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TM 9-8000
mined by use of the left-handed rule, imagining the
15-1.  Function. Ignition of the fuel-air mixture in the
expanding lines of force from the first loop cutting across
engine cylinder may be accomplished by either of two
the second loop. Note that the lines of force cut across
methods: heat of compression, as in diesel engines; or
the second loop in a direction that will induce an
electric spark, as in gasoline engines.  Spark ignition
opposing current.
may be subdivided into two classes: battery and
magneto. These two systems are essentially the same.
(3)  It would be more correct to say that an
With either, the fundamental job is to step up low voltage
opposing voltage is induced. The voltage must come
to a much higher value, and to deliver the high voltage to
first, then the current. That is, there first must be an
the spark plugs with the proper timing. The high voltage
electron concentration and an electron shortage in a
is capable of pushing current through the high resistance
circuit for current (or electrons) to flow.  With this
set up by the pressure in the combustion chamber and
electron imbalance (or this voltage) in existence, there
across from one spark plug electrode to the other. The
will  be  an  electric  current.
The  opposing  or
hot spark created ignites the fuel-air mixture.  This
countervoltage that is self-induced by the expanding
section pertains to battery ignition, its theory and
lines of force is opposite in direction to the voltage from
Paragraphs 15-9 thru 15-15 describe
the external source that is forcing current through the
magneto ignition.
loops of the electromagnet.
Operating Principles.
(4) The effect of the countervoltage is to prevent
an  immediate  magnetic  buildup.
That  is,  the
a. Self-Induction(Fig. 15-1).
countervoltage slows down the rate at which the current
flow can increase and the magnetic field can expand and
(1)  The principle of electromagnetic induction
strengthen. The increase of the magnetic field produces
has been described in paragraph 11-15. When a wire is
the countervoltage; this countervoltage opposes further
moved through a magnetic field, or a magnetic field is
increases of current flow. However, the external voltage
moved past a wire, the wire will have current induced in
is stronger and therefore continues to increase the
it.  The magnetic field can be moved past the wire
strength of the current until it reaches the value
mechanically, or it can be made to move (if it is from an
determined by the resistance of the circuit, including the
electromagnet) by turning the current on and off. This
action causes the magnetic field to build up and collapse.
Any wire held in this magnetic field will have current
Actually, in most electromagnets, the
induced in it.
countervoltage effect slows buildup time only a very
small fraction of a second. But with electromagnets of
(2) The turns of wire in the electromagnet itself
many hundreds or thousands of turns, it does take time
will have current induced in them by the moving
for buildup to occur. In the ignition system, if buildup
magnetic field.  For example, figure 15-1 shows two
time took too long, the high-voltage sparks would not be
loops, or turns, of an electromagnet. When current first
produced fast enough and high engine speed could not
starts to flow, it enters the first loop and a magnetic field
be attained. However, the ignition coil can function with
begins to build up along with the increase of current.
adequate speed for normal ignition even at high speed.
The magnetic field consists of circular lines of force
surrounding the wire, and the lines of force move
(6)  Not only does the countervoltage appear
outward and cut through the adjacent loops. These lines
during buildup, it also occurs during magnetic collapse.
of force produce a current in the adjacent loops as they
That is, when the electromagnet is disconnected from
cut through.  It is important to note that the induced
the voltage source so that
currents oppose the original currents. This can be deter-


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