MAGNETS, COILS, ARMATURES, ETC.
THE TWO KINDS OF MAGNET.--Generally speaking, magnets are of two kinds,
namely, permanent and electro-magnetic.
PERMANENT MAGNETS.--A permanent magnet is a piece of steel in which an
electric force is exerted at all times. An electro-magnet is a piece of
iron which is magnetized by a winding of wire, and the magnet is
energized only while a current of electricity is passing through the
wire.
ELECTRO-MAGNET.--The electro-magnet, therefore, is the more useful,
because the pull of the magnet can be controlled by the current which
actuates it.
The electro-magnet is the most essential of all contrivances in the
operation and use of electricity. It is the piece of mechanism which
does the physical work of almost every electrical apparatus or machine.
It is the device which has the power to convert the unseen electric
current into motion which may be observed by the human eye. Without it
electricity would be a useless agent to man.
While the electro-magnet is, therefore, the form of device which is
almost wholly used, it is necessary, first, to understand the principles
of the permanent magnet.
MAGNETISM.--The curious force exerted by a magnet is called magnetism,
but its origin has never been explained. We know its manifestations
only, and laws have been formulated to explain its various phases; how
to make it more or less intense; how to make its pull more effective;
the shape and form of the magnet and the material most useful in its
construction.
[Illustration: _Fig 5._ PLAIN MAGNET BAR]
MATERIALS FOR MAGNETS.--Iron and steel are the best materials for
magnets. Some metals are non-magnetic, this applying to iron if combined
with manganese. Others, like sulphur, zinc, bismuth, antimony, gold,
silver and copper, not only are non-magnetic, but they are actually
repelled by magnetism. They are called the diamagnetics.
NON-MAGNETIC MATERIALS.--Any non-magnetic body in the path of a magnetic
force does not screen or diminish its action, whereas a magnetic
substance will.
In Fig. 5 we show the simplest form of magnet, merely a bar of steel (A)
with the magnetic lines of force passing from end to end. It will be
understood that these lines extend out on all sides, and not only along
two sides, as shown in the drawing. The object is to explain clearly how
the lines run.
[Illustration: _Fig. 6._ SEVERED MAGNET]
ACTION OF A SEVERED MAGNET.--Now, let us suppose that we sever this bar
in the middle, as in Fig. 6, or at any other point between the ends. In
this case each part becomes a perfect magnet, and a new north pole (N)
and a new south pole (S) are made, so that the movement of the magnetic
lines of force are still in the same direction in each--that is, the
current flows from the north pole to the south pole.
WHAT NORTH AND SOUTH POLES MEAN.--If these two parts are placed close
together they will attract each other. But if, on the other hand, one of
the pieces is reversed, as in Fig. 7, they will repel each other. From
this comes the statement that likes repel and unlikes attract each
other.
REPULSION AND ATTRACTION.--This physical act of repulsion and attraction
is made use of in motors, as we shall see hereinafter.
It will be well to bear in mind that in treating of electricity the
north pole is always associated with the plus sign (+) and the south
pole with the minus sign (-). Or the N sign is positive and the S sign
negative electricity.
[Illustration: _Fig. 7._ REVERSED MAGNETS]
POSITIVES AND NEGATIVES.--There is really no difference between positive
and negative electricity, so called, but the foregoing method merely
serves as a means of identifying or classifying the opposite ends of a
magnet or of a wire.
MAGNETIC LINES OF FORCE.--It will be noticed that the magnetic lines of
force pass through the bar and then go from end to end through the
atmosphere. Air is a poor conductor of electricity, so that if we can
find a shorter way to conduct the current from the north pole to the
south pole, the efficiency of the magnet is increased.
This is accomplished by means of the well-known horseshoe magnet, where
the two ends (N, S) are brought close together, as in Fig. 8.
THE EARTH AS A MAGNET.--The earth is a huge magnet and the magnetic
lines run from the north pole to the south pole around all sides of the
globe.
[Illustration: _Fig. 8._ HORSESHOE MAGNET]
The north magnetic pole does not coincide with the true north pole or
the pivotal point of the earth's rotation, but it is sufficiently near
for all practical purposes. Fig. 9 shows the magnetic lines running from
the north to the south pole.
WHY THE COMPASS POINTS NORTH AND SOUTH.--Now, let us try to ascertain
why the compass points north and south.
Let us assume that we have a large magnet (A, Fig. 10), and suspend a
small 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 (B) wound around it. The current, through the wire (B), as
shown by the darts (D), moves in one direction, and the induced current
in the core (C) travels in the opposite direction, as shown by the darts
(D).
[Illustration: _Fig. 16._ DIRECTION OF INDUCTION CURRENT]
PARALLEL WIRES.--In like manner, if two wires (A, B, Fig. 16) are
parallel with each other, and a current of electricity passes along the
wire (A) in one direction, the induced current in the wire (B) will move
in the opposite direction.
These fundamental principles should be thoroughly understood and
mastered.