18WuD2x

vacuum level • V antenna 717
vacuum level 1. The pressure, in millimeters of
mercury (mm/Hg). Normal atmospheric pressure
is 760 mm/Hg. A perfect vacuum would be represented
by 0 mm/Hg. 2. The proportion, or percentage,
of normal atmospheric pressure in a
given environment.
vacuum phototube A phototube enclosed in an
evacuated envelope, as opposed to one that is
gas-filled.
vacuum pump A pump for removing air and gases
from electron tubes, X-ray tubes, lamp bulbs, etc.
Such pumps include mechanical types, diffusion
types, and combinations of these. Also see DIF-
FUSION PUMP.
vacuum range The range of a communications
system if propagation takes place through a
perfect-vacuum (theoretical) medium.
vacuum seal An airtight seal between adjoining
parts in an evacuation system.
vacuum switch A switch that is enclosed in a vacuum
bulb or tube to reduce contact sparking.
vacuum thermocouple A thermocouple enclosed
in a vacuum bulb with a small heater element.
Radio-frequency current passed through the
heater raises its temperature and causes the
thermocouple to generate a proportional directcurrent
voltage.
vacuum tube An electron tube from which virtually
all air and gases are removed. Also see ELEC-
TRON TUBE.
vacuum-tube amplifier An amplifier using one or
more vacuum tubes, rather than semiconductor
devices.
vacuum-tube bridge A special bridge for determining
vacuum-tube characteristics by null methods.
vacuum-tube characteristics The operating parameters
of a vacuum tube, such as plate current,
grid voltage, input resistance, interelectrode
capacitances, amplification factor, transconductance,
etc., that describe tube performance.
vacuum-tube coefficients See VACUUM-TUBE
CHARACTERISTICS.
vacuum-tube current meter An ammeter, milliammeter,
microammeter, or nanoammeter embodying
an amplifier that uses one or more
vacuum tubes. Also see ELECTRONIC CURRENT
METER.
vacuum-tube modulator A circuit using one or
more vacuum tubes to impress a modulating signal
on a carrier.
vacuum-tube rectifier 1. An alternating-to-directcurrent
converter circuit using one or more vacuum
tubes, rather than gas tubes or
semiconductor devices. 2. A rectifier tube.
vacuum-tube sweep A sweep oscillator using a
vacuum tube, rather than a gas tube. Also called
HARD-TUBE SWEEP.
vacuum-tube voltmeter Abbreviation, vtvm. A
voltmeter using a tube-type amplifier. Also see
ELECTRONIC VOLTMETER.
val Abbreviation of VALUE.
valence A unit showing the degree to which elements
or radicals (replaceable atoms or groups of
atoms) will combine to form compounds.
valence band In the energy diagram for a semiconductor,
the band of lowest energy. This band lies
below the forbidden band (energy gap), which is
below the conduction band. Also see ENERGY
BAND DIAGRAM.
valence bond In a semiconductor material, an interatomic
path over which shared electrons
travel.
valence electrons Electrons in the outermost orbits
of an atom. These electrons determine the
chemical and physical properties of a material.
Also see FREE ELECTRON.
valence shells See ELECTRON SHELLS.
validate To check data for correctness.
validity 1. Correctness or accuracy of data. 2. The
logical truth of a derivation or statement, based
on a given set of propositions.
valley A dip between adjacent peaks in a curve or
wave.
valley current In a tunnel diode, the current at the
valley point.
valley point The lowest point of finite current on
the current-voltage response curve of a tunnel
diode. Immediately before this point, current decreases
with increasing applied voltage (an indication
of negative resistance). Beyond this point,
however, the current again increases with increasing
voltage. Compare PEAK POINT.
valley voltage In a tunnel diode, the voltage at the
valley point.
value 1. The level or magnitude of a quantity (e.g.,
voltage value). 2. The worth of a system, procedure,
device, etc., in terms of goal fulfillment or
other criterion.
valve Variation (Brit.) of ELECTRON TUBE. The
term was applied to the first tube, a diode (the
Fleming valve), and is descriptive of the action of
a tube (a controller of electric current), rather
than its appearance.
vanadium Symbol, V. A metallic element. Atomic
number, 23. Atomic weight, 50.94.
Van Allen radiation belts Two high-altitude zones
that surround the earth and consist of highenergy
subatomic particles. These belts were
once thought to preclude space travel by humans
beyond the immediate vicinity of the earth.
Van de Graaff generator See BELT GENERATOR.
vane-anode magnetron A magnetron having plane
parallel walls between adjacent cavities.
vane attenuator A waveguide attenuator consisting
essentially of a slab (vane) of resistive material
that slides laterally through the waveguide.
vane instrument See IRON-VANE METER.
vane-type magnetron See VANE-ANODE MAG-
NETRON.
vane-type meter See IRON-VANE METER.
V antenna See VEE ANTENNA.