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CHAPTER 6
ELECTROMAGNETISM
MAGNETS FROM ELECTRICITY
You probably have been warned not to bring your
watch near an a.c. or d.c. generator, because the presence
of the MAGNETIC FIELD may magnetize your watch. Or
you may have seen how a compass needle behaves when a
street car or an electric train approaches.
The use of electricity to obtain a MAGNETIC FIELD has
so many common applications that we are apt to overlook
its importance. The widespread use of electromagnets
demonstrates their importance in our daily lives.
Electromagnets have two big advantages in their favor.
First, you can turn them ON and OFF as you wish. That
is not possible with permanent magnets. Bells, lifting
devices, relays, and telegraph sounders use magnets that
can be turned on and off.
The second advantage is the added field strength possible with electromagnets. And the STRENGTH of the
field can be REGULATED by controlling the flow of current
through the coils.
You can detect the presence of a magnetic field around a
wire connected to the terminals of a battery. Just dip
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the wire into a pile of iron filings. (See figure 45.)
Some of the filings will "stick" to the wire as they do the
permanent magnets.
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Figure 45.-Magnetism produced by a current.
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Disconnecting one end of the wire from the battery
stops the current flow and the filings fall off. When you
connect the wire to the battery again, the magnetic effect
is restored. This little experiment shows that a CONDUCTOR CARRYING A CURRENT IS SURROUNDED BY A MAGNETIC FIELD.
You may observe more of the field about a conductor
by sprinkling iron filings on a piece of cardboard through
which the conductor passes. Actually, you are observing a cross-section of the magnetic field. You see a
CIRCULAR FIELD, with the greatest concentration of filings
near the center, where the field is strongest.
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Figure 46.-Magnetic field about a conductor.
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The small compasses placed in the field show that the
magnetic flow also has DIRECTION.
In figure 47A, the CURRENT is flowing DOWN, and the
compasses indicate the flux to be moving in a COUNTER-CLOCKWISE direction.
When the CURRENT is REVERSED, the needles of the compasses turn around, indicating that the FIELD is ALSO
REVERSED.
It boils down to this-a conductor carrying a current
is surrounded by a magnetic field, whose DIRECTION depends on the direction of the current flow.
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Figure 47.-Direction of the field about a conductor.
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You can always find the direction of the field-grasp
the conductor in your LEFT HAND with your THUMB pointing in the DIRECTION the CURRENT is flowing. Your FINGER S will POINT in the direction of the field around the
conductor. This rule is demonstrated in figure 48.
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Figure 48.-Left-hand rule.
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In some diagrams of electrical equipment it is necessary
to "cut" conductors so that you view them from the ends.
In such cases, it is impossible to use arrows to indicate
the direction of current flow. Instead., you use a system
of dots and crosses.
The DOT (figure 49A) indicates the current to be flowing
OUT of the conductor (TOWARD you) . The CROSS indicates the current to be flowing INTO the conductor (AWAY
from you) . Think of the DOT as the "point" of the arrow
coming OUT of the wire, and the cross as the "tail" of the
arrow ENTERING the conductor.
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Figure 49.-Symbols used to indicate direction of current flow.
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The two drawings at the bottom of figure 49 shows the
direction of the field with the "dot" and "cross" system
of indicating current flow.
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Figure 50.-Magnetic polarity of a loop.
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699198°-46-5
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A LOOP OF WIRE CARRYING A CURRENT HAS POLARITY
When a conductor is twisted into a loop and connected
to a battery, as indicated in 50, the MAGNETIC FIELD ABOUT
the loop will have north and south poles. Notice the
direction of the field about the loop. It enters at the left
and leaves at the right. Since magnetic lines of force
enter at the south pole and leave at the north pole, the
LEFT side of the loop will have SOUTH magnetism and the
RIGHT side NORTH magnetism.
MAGNETIC FIELD OF A COIL
Several turns of wire, placed side by side, form a coil.
The INDIVIDUAL MAGNETIC FIELD of each turn combines
with fields of the other turns to give the coil north and
south poles. In figure 51, the lines of force enter at the
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Figure 51.-Magnetic field of a coil.
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south poles. In figure 51, the lines of force enter at the
south pole and leave at north pole, just as they do with
permanent magnets.
The north pole of a coil can be found easily, if you know
the direction the current is flowing. Figure 52 shows
you how. Wrap your LEFT hand about the coil with your
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Figure 52.-Left-hand rule for coils.
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FINGERS pointing in the direction of the current flow.
Your THUMB POINTS toward the NORTH POLE.
STRENGTH OF A COIL
The MAGNETIC FIELD STRENGTH of a coil is determined
by two things-the AMOUNT of CURRENT flowing through
the individual turns, and the NUMBER of turns. The
more turns and the larger the current, the greater
the strength of the coil.
The field strength is expressed in AMPERE-TURNS. ONE
AMPERE TURN is one loop of wire carrying one ampere of
current. Two loops of wire carrying a half-ampere of
current is also one ampere-turn.
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Figure 53.-Equal ampere-turns.
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In other words, the PRODUCT of the CURRENT times the
NUMBER OF TURNS gives you the strength of the coil in
AMPERE-TURNS.
Here are two examples. Coil A with 10 turns carrying
5 amperes has a strength of-
5 X 10 = 50 ampere turns,
Coil B with 20 turns carrying 2.5 amperes has the SAME
strength-
2.5 X 20 = 50 ampere-turns.
A very strong coil can be made by using MANY TURNS
of fine wire carrying a small current. And an equally
strong coil can be made by using only a few turns carrying a high current.
IRON CORES
The RELUCTANCE of SOFT IRON is low, compared to that
of air. And soft iron also has a low RESIDUAL MAGNETISM.
Put those two properties of soft iron together, and you
have the reasons for using soft iron cores in electromagnets.
The iron core CONCENTRATES the magnetic flux into a
SMALL USEFUL AREA, but does not increase the ampere-turn
strength of the coil. It simply holds most of the
flux inside the coil, increasing the useful magnetism.
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Figure 54.-Iron core electromagnet.
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The core of an electromagnet is usually made of
BUNDLES of SOFT IRON wires. If steel were used, a great
deal of energy would be required to magnetize the cores.
And when the current was turned off, the core would
remain magnetized (HIGH residual magnetism).
Both conditions are undesirable, since you want the
electromagnet to assume FULL STRENGTH the instant the
current is turned on, and lose it IMMEDIATELY when the
current is turned off.
You will hear more about iron cores of electromagnets
when transformers are discussed.
USE OF ELECTROMAGNETS
Electromagnets have an almost endless list of applications in electric motors, generators, bells, telephones, telegraphs, and in thousands of other electrical devices.
Radios use a number of electromagnets, for example in
the earphone and loudspeaker.
Shipboard radio has a special application-RELAYS.
They are used to control the operation of transmitters
and receivers from remote points on your ship. The
system used to "key" your transmitter is an example of
this. You know that you may touch the hand key any
place without getting a serious shock. Why? Because
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Figure 55.-Basic parts of a simple relay.
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an electromagnet in the form of a RELAY is used to open
and close a high voltage circuit.
RELAY
A basic relay has three major parts-an ELECTROMAGNET, a movable iron bar called an ARMATURE, and CONTACT
POINTS. See figure 55. The spring attached to the
armature holds the contacts open when no voltage is being applied to the electromagnet.
When a voltage is applied to the electromagnet, the
magnetic field draws the armature toward the core,
closing the contact points. Removing the current demagnetizes the magnet, and the spring pulls the armature
upward, breaking the circuit again.
A typical arrangement for "keying" a transmitter is
indicated in figure 56. The hand key is supplied with a
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Figure 56.-A simple transmitter keying circuit.
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low voltage to energize the electromagnet. The connecting wires from the operator's station to the transmitter
may be several hundred feet long. The relay itself is
usually located inside the transmitter cabinet so that none
of the high voltage wire need be strung about the ship.
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A relay not only makes it possible to keep high voltages
away from the key, but also permits the installation of
the transmitter in some out-of-the-way space instead of
in the radio room.
Most relays are not as simple in design as those given
in figures 55 and 56. Many have two or more sets of contact points. Some have a set of contacts above and
another below, so that when one circuit is closed the other
is opened.
Still other relays have a DELAYED-ACTION mechanism
built into them to prevent the circuit from being opened
or closed until a definite amount of time has elapsed.
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