18WuD2x

292 fluorescent screen • flyback time
fluorescent screen A transparent or translucent
plate (such as the end of a cathode-ray tube or
fluoroscope) coated with phosphors that glow
when struck by an electron beam, or by highenergy
electromagnetic radiation, such as ultraviolet
or X rays.
fluorescent tube A mercury-vapor glow lamp distinguished
by having a glass tube whose inner
wall is coated with a phosphor that emits light
when excited by the ultraviolet glow discharge in
the vapor.
fluorescent X rays X rays reradiated by the atoms
of a material that has absorbed X radiation. During
initial exposure, energy absorbed from the radiation
raises the energy level of electrons in the
atoms; when the electrons return to their normal
energy levels, they reradiate some of the absorbed
energy.
fluorine Symbol, F. A gaseous element of the halogen
family. Atomic number, 9. Atomic weight,
18.998.
fluoroscope A device used for viewing the internal
structures of objects. A screen coated with material
that fluoresces when exposed to X rays is
mounted in one end of a light-tight viewing hood.
When an object is placed between the screen and
an X-ray tube, an image is produced on the
screen. In medical applications, this device has
been supplanted by methods that do not use ionizing
radiation; nuclear magnetic resonance imaging
(NMRI) is one example.
fluoroscopy The art of using a fluoroscope in the
inspection of materials and parts or in medical
examinations.
flush A form of mounting in which there is little or
no protrusion from the panel surface.
flutter 1. In a high-frequency superheterodyne receiver,
a rapid fluctuation in signal strength,
caused by tuning and detuning of the oscillator
stage. This usually results from poor directcurrent
(dc) power-supply regulation. 2. Repetitive,
rapid fluctuations in the output of a sound
reproducer. Also see WOW. 3. An echo effect
sometimes observed in rooms or auditoriums of
poor acoustic design.
flutter bridge A bridge-type instrument for measuring
flutter in constant-speed machines, such
as sound recording and reproducing devices.
flutter rate The frequency of flutter, in cycles per
second (Hertz).
flux 1. Theoretical lines of force that extend in all
directions from an electric charge (electric flux)
or from a magnetic pole (magnetic flux). 2. A
material that makes metals more amenable to
being joined by soldering. 3. The number of
photons that pass through a surface for a given
time.
flux density Symbol, B. Unit, tesla. The degree of
concentration of magnetic lines of force. One tesla
represents a flux density of one volt-second per
square meter (V•s/m 2 ).
fluxgate A device that controls the azimuth bearing
of a directional system by means of interaction
with the geomagnetic field.
fluxgate magnetometer A magnetic compass for
robot guidance. Uses coils to sense changes in artificially
generated reference fields. Output from
the sensors is sent to a computer that calculates
the robot’s position, based on the orientation and
intensity of the lines of flux in the reference fields.
flux graph A device that graphically records the intensity
of a magnetic field around a permanent
magnet or electromagnet, or around an inductor
carrying a current.
flux leakage See MAGNETIC LEAKAGE.
flux lines The theoretical lines of force in an electric
or magnetic field.
flux linkage The passage of lines of force set up by
one component through another component, so
as to enclose most of the penetrated component’s
volume.
In
flux linkage
Out
Flux
lines
fluxmeter An instrument for measuring magnetic
flux density. Also called gaussmeter.
flux refraction The tendency for magnetic lines of
flux to change direction at the boundary between
substances having different permeability. Flux
refraction resembles refraction of electromagnetic
radiation in or at a boundary between substances
having different indices of refraction.
flyback 1. The abrupt drop or reversal of a current
or voltage that was previously increasing (e.g., the
rapid fall of a sawtooth wave). Also see KICK-
BACK. 2. The duration of the drop of a current or
voltage that was previously increasing, for a sawtooth
or similar wave. 3. In an oscilloscope or picture
tube, the rapid return of the beam to its
starting position.
flyback checker An apparatus that senses the
presence of short or open circuits in motors,
transformers, and generators, by measuring the
amount of flyback (kickback).
flyback power supply See KICKBACK POWER
SUPPLY.
flyback time The time taken for the electron beam
in an oscilloscope tube, picture tube, or camera
tube to return to its starting point after it has
reached the point of maximum deflection.