of a magnet is
that, while apparently the magnetic field flows out at one end of the

magnet, and moves inwardly at the other end, the power of attraction is
just the same at both ends.
In Fig. 12 are shown a bar (A) and a horseshoe magnet (B). The bar (A)
has metal blocks (C) at each end, and each of these blocks is attracted
to and held in contact with the ends by magnetic influence, just the
same as the bar (D) is 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 is called a conduction current. Remember these terms.
In this connection there is also another thing which you will do well to
bear in mind. In Fig. 15 you will notice a core (C) and an insulated wire
coil