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fire and takes other stuff with it, if it
doesn’t actually burn its way through
the airframe and fall out.
If you spill any electrolyte from a
NiCad, you can neutralise it with
dilute boric acid.
The Battery Master Switch controls the
power to all circuits, and there will
be other switches to control smaller
groups of equipment, such as the
Avionics Master Switch, for the radios
and navaids. The Battery switch may
well be in two parts, one for the
battery itself, and the other for the
alternator circuit. Circuits will be
protected by fuses or circuit
breakers, which should only be reset
once, since there is a reason for them
blowing the first place.
A fuse is a deliberately weak part of
a circuit that is designed to fail if a
problem should happen, thus
protecting the rest of the circuit and
saving the trouble of replacing
wiring in odd places.
A more modern replacement is the
circuit breaker, which is a button that
pops out when a fuse would
otherwise break. A trip-free circuit
breaker is one that will trip even if it
is held in.
Electricity & Radio 117
The voltage regulator is there to stop
the battery being overcharged or the
system being overloaded by the
generator or alternator. There will be
a warning light in the cockpit to
indicate that this is happening, and
that you are getting battery power
only. The over voltage sensor may be
reset with the Master Switch. It may
be tested by turning the ALT half of
the battery switch off for a moment.
When starting, the starter switch will
activate a solenoid, which is just a
bigger switch that can handle more
current, to actually turn the engine.
Since the current is large (60 amps),
there is no fuse protection, which is
why there is a starter light.
Amps
The flow of electrons in a conductor
(i.e. the current) is expressed in terms
of amperes, or amps, which are defined
as the movement of 1 coulomb per
second (a coulomb is the
accumulated charge of a large
number of electrons, actually 6.28 x
1018). In an aircraft, amps would be
measured with an ammeter, or
loadmeter, a useful device for checking
if your battery is being charged. An
ammeter needle should always be
showing in the + side of the gauge,
showing a positive charge.
Volts
The work done to add electrons to
an atom is expressed in volts, which
move from high to low pressure, like
air does. Once a body is charged this
way, it is "pressurised" (for want of a
better word), and the potential
energy it contains is called the
potential difference when it refers to a
difference in energy, or pressure,
between two points. You can look
on volts as the equivalent of water
pressure.
Another (older) name for volts is
electromagnetic force, or emf. It is
measured with a voltmeter, which you
might use to check the state of your
battery before starting a jet engine.
Resistance
Even a good conductor slows
electrons down. The longer and
thinner the wire is, the more the
opposition, called resistance, expressed
in ohms. 1 ohm allows 1 amp to flow
when 1 volt is applied.
All this work causes heat, due to the
friction of electrons moving against
each other, and the more work you
make electricity do, the hotter things
get, which is how electric fires work.
If you make it work harder, you get
light as well, hence light bulbs.
When you start using AC, however,
the current flows on the outside of the
cable, increasing the resistance
(many times) because the effective
cross-sectional area is reduced,
which is called the skin effect.
Magnetism
A magnet is a substance, typically a
soft iron bar, that has lines of force
running through and around it (the
Earth is a magnet as well). All
magnets have a North and a South
pole, and two North poles will repel
each other. North and South poles
attract each other. If you therefore
had a bar magnet, its South Pole
(traditionally red) would point
towards the Earth's (magnetic)
North pole. This is what a compass
118 JAR Private Pilot Studies
is all about, discussed more fully
under Instruments. The thing to
remember, though, is that the South
Pole is marked as North.
Magnetic reluctance is the ability of a
substance to pass lines of flux within
itself. Hard iron in this respect will
not pass flux easily, so it has a high
reluctance and is therefore not easily
　
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