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vinylidene chloride • visible spectrum 729
vinylidene chloride A plastic insulant. Dielectric
constant, 3 to 5. Dielectric strength, 20 kV/mm.
Vinylite-A A brand of the plastic polyvinyl acetate.
Vinylite-Q A brand of the plastic polyvinyl chloride.
vinyl resin A synthetic resin resulting from the
polymerization of compounds that contain the
group CH 2 = CH–.
violation In a security system, a condition that results
in the actuation of an alarm. Example: an
intruder breaks the light beam in an electric eye.
virgin magnetic material Core or shield material
that has never been magnetized. When virgin material
is first subjected to magnetization, the hysteresis
loop starts at zero, but never returns to
zero (see BOX-SHAPED LOOP).
virgin neutron Any neutron that has not been involved
in a collision with another particle subsequent
to its initial generation.
virgin record See BLANK RECORD.
virgin tape See BLANK TAPE.
virtual address In computer operations, an address
that must be modified to refer to a location
in main memory.
virtual decision value In quantizing or encoding,
a method of expressing the maximum input amplitude.
Two values are extrapolated from actual
decision values—one at either end of the working
range used.
virtual height The altitude that a vertically propagated
electromagnetic wave would reach before
reflection if its path in the ionosphere were a
straight line. The actual distance at which the
wave penetrates the ionosphere before reflection
is less than the virtual height.
virtual image The image formed when rays from a
scene diverge after passing beyond the focal point
of a convex lens. The scene appears inverted to an
observer. Compare REAL IMAGE.
virtual memory 1. In a computer system, a means
of using two or more memory stores simultaneously.
2. Auxiliary memory used in conjunction
with the main, or core, memory.
virtual ppi reflectoscope A device used to superimpose
a virtual image of a chart or map onto a
plan-position indicator (ppi) radar display. Also
see VIDEO MAPPING.
virtual reality Abbreviation, VR. A general term for
any of various high-level computer simulation or
remote-control programs. The user often wears a
head-mounted display that provides vivid, threedimensional
imagery and binaural sound. It can
be used in robotic telepresence systems for precision
remote control.
virus See COMPUTER VIRUS.
vis 1. Abbreviation of visibility. 2. Abbreviation of
visual.
viscometer An instrument for measuring viscosity.
There are several electronic versions of this
device. In one, a steel ball falls through a material
(such as an oil) under test and distorts the magnetic
field of a pickup coil, causing the deflection
of a meter by an amount proportional to the
speed of the ball and, consequently, to the viscosity
of the fluid.
viscosimeter See VISCOMETER.
viscosity The resistance offered by a fluid (liquid
or gas) to objects passing through it. The viscosity
of pure water is low; that of heavy oil is high.
Expressed in newton-seconds per meter squared.
viscosity index Abbreviation, VI. A number indicating
how well an oil retains its viscosity with
temperature changes; larger indexes are assigned
to oils that are little influenced by variations in
temperature.
viscous-damped arm A phonograph pickup arm
with an oil dashpot to prevent arm resonance and
to slow the descent of the arm to the disc.
viscous damping The use of a viscous fluid in the
dashpot of a device (such as a relay, timer, or
pickup arm) to provide damping. See, for example,
DASHPOT RELAY.
viscous hysteresis A slow, slight increase in the
magnetization of a material when the magnetizing
field is constant. Compare STATIC HYSTERESIS.
visibility factor See DISPLAY LOSS.
visible radiation Electromagnetic radiation that is
perceptible to the eye. Also see VISIBLE SPEC-
TRUM.
visible spectrum The band of electromagnetic
wavelengths that the human eye perceives as visible
light. For most people, this band extends
from approximately 750 nanometers (nm), representing
red light, down to 390 nm, representing
violet light. The visibility curve peaks in the
yellow-green region at about 560 nm; 1 nm = 0.000
Wavelength
(angstroms)
10 0
X rays
10 1
10 2 Ultraviolet
10 3
Visible light
10 4
10 5 Infared
10 6
10 7
EHF
10 8
SHF waves
10
UHF}Radio 9
10 10
visible spectrum