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Table of
Contents. • U.S. Government Printing Office; 1945  618779
Chapter 13 INDUCTION
MAGNETISM TO ELECTRICITY
In the last chapter you witnessed the production of a MAGNETIC FIELD by an ELECTRIC
current. This illustrated onehalf of the tieup between electricity and magnetism.
The other half of the picture is the production of an ELECTRIC current by a MAGNETIC
FIELD.
HOW IT'S DoNE
Set up a magnetic field from a horseshoe magnetcut through this field with
a conductor. A voltage is induced in the conductor. That's the gist of producing
a current from magnetism. But, for a complete understanding of this process, you'll
have to first know something about a GALVANOMETER.
The galvanometer is a sensitive meter which. measures very small currents. It
is used instead of an ammeter when the value of current is small enough to be measured
in microamperes or milliamperes. You would use this instrument to measure the small
current produced in cutting the field of ONE magnet with ONE conductor.
Figure 106.  lnducing EMFdownward motion.
Set up the circuit shown in figure 106  you are ready to produce a current
from magnetism. notice that when the conductor is forced DoWNWARD through the field,
the galvanometer is deflected to the right, which indicates that the current of
the conductor is IN. Now, as in figure 107, force the conductor UPWARD through the
magnetic field. The galvanometer is deflected to the left, which indicates that
the current of the conductor is OUT. The fact that the direction of galvanometer
deflection REVERSES for a reversal of the direction of flux cutting by the conductor
shows that 
the DIRECTION OF the INDUCED current DEPENDS ON the DIRECTION OF FLUX CUTTING.
currents which are produced by a conductor cutting magnetic lines are called
INDUCED currentS. Actually it is not current which is induced. nothing can create
current because current is electrons and electrons are matter. You cannot CREATE
nor DESTROY matter. What really happened is this  cutting the lines of force transferred
some of the magnetic energy to the conductor. This energy then became an EMF  an
INDUCED EMF. The induced EMF forced the electrons (already in the wire) to flow.
It's perfectly OK to call it INDUCED current so long as the EMF is INDUCED EMF.
Most electricians make use of the term "induced current" and it has become
pretty well accepted. Just another one of those things!
Figure 107.  lnducing EMFupward motion.
Compare A and B of figure 108. These diagrams differ in two ways 
(1) A has the N pole on the left and B has the N pole on the right.
This means that flux is traveling
to the right in A and to the left in B. Check it!
(2) The induced current in A is IN and the induced current in B
is OUT. Connect these two items
together and you have
the DIRECTION OF the INDUCED current DEPENDS ON the DIRECTION OF the MAGNETIC
FIELD.
Figure 108.  Magnetic field reversalinduced EMF
reversed.
This makes three "directions" involved in the process of inducing
an EMF 
(1) The direction of the CONDUCTOR in cutting flux.
(2) The direction of the FLUX FIELD.
(3) The direction of the INDUCED EMF.
All three "directions" are interdependent, and are connected together
by another hand rule  the generator hand rule. The GENERATOR HAND RULE states 
PLACE the THUMB, FIRST, AND MIDDLE FINGERS OF the LEFT HAND ALL AT RIGHT ANGLES
TO EACH OTHER (Figure 109). Now, the FIRST FINGER POINTS IN the FLUX DIRECTION,
the THUMB POINTS IN the DIRECTION OF the MOTION OF the CONDUCTOR, AND the MIDDLE
FINGER POINTS IN the DIRECTION OF the INDUCED EMF.
Figure 109.  Fingers in the generator hand rule.
Figure 110 illustrates three examples of changing one of the directions. note
that the direction of EMF changes every time either the conductor motion or the
magnetic field changes direction.
Figure 110.  Generator hand rule.
The generator hand rule tells you the third "direction" anytime you
know the other two "directions." Sometimes it will be difficult to get
your fingers lined up with the known directions. Just remember that it makes no
difference if you face the conductor, stand to one side of the conductor, or turn
your back to the conductor. As long as your thumb points in the direction of motion,
your first finger in the direction of the flux, then your middle finger must point
in the direction of the induced EMF. Stand on your head if you must  but get those
fingers lined up! It might help you to construct a drawing like figure 111. Draw
a circle for the crosssection of the conductor. Then run arrows out in the direction
of the flux and the motion. You can apply the generator hand rule directly to the
diagram. Your middle finger tells you whether a • or a + goes in the crosssection
of the wire.
Figure 111.  Model for the generator hand rule.
What would happen if an electromagnet replaced the artificial magnet in producing
an induced EMF? It's perfectly clear that the electromagnetic field is stronger.
Therefore, the wire cuts MORE FLUX  and a STRONGER EMF is induced.
It has been calculated that 100,000,000 lines of flux must be cut per second
to produce ONE volt. Now it's time to do a little mathematical thinking. If 100,000,000
lines cut per second would produce one volt, then 200,000,000 lines cut per second
would produce two voltsand so on. In order to produce 10 volts, it would be necessary
to cut 1,000,000,000 lines per second. The key to understanding this is in the term,
PER SECOND. What , methods can be used to cut more lines PER SECOND? There are three:
(1) cut faster, which simply means speeding up the moving conductor (2) put more
lines there to be cut, which means increasing the magnetic strength, or (3) cut
with more than one conductor, that is, coil the conductor so that many TURNS of
wire cut the field.
Many generators employ the SPEEDUP principle to increase voltage output. This
explains the increased output of an automobile or a motorlaunch generator when
the engine is raced.
The MAGNETIC FIELD STRENGTH can be increased by two methods  either increase
the current through the coil or put more turns on the electromagnet. Either method
increases the NI of the coil and you know that the magnetic strength of an electromagnet
depends on the number of ampereturns.
When a CONDUCTOR IS COILED each turn is in series with the other turns. Therefore,
voltages add. Suppose one conductor cutting a field produces 10 volts. This same
conductor, coiled into 5 turns, and cutting the same field produces 50 volts.
MUTUAL INDUCTION
"Mutual" means that something. is shared. MUTUAL INDUCTION means that
TWO circuits share the energy of one. An example of mutual induction is pictured
in figure 112. Coil A is the PRIMARY circuit and gets its energy from the battery.
Coil A changes the ELECTRICAL energy of the battery into the MAGNETIC energy of
a magnetic field. Then this field is cut by coil B (the SECONDARY circuit), inducing
a voltage. And the galvanometer registers the current produced by the induced EMF.
Here is an interesting fact  the induced voltage MIGHT have resulted from moving
coil B through the flux  but not NECESSARILY. When the switch to A was open, A
had no current and no field. But as soon as the switch was closed, current surged
through the coil and a field blossomed out. This moving field "breaks itself"
across the wires of coil B  thus lines are cut and a voltage is induced, WITHOUT
MOVING COIL B. It only takes a fraction of a second for the field to become STATIONARY
at its maximum size  cutting stops and induction ceases  the galvanometer returns
to zero. If the switch is opened, the field collapses back to the wires of coil
A. Again the field breaks itself across the wires of coil B. The galvanometer deflects,
but in the opposite direction, indicating that the induced voltage has reversed
direction. The important point here is that induction occurs only when the field
is movingeither building up or collapsing. This principle of holding the coils
steady and forcing the field to move is used in all MAKEBREAK circuits. The spark
coil and distributor points of a gasoline engine is a makebreak induction circuit.
Figure 112.  Mutual induction circuits.
Review the circuits of figure 112. Did you notice the rheostat R in the primary
circuit? When the switch to coil A is closed, the coil's current rises to its I
= E/R value. The field becomes stationary. But for any change in R, the current
also changes. And for every change in current, there is a corresponding field change.
Suppose to be resistance of the rheostat is decreasedcurrent increases. The flux
expands and cuts across coil B inducing a voltage. Now suppose that the resistance
of the rheostat is increasedthe current decreases. The flux contracts and again
cuts across coil B inducing an opposite voltage.
All of the examples used in connection with figure 112 illustrate MUTUAL INDUCTION.
You can always spot a mutual induction setup by its TWO circuits. One circuit 
the primary  gets its energy from a voltage source (generator or battery) and the
other circuit  the secondary  gets its energy by induction from the field of the
primary. Two methods of mutual induction stand out 
(1) Move the secondary coil through the field of the primary coil.
(2) Cause the field of the primary to fluctuate, thus breaking
it across the conductors of the secondary.
LENZ'S LAW
There are four diagrams in figure 113. Each successive diagram adds to the complete
picture shown in D. The first diagram, A, shows a conductor at rest in a stationary
magnetic field. The second diagram, B, shows this conductor moving as a result of
a downward push. note that two items have been added  the downward push and the
resulting induced current in the conductor. ANY CONDUCTOR CARRYING current HAS A
FIELD OF ITS OWN. This conductor is no exception. The generator hand rule, proves
this field to be in a counterclockwise direction. The third diagram, C, shows the
field of the conductor only. There are two fields involved  the one from the MAGNET
and the one from the CONDUCTOR. The first is a straight line field traveling from
the N pole to the S pole. The second is a circular field surrounding the conductor.
Figure 113.  Lenz's law.
Magnetic lines never cross. Therefore, the lines of these two fields must either
BLEND together producing a STRONG resultant field or else they must CANCEL each
other producing a WEAK resultant field. A of figure 114 shows what happens above
the wire. The two magnetic fields are meeting headon. It's like two autos meeting
headon  the forces cancel each other. The cancellation of flux lines results in
a WEAK field ABOVE the conductor.
Figure 114.  Conductor's and magnet's fields.
B of figure 114 shows what happens below the wire. The two magnetic fields are
blending together. It's like two autos meeting front to rear  their forces add.
This addition of flux lines results in a STRONG and BENT field BELOW the conductor.
There is a weak field above and a strong bent field below the conductor. Remember
that flux lines are like rubberbands  they tend to spring back into shape. But,
before the distorted lines below the conductor can spring back into shape, they
must push the conductor up out of the way. D of figure 113 shows ALL the conditions
present during induction. Better review them 
1. the DISTORTED FIELD resulting from the combination of the straight
field of the poles and the circular field
of the conductor.
2. the DoWNWARD FORCE added by a push on the conductor.
3. the UPWARD FORCE which results from the distorted field. This
upward force opposes the downward push.
Numbers 2 and 3 above are of prime importance.
They tell you that whenever you add a push to move a conductor in a magnetic
field, there is induced a current which sets up a field that tries to move the conductor
back against the push. This is Lenz's law 
IN ALL CASES OF ELECTROMAGNETIC INDUCTION, the DIRECTION OF the INDUCED EMF
IS SUCH THAT the MAGNETIC FIELD SET UP BY the RESULTING current TENDS TO STOP the MOTION PRODUCING the EMF. Let's see what this means in everyday English.
Suppose you try to push a conductor UPWARD through a magnetic field. Immediately
the induced current sets up a field that tries to push the conductor DoWNWARD. The
force you use in the upward push must buck the magnetic downward push. If you push
harder, the conductor goes faster. But this only produces more induced current and
a stronger conductor field. Consequently, there is a stronger DoWNWARD force to
buck your stronger UPWARD force.
You might state Lenz's law this way  FOR EVERY FORCE THERE IS AN OPPOSITE FORCE
SET UP WHICH TENDS TO CANCEL the FIRST FORCE.
The whole business of Lenz's law is quite reasonable. Look at it this way. You
want to increase an induced voltage from 50 volts to 100 volts. In short, you want
to double the output. If you want TWICE as much output you're going to have to furnish
twice as much input. You'll have to push twice as hard against the conductor to
get your 100 volts.
Have you ever heard a motordriven welding generator? When the welding arc is
struck the motor whines and labors. Lenz's law is working. The arc increased the
output load and the motor is working against the increased opposing, force which
was set up by the increased load. The motor must increase its input to balance the
increased output of the arc.
SELF INDUCTION
There are only three items required to generate an induced voltage  (1) a conductor,
(2) a magnetic field, (3) motion between the conductor and the field. These three
items give you LINES OF FORCE CUT BY A CONDUCTOR. Look at figure 115  are these
three items present in this circuit?
Conductors?  The coil has plenty of them.
Magnetic Field?  The coil sets it up whenever current flows.
Motion?  Occurs only when the field is moving.
And to make the field move, you'll have to expand it or contract it by changing
its current. It's easy to make the coil induce a voltage in ITSELF by opening and
closing the switch. This kind of induction is called SELF INDUCTION. And here is
how it works. At the instant the switch is closed the current starts and magnetic
lines expand from the center of each conductor. As these lines blossom outward,
they are cut by the other conductors of the coil. An EMF is induced in each conductor
cutting flux.
Figure 115.  Self induction circuits.
Figure 116 shows an enlargement of only two turns of the coil in figure 115.
Flux is pictured blossoming out, from one of the turns. notice how these lines are
cut by the next turn. Now, applying the generator hand rule, determine the direction
of the induced voltage. It's easier to use the rule on a crosssection of the coil
like figure 117. Flux direction is down (first finger). Motion is to the RIGHT (thumb).
(ATTENTION  the flux is moving across the conductor to the LEFT  the effect is
AS THOUGH the CONDUCTOR were moving to the RIGHT). Induced voltage is OUT (middle
finger). It means exactly what it saysthe induced voltage OPPOSES the flow of current.
Figure 116.  Self induction in one turn.
What happens when the switch is opened? The field collapses and cuts across
the conductor in the opposite direction. Because the direction of motion has reversed,
the induced EMF is now IN. Thus, in a collapsing field, the induced EMF AIDS the
flow of current.
Figure 117.  Self induction  crosssection.
These are the characteristics of selfinduction 
1. Any coil will induce a voltage in itself whenever its current
value exchanges because current controls
the size and strength of the field.
2. When the current is increasing (field expanding), the induced
EMF opposes current flow.
3. When the current is decreasing (field contracting) , the induced
EMF aids the current flow.
This, after all, is another manifestation of Lenz's law. The first force is
applied voltage (from a battery). The second force is the induced voltage. The induced
voltage opposes the applied when the current is increasing and aids the applied
when the current is decreasing. Thus the induced voltage opposes any changes in
the current value.
The voltage of self induction can be very troublesome. Imagine that you are
operating the switch controlling the field coils on a large motor. These coils have
thousands of turns. When the switch is closed, the voltage of self induction does
little damage. It opposes the increase of current flow for an instant (perhaps 0.1
second), but as soon as the field is built up and stationary, the induced voltage
ceases. On the other hand, when the switch is opened, the field rapidly contracts.
The induced voltage on collapse may be hundreds of times as strong as the applied
voltage. This tremendous induced voltage drives current across the opening switch
terminals in the form of an arc  it CAN burn both the operator and the switch very
badly. All switches subject to high induced voltages are protected by discharge
rheostats to absorb and dissipate the induced voltage, which might otherwise cause
dangerous arcs.
So far in this book, current has been understood as a STEADY flow of electrons.
Apply a voltage  it pushes steadily  current flows in a steady stream of electrons.
Technically, this type of current is known as DIRECT current (D.C.).
Telephones, ignition coils, and radios make use of a special type of direct
current. By means of rheostats, or makebreak switches, the current is alternately
turned on and off. This results in a PULSATING D.C. Pulsating d.c. is like the blood
in your body. The blood gets a push (or pulsation) every time your heart beats.
In a circuit this means that the current flows in SURGES. The surges may be all
of the same strength and regularly spaced, or they may be of varying strength and
irregularly spaced. The exact type of pulsating d.c. depends on the electrical machinery
producing the pulsations. Figure 118 is two graphs of pulsating d.c. A is the current
in a gasoline engine ignition coil. It is regular and the surges are of equal strength.
B is the current in a telephone circuit. It is irregular and the surges are unequal.
Figure 118.  Pulsating d.c.
When pulsating d.c. is fed into a coil, its magnetic field does some tricky
things. Every time the current goes up the field expands, and every time the current
goes down the field contracts. In short, the field is almost constantly in MOTION.
And moving fields produce a lot of induced voltage. Pulsating d.c. produces a field
like that of a closing and opening circuit switch  ONLY  it is much more rapid.
In mutual induction, if the primary is energized with pulsating d.c., the secondary
is alternately cut by the expanding and contracting flux. This produces a high induced
voltage on the secondary coil. In the gasoline engine ignition coil, pictured in
figure 119, the primary circuit is energized from a 6volt battery through the makebreak
switch of the distributor points. When the points close the flux field expands,
and when the points open, the field rapidly collapses. This collapse is so rapid
that the induced voltage in the secondary is often 20,000 VOLTS. This high voltage
is used in jumping current across the air gap at the spark plugs. If you've ever
inadvertently taken the "poke" off a spark plug you know it's plenty hot!
Figure 119.  Gasoline engine ignition coil.
In self induction, a coil carrying pulsating d.c. is a confusing mixture of
current values, applied voltage values, and induced voltage values. Simplified,
it's like thiswhen the current is on the increase, the voltage of self induction
opposes the applied voltage. This makes the net voltage (applied minus induced)
low and the current is slow, in building up. But on collapse  the field cuts in
the opposite direction and the induced voltage aids the applied. This makes the
net voltage high and produces a surge of current. Surging current is dangerous and
must be guarded against with shields, insulators, and resistors. The ordinary coils
of a small electrical motor may produce one or two thousand volts of self induction
if their feeder circuit is opened rapidly.
ALTERNATING current
Direct current, either pulsating or regular, is a ONE WAY flow of electrons.
A TWO WAY flow of electrons  a current which first flows in one direction and then
reverses and flows in the opposite direction  is an ALTERNATING current (A.C.).
Alternating current voltage cannot be obtained directly from batteries, but
usually originates in a special kind of generator called an ALTERNATOR. The alternator
starts out with a zero voltage. It then builds up a voltage which pushes in the
POSITIVE DIRECTION. This positive voltage increases until the maximum is reached,
then decreases again to a zero value. The voltage then builds up again to a maximum
value, but in the NEGATIVE DIRECTION, then decreases to zero. The period of time
required to go from zero, to positive maximum, to zero, to negative maximum and
again to zero is called a CYCLE. And the number of cycles occurring per second is
the FREQUENCY.
Figure 120 is a graph of one cycleof a.c. voltage. In this graph the voltage
strength is measured on the ordinate and the time of one cycle is measured on the
abscissa. Point 1 is the beginning of the cyclezero voltage. From point 1 to point
2, the voltage steadily increases from 0 to 10 to 20 to 30 to 40 to 50 to 60 volts.
Point 2 is the positive maximum (60 v.). Between points 2 and 3 the voltage decreases
to zero in the same steady fashion that it built up. From point 3 to point 4 the
voltage rises again, but in the negative direction. Point 4 is the negative maximum
 again 60 volts. Between points 4 and 5, the voltage falls back to zero. Usually
a cycle takes a lot less time to happen than to tell about  normally about 1/60th
of a second. A cycle takes 1/60th of a second when the frequency equals 60  because
a frequency of 60 means 60 cycles per second. Ohm's law tells you that the current
varies and changes direction exactly the same as the voltage. For every instant
there is an I = E/R value of current. The I changes. in exact proportion to every
change of E.
Figure 120.  Graph of a.c. voltage.
SUMMARY OF A.C. AND D.C. INDUCTION
ACTION 
D.C. 
PULSATING D.C. 
A.C. 
current direction 
Always in one direction. 
Always in one direction. 
Changes direction regularly. 
current steadiness 
Always steady. 
Rises and falls. 
Rises and falls. 
Magnetic fields produced 
Build upthen steady as long as current is steady.
Always the same direction. 
Constantly expanding and contracting. Always the same
direction. 
Constantly expanding and contracting. Reverse direction
regularly. 
Mutual induction 
Occurs only when circuit is opened, closed, or when
current value changes. Induced voltage varies in direction depending on primary
current. 
Occurs constantly. Varies in direction constantly. 
Occurs constantly. Varies in direction constantly. 
Self induction 
Occurs only when circuit is open or closed or when
current value changes. Varies in direction. 
Occurs constantly. Varies in direction. 
Occurs constantly. Varies in direction. 
HOW A.C. ACTS IN INDUCTION
Alternating current is constantly changing value and direction. Therefore, the
fields produced by a.c. are constantly expanding and contracting  also constantly
reversing polarity.
In mutual induction, a.c. on the primary produces a CONTINUOUS a.c. on the secondary.
The TRANSFORMER is an a.c. mutual induction circuit.
In self induction, a.c. produces a CONTINUOUS voltage. The INDUCED voltage opposes
the APPLIED and some coils are designed so that the EMF of self induction is strong
enough to almost completely stop current flow.
COMPARISON OF A.C. AND D.C.
The table on page 171 compares the action of a.c., pulsating d.c., and regular
d.c. in mutual and self induction. Study itif there are points you don't understand,
go back over this chapter and get 'em cleared up.
Chapter 13 Quiz
(click here)
