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second.
heptane. A liquid organic hydrocarbon compound, CH3(CH2)5CH3. Heptane, which has a very low
critical pressure and temperature, is used as the low reference fuel (octane zero) in determining the octane
rating of a reciprocating engine fuel.
heptode (electron tube). An electron tube, or vacuum tube, that has seven active electrodes. The electrodes
in a heptode are the anode, the cathode, the control grid, and four special purpose grids, such as a screen
grid, suppressor grid, and beam-forming grids.
hermaphrodite calipers. Calipers having one sharp-pointed leg, similar to that used on dividers, and one
leg of an outside caliper. Hermaphrodite calipers are used to scribe a mark along a piece of material a
specific distance from the edge.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 296
hermetic seal. An airtight seal used to protect components or devices from the effect of moisture in the air.
To achieve a hermetic seal, air is pumped out of the container or displaced by a dry inert gas, such as
nitrogen, and the container is sealed so no moisture can get into it.
herringbone gear. A gear whose teeth are cut across its face in the shape of a V. A herringbone gear is
similar to two helical gears whose teeth run in opposite directions, mounted side-by-side on the shaft.
hertz. The term used for the frequency of any cyclic repetition. One hertz, Hz, is one cycle of events each
second.
Hertz antenna (radio antenna). An ungrounded, half-wave antenna whose resonant frequency is
determined by its distributed capacity. This capacity, and therefore its resonant frequency, is determined by
the antenna length.
heterodyne. To mix two electrical or acoustic signals having different frequencies so they produce two
new signals whose frequencies are different from either of the two original signals.
If a signal with a frequency of 1,000 hertz is mixed with a signal having a frequency of 800 hertz,
two new frequencies are produced: the sum of the two frequencies, 1,800 hertz, and the difference between
the two, 200 hertz.
heterodyne-type frequency meter. An instrument used to determine the frequency of alternating current.
The frequency of an accurately calibrated internal oscillator is varied and compared with the frequency
being measured.
When the two frequencies are mixed, a third frequency, called the beat frequency, is produced.
This beat frequency is the difference between the two.
The variable frequency is changed until the beat frequency disappears. When it disappears, the
frequency being measured is the same as that being produced by the oscillator.
heterogeneous mixture. A physical mixture of dissimilar ingredients. Concrete is a heterogeneous mixture
of sand, gravel, and portland cement.
hexadecimal number system. A number system that uses 16 as its base. The values from 0 through 9 are
represented by the numerals 0 through 9, and values from 10 through 15 use the letters A through F.
Hexadecimal numbers can easily be converted into binary numbers for use in a computer. Values of up to
decimal 255 or binary 1111 1111 can be expressed by using only two hexadecimal digits, FF.
Decimal Hex Binary Decimal Hex Binary
0 0 0000 8 8 1000
1 1 0001 9 9 1001
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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2 2 0010 10 A 1010
3 3 0011 11 B 1011
4 4 0100 12 C 1100
5 5 0101 13 D 1101
6 6 0110 14 E 1110
7 7 0111 15 F 1111
hexagon. A closed, plane figure that has six sides.
hexagon head bolt. A bolt whose head has six sides (a hexagon). A hex-head bolt can be turned with an
open-end wrench or with either a six-point or a 12-point socket wrench. The six flat sides of a hex-head
bolt allow the wrench to be repositioned more often than could be done with a square-head bolt, and this
makes for easier installation and removal.
hexode (electron tube). A form of electron tube having six active electrodes. The electrodes are the
cathode, the anode, the control grid, and three other grids that serve special functions in controlling the
passage of electrons between the cathode and the anode.
hidden line (aircraft drawing). A medium-weight line made up of short dashes that shows a surface of a
part that is not visible in the view shown.
hidden surface (aircraft drawing). Any surface on an object that cannot be seen in the view depicted by a
drawing. A hidden surface is represented in outline form by a hidden line, a series of medium-weight
dashes.
high (meteorology). An area of high barometric pressure with its attendant system of anticyclonic winds,
winds that circulate in a clockwise direction in the northern hemisphere and counterclockwise in the
　
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