18WuD2x

58 back scatter • bail
back scatter Scattering of a wave back toward a
radio transmitter from points beyond the skip
zone. This phenomenon is caused by ionospheric
reflection. Compare FORWARD SCATTER.
backstop A contact or barrier (such as a screw or
post) that serves to limit the BACKSWING of the
armature of a relay.
backswing 1. The tendency of a pulse to overshoot,
or reverse direction after completion. Backswing
is measured in terms of the overshoot amplitude
as a percentage of the maximum amplitude of the
pulse. 2. The extent to which a relay armature
moves back from a contact when the relay contacts
are open.
back-to-back connection The connection of
diodes or rectifiers in reverse parallel (i.e., the anode
of one to the cathode of the other) across a
signal line to pass both half cycles of ac in certain
control circuits.
back-to-back sawtooth A symmetrical sawtooth
wave in which the rise slope is equal to the fall
slope. Also called triangular wave and pyramidal
wave.
backup 1. An element, such as a circuit component,
that is used to replace a main component,
in case of main-component failure. 2. Any process
or scheme that serves to maintain operation
of a system in case of main-component
failure. 3. A battery that maintains volatile
memory data stored in one or more integrated
circuits. 4. A computer file, or set of files, stored
in a nonvolatile medium, such as diskettes or
magnetic tape, to prevent catastrophic data loss
in the event of hard-disk failure. 5. A battery or
alternative power source that keeps an alarm
system operational in the event of a utility power
failure.
backup battery 1. In a computer or microcomputer-controlled
electronic device, a source of
voltage to preserve volatile memory data if the
power is removed. 2. A battery used for powering
a system in the event that the main power source
should fail.
backup facility In an electrical or communications
system, a facility that is intended for use when
the primary, or main, facility is not operational.
back voltage 1. Voltage induced in an inductor by
the flow of current through the inductor, so called
because its polarity is opposite to that of the applied
voltage. Also called counter emf. 2. A voltage
used to obtain bucking action (e.g., the voltage
used to zero the meter in an electronic voltmeter
circuit). 3. Reverse voltage applied to a semiconductor
junction.
backwall In a pot core, the plate or disk that connects
the sleeve and center post to close the magnetic
circuit.
backward diode A semiconductor diode manufactured
in such a way that its high-current flow occurs
when the junction is reverse biased. Such a
diode is also a negative-resistance device.
backward-wave oscillator Abbreviation, BWO. A
microwave oscillator tube similar to the travelingwave
tube. Like the traveling-wave tube, the BWO
contains a helical transmission line. In the electron
beam, electron bunching results from interaction
between the beam and the electromagnetic
field, and reflection occurs at the collector. The
wave moves backward from collector to cathode,
and oscillation is sustained because the backward
wave is in phase with the input. Output is
taken from the cathode end of the helix.
Electron
gun
Output
backward-wave oscillator
Collector
Helical line
back wave The oscillator signal present in an amplifier-keyed,
continuous-wave (CW), Morse-code
transmitter. Normally, this signal is at the same
frequency as the transmitter output, but is not
sufficiently strong to be radiated over the air.
back-wave radiation The condition wherein a back
wave is strong enough to be heard on a continuous-wave
(CW) keyed signal at the receiving station.
This results from ineffective amplifier keying.
baffle A board on which a loudspeaker is mounted
to separate acoustic radiation from the back of
the cone from radiation emanating from the front.
The baffle improves bass response by increasing
the wavelength (lowering the frequency) at which
phase cancellation occurs.
baffle plate 1. See BAFFLE. 2. A metal plate
mounted in a waveguide to reduce the crosssectional
area.
bail A wire loop or chain that holds one member of
a two-member assembly to prevent loss (e.g., the
short chain holding the dust cap of a jack).