BASIC ELECTRICITY
the circuit closes. When the circuit is open, the
current cannot flow and its value, or voltage,
drops to zero until the circuit closes again.
You should be familiar with the aircraft
Alternating current is current that changes
electrical system in general. You must become
direction in the circuit, flowing first in one
familiar with the different engine electrical systems
direction and then in the other. A cycle is two
and components that support your type of engine.
Some of the electrical systems that ADs maintain
complete alternations within a period of time. The
hertz (Hz) indicates one cycle per second. For
include ignition, starting, thermocouple, tempera-
example, one cycle per second is 1 hertz. The
ture control, and constant-speed drive (CSD)
standard unit of electricity used in the United
systems. To understand the operation of these
States is 60 Hz ac.
systems, you must understand basic electricity.
The electromotive force is the force that causes
The following paragraphs cover some of the basic
electrons to flow from atom to atom in a
facts and laws that will be helpful in under-
conductor. Electrons flow from atoms with an
standing electrical principles.
excess of electrons to atoms with less electrons.
Electricity is a form of energy. Energy is the
The practical electrical units are the volt, the
ability of a body to do work. It does not occupy
ampere, and the ohm. The volt is the unit of
space, but it can be measured. There are two
electromotive force necessary to cause electrons
forms of electricity-static and dynamic. Static
to flow in a circuit. The ampere is the rate of flow
electricity is electricity at rest. It is produced by
of these electrons in a conductor. The ohm is
friction, which causes one body to give up
called the unit of electrical resistance. This
electrons to another. The body that lost the
resistance varies according to the kind of material
electrons will have a positive charge. The body
used as a conductor, the length of the conductor,
that gained the electrons will have a negative
and the cross-sectional area of the conductor.
charge. As a result, the positively charged body
Resistance also varies with temperature. Other
will try to gain electrons. The negatively charged
factors being equal, the resistance increases if the
body will try to cast off its surplus electrons to
length of the conductor is increased, and decreases
an oppositely charged body or a neutrally charged
if the cross-sectional area of the conductor is
body. Lightning is an example of static electricity
increased. Resistance increases as the temperature
during the discharge of electrons. Dynamic
increases.
electricity is electricity y in motion. This is the most
useful form of electricity, and it is produced,
controlled, and measured with relative ease. It is
called electricity in motion because it will flow
IGNITION, STARTING, BLEED
along a definite path, called a circuit. Static
AIR, AND AUXILIARY POWER
electricity will remain in the body containing it
UNIT SYSTEMS
until discharged.
There are three methods of producing elec-
As an Aviation Machinist's Mate, your
tricity. These methods are heat, chemical, and
primary responsibility is to maintain power plants
mechanical means. When two dissimilar metals
and related systems. You should know that engine
in contact with each other are heated, an electron
systems support the entire aircraft. Engine systems
flow takes place between them. Thermocouples
support more than just the engine. This means
are a good example of this flow. The storage
that the maintenance of these systems are the
battery is a good example of converting chemical
responsibility of more then one work center.
energy into electrical energy. The mechanical
Electrical or pneumatic systems that other work
means of producing electricity will be of the most
centers help maintain include ignition, starting,
interest to you. The generator and magneto are
bleed-air, and auxiliary power units (APU).
two methods used to mechanically produce
electricity.
AIRCRAFT IGNITION SYSTEMS
There are three types of current that these
mechanical devices produce. They are direct,
Jet engine ignition systems are simple com-
pulsating direct, and alternating current. Direct
pared to automobile ignition system. They are
current is current that always flows in the same
simple because jet engine ignition systems require
direction. Pulsating direct current is direct
no ignition timing. Since jet engine combustion
current that is interrupted by a set of breaker
is a self-sustaining process, a spark is needed only
points. The current will flow in one direction when
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