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across the contacts.
spark test (metal identification method). A quick test to identify types of ferrous metal. A sample of the
metal is pressed against a grinding wheel, and the pattern of sparks thrown off indicates of the type of metal.
When wrought iron which contains no carbon is held against the wheel, the sparks stream out in a straight
line. The lines of sparks grow wider and brighter as they stream out behind the wheel.
Mild steel, with its low carbon content, has a spark pattern similar to that of wrought iron, except
some of the sparks have tiny forks on them. The spark pattern for tool steel with its high carbon content is
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 543
similar to that for mild steel, except there are more forks in the stream, and the forked sparks form close to
the wheel. Cast iron produces a large number of sparks close to the wheel, but they die out quickly. The
spark pattern for cast iron is very short.
spar varnish. A hard, durable finish for wood that is to be kept outdoors where it is exposed to the weather.
Spar varnish is made of a resin mixed with drying oils and thinners, and it is brushed on the surface to be
protected. When the drying oils cure, they leave a hard, protective resin film on the wood. Spar varnish is
much more durable than spirit varnish.
spatula. A tool that looks much like a knife, but has no cutting edge. The thin, flexible, metal blade of a
spatula is used to mix, spread, or move soft materials, such as resins.
speaker voice coil. A small coil that rides inside an air gap in a permanent magnet or electromagnet inside
a radio speaker. The voice coil is physically attached to the speaker cone, and alternating current from the
output of an audio amplifier flows through it.
Current flowing in the voice coil produces a magnetic field that either aids or opposes the
magnetic field in which the coil is mounted. This interaction between the magnetic fields moves the coil in
and out, and as it moves, it vibrates the speaker cone and produces sound waves in the air.
special emergency (air traffic control). A condition of air piracy or other hostile act by a person or
persons aboard an aircraft which threatens the safety of the aircraft or its passengers.
special VFR. 14 CFR Part 1: “Meteorological conditions that are less than those required for basic VFR
flight in controlled airspace and in which some aircraft are permitted flight under visual flight rules.”
special VFR operations. 14 CFR Part 1: “Aircraft operating in accordance with clearances within
controlled airspace in meteorological conditions less than the basic VFR weather minima. Such operations
must be requested by the pilot and approved by ATC.”
specific fuel consumption. A measure of the amount of fuel burned in a heat engine to produce a given
amount of power. Specific fuel consumption of a reciprocating engine is expressed in pounds of fuel
burned per hour for each horsepower produced.
specific gravity. The ratio of the density of a material to the density of pure water. If the specific gravity of
a material is less than 1.0, it will float in water, but if it is greater than 1.0, it will sink.
specific heat. The ratio of the amount of heat energy needed to raise the temperature of a specific mass of
material one degree Celsius, to the amount of heat energy needed to raise the temperature of the same mass
of pure water one °C. Water has a specific heat of 1.0, and copper has a specific heat of 0.093. Therefore, it
takes 10.75 (1 / 0.093) times as much heat to raise the temperature of one gram of water one °C as it does to
raise the temperature of one gram of copper one °C.
specific humidity (meteorology). The ratio, by weight, of the water vapor in a sample of air to the
combined weight of the water vapor and the dry air.
specific weight (aircraft engine measurement). The ratio of the weight of an aircraft engine to the brake
horsepower it produces.
spectrometric oil analysis. See SOAP (Spectrometric Oil Analysis Program).
speech scrambler (communications device). A security device used with a radio transmitter to make
voice transmissions unintelligible to anyone not having the correct descrambling equipment and the correct
code for the day. The scrambler circuit modifies the voice signals being transmitted so they no longer
resemble the original sound. When the signals are received by the intended receiver, they are passed
through a descrambler that changes them back into their original form.
speed. The rate of motion. Speed, which is the distance an object travels in a given length of time, is
expressed in such terms as miles per hour, kilometers per hour, knots (nautical miles per hour), feet per
　
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