18WuD2x

336 helical-beam antenna • Herschel-Quincke tube
helical-beam antenna See HELICAL ANTENNA.
helical line The helix in a backward-wave oscillator
or traveling-wave tube.
helical potentiometer A potentiometer whose resistance
element is a wire wound into a coil of
several turns. The slider moves over the wire (or
the larger coil) from one end to the other as the
slider or coil is turned through several complete
revolutions. Also called MULTITURN POTEN-
TIOMETER.
helical scanning Radar scanning by an antenna
that moves vertically as it moves horizontally,
producing a spiral motion to the radiated beam.
helical sweep See SPIRAL SWEEP, 1, 2.
helical transmission line See HELICAL LINE.
helicone An antenna used at ultra-high and microwave
frequencies, consisting of a helical radiator
within a cone-shaped reflector.
helix recorder An information recorder using a
spiral method of scanning. The recording
medium is usually drum-shaped.
Helmholtz coil A device consisting of two crossedfield
primary windings in which an inductively
coupled secondary winding rotates. The primary
windings carry currents that differ in phase by 90
degrees. Rotating the secondary coil provides 360
degrees of continuously variable phase shift.
Input
V 2
H S
2 H
2
V 2
Output
Signal
helicone
Feed point
Helix
Reflecting
cone
helionics The science of converting solar heat into
electrical energy. The term is an acronym from
helio- (pertaining to the sun) and electronics.
heliostat 1. A servo-controlled motor-driven device
that drives a mirror to keep sunlight
trained upon a specific target. 2. By extension,
any similar device to keep a solar cell pointed to
the sun.
helitron A form of oscillator used at ultra-high and
microwave frequencies. The output frequency is
variable over a wide range.
helium Symbol, He. A gaseous element. Atomic
number, 2. Atomic weight, 4.0026.
helium group The six inert gases in group 0 of the
periodic table: argon, helium, krypton, neon,
xenon, and radon.
helium-neon laser A laser in which the lasing substance
is a mixture of helium and neon. Produces
a characteristic brilliant red visible output. Also
see HELIUM and NEON.
helix 1. A single-layer coil. 2. That which is coilshaped
(i.e., spiral in configuration). 3. See HELI-
CAL ANTENNA. 4. See HELICAL LINE.
helix line See HELICAL LINE.
Helmholtz coil
Helmholtz double layer An intermolecular layer
between a metal and an electrolyte in which it is
immersed. It is formed when the adhesive force between
the metal and electrolyte decreases the surface
tension of the metal, causing positive ions to
migrate from the metal into the liquid. The metal,
charged negatively, and the electrolyte, charged
positively, form a capacitor whose dielectric is the
Helmholtz layer.
Helmholtz resonator An acoustic (sound) chamber
whose geometry, in combination with the size
of a small opening, results in resonance at a specific
frequency.
HEM Abbreviation of hybrid electromagnetic (see, for
example, HYBRID ELECTROMAGNETIC WAVE).
hemimorphic Pertaining to an object with ends
that have unlike faces.
He-Ne laser See HELIUM-NEON LASER.
henry Symbol, H. The standard unit of inductance.
It is the inductance exhibited by a closed circuit
in which one volt is produced by a current changing
uniformly at one ampere per second. This is a
large unit of inductance; more common units are
the MILLIHENRY and the MICROHENRY.
hermaphroditic plug A plug that has the prongs of
a male plug and the recessed contacts of a female
plug. Compare FEMALE PLUG and MALE PLUG.
hermetically sealed Constructed in manufacture
so as to be permanently closed against the entry
of air or other gases, dust, and moisture.
hermetic seal A permanent, air-tight seal that effectively
prevents corrosion from elements in the
outside environment.
herringbone pattern A pattern of interference in a
television picture, so named because of its resemblance
to the skeleton of a fish.
Herschel-Quincke tube An acoustic device that
demonstrates sound interference. The device has