18WuD2x

hygroscopic material • hyperpolarization 349
hygroscopic material A material that absorbs
moisture from the air, but not enough to get wet
(e.g., lime and silk). Compare DELIQUESCENT
MATERIAL.
hygrostat A humidity-sensitive relay or switching
circuit.
hygrothermograph A graphic recorder indicating
humidity and temperature on the same chart,
both as functions of time.
hyperacoustical zone In the upper atmosphere, a
region in which the distance between air
molecules is comparable to the wavelengths of
audible sound. As the altitude increases within
this zone, the upper-frequency limit of effective
sound propagation decreases. At altitudes above
this zone, no audible sound can be propagated.
hyperbola A conic-section curve satisfying the
equation:
(x – x 0 ) 2 /a 2 – (y – y 0 ) 2 /b 2 = 1
where x and y are the independent and dependent
variables, x 0 and y 0 are the coordinates of
the center, a is half the length of the major axis,
and b is half the length of the minor axis.
Plane
Double
cone
(x 0, y 0 ) y
hyperbola
x
Cone
axis
hyperbolic angle An angle subtended by a sector
of a hyperbola in a manner analogous to that in
which a circular angle is subtended by an arc of a
circle.
hyperbolic-cosine horn See CATENOIDAL HORN.
hyperbolic error 1. In an interferometer, a miscalculation
in the direction of arrival of a signal. The
signal from one antenna in the system can be as-
sumed to be in phase with the signal from another
antenna, when actually the two components
differ by an integral number of whole
wavelengths. 2. The angular error, in degrees,
minutes, or seconds of arc, resulting from a miscalculation
of phase in an interferometer.
hyperbolic face contour See HYPERBOLIC GRIND.
hyperbolic functions The nonperiodic functions of
a hyperbolic angle. Hyperbolic functions are related
to the hyperbola in the same manner that
common trigonometric functions are related to
the circle. Hyperbolic functions are useful in calculations
involving traveling waves on transmission
lines, attenuator design, and wave-filter
design.
hyperbolic grind The shape (approximately hyperbolic)
to which the face of a magnetic recording
head is ground. It provides optimum contact with
the tape and ensures good high-frequency response.
hyperbolic horn A horn antenna whose crosssectional
area is a hyperbolic function of the
distance along the axis.
hyperbolic logarithm See NAPIERIAN LOGA-
RITHM.
hyperbolic navigation A radionavigation system
in which the operator of an aircraft or boat determines
position by comparison of two received signals.
The two transmitters radiate signals from
known positions and with known timing characteristics.
The time delay from each transmitter is
determined, resulting in two hyperbolic curves on
a map. The point of intersection of the curves is
the location of the aircraft or ship.
hyperbolic radian A unit of measure derived from
a hyperbolic angle. A hyperbolic radian is the hyperbolic
angle that encloses an area of 0.5 when
the distance along the x-axis to the hyperbola is
unity. Also see HYPERBOLIC ANGLE. Compare
CIRCULAR RADIAN.
hyperbolic trigonometry The branch of mathematics
dealing with the theory and application of
hyperbolic angles and their functions.
hypercardioid microphone A unidirectional microphone
with exceptional sensitivity in front,
and minimal responsiveness from the sides and
rear.
hypercardioid pattern A directional CARDIOID
PATTERN with accentuated responsiveness or
emission in the favored direction (front), and
greatly suppressed responsiveness or emission in
other directions (sides and rear).
hyperfocal distance The shortest distance to
which a lens can be focused without degrading
definition at infinity.
hyperfrequency waves See MICROWAVES.
hypernik See HIPERNICK.
hyperon Any one of various particles having a
mass greater than that of a neutron or proton.
hyperpolarization The production of an increased
voltage across a biological membrane.