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616 selective fading • self-excited oscillator
to arrive in various phase relationships, with resulting
distortion in the received signal.
selective interference Interference confined to a
narrow band of frequencies.
selective polarization See POLARIZATION SE-
LECTIVITY.
selective reflection In the reflection of electrons
directed into a crystal by means of an electron
gun, the tendency of the electrons to be reflected
more readily when they strike the crystal at certain
speeds.
selective relay 1. A relay or relay circuit tuned to
open or close at one signal frequency. 2. A relay
or relay circuit adjusted to open or close at one
value of current or voltage.
selective trace In computer operations, a diagnostic
program used to analyze certain areas of
memory or specific kinds of program instructions,
for debugging purposes.
selectivity 1. The ability of a circuit or device to
pass signals of one frequency and reject signals
at other frequencies. 2. The degree to which a circuit
or device passes signals of one frequency and
rejects signals at other frequencies.
selectivity control In some equipment (such as
receivers, crystal filters, wave analyzers, and vibration
meters) an adjustment that permits variation
of selectivity.
Selectoject A fully electronic, continuously tunable,
notcher-peaker that is resistance-capacitance
tuned. The name is an acronym for select or
reject.
selector 1. A channel switch in a radio or television
receiver. 2. See SELECTOR SWITCH.
selector channel In data-processing and computer
systems, a data transmission channel controlling
the information flow between peripherals
and a central processing unit.
selector pulse In digital communications, an identifying
pulse that represents a certain group of
bits or data.
selector relay A device, such as a stepping switch,
that actuates one of a number of available circuits
on receipt of a predetermined number of
pulses.
selector switch A (usually rotary) multiposition
switch that allows an operator to select from
among several options (such as frequency channels,
frequency bands, or selective filters).
selenium Symbol, Se. A nonmetallic element.
Atomic number, 34. Atomic weight, 78.96. It is
used in the manufacture of some diodes, rectifiers,
and photocells.
selenium cell A photoelectric cell that uses specially
processed selenium as the light-sensitive
material. It can be operated as a photoconductive
cell or a photovoltaic cell.
selenium diode A junction diode in which the
semiconductor material is specially processed selenium.
Also see JUNCTION DIODE.
selenium photocell See SELENIUM CELL.
selenium rectifier A disk- or plate-type power rectifier
utilizing the junction between selenium and
aluminum or selenium and iron.
self-bias For a transistor or vacuum tube, inputelectrode
bias voltage resulting from the flow of
output-electrode current through a resistor common
to both circuits. Also called AUTOMATIC
BIAS.
self-bias
self-capacitance The inherent internal capacitance
of a device other than a capacitor.
self-checking number A number whose digits
have a value that determines the check digit attached
to it; thus, it can be verified following its
transfer between storage locations or peripherals.
self-cleaning contacts Switch or relay contacts
that clean themselves automatically by means of
wiping action.
self-contained device A device containing all
parts and sections (e.g., main circuit, power supply,
meter, loudspeaker, etc.) needed for full operation
(i.e., no auxiliary equipment is needed).
self-controlled oscillator See SELF-EXCITED OS-
CILLATOR.
self discharge The tendency of an electrochemical
cell or battery to gradually lose stored energy
when not in use.
self-discharge rate A quantitative expression of
the speed with which SELF DISCHARGE occurs
in an electrochemical cell or battery when it is
stored without being used.
self-energy Symbol, E. The energy mc 2 , in joules,
of a particle traveling at the speed of light c
(2.9979 × 10 8 meters per second) and whose
mass is m kilograms.
self erasing In a magnetic tape, the unwanted
erasing of data near a highly magnetized region.
self-excited generator An alternating-current
generator in which the field coils are supplied
with direct current produced by the machine itself.
Compare SEPARATELY EXCITED GENERA-
TOR.
self-excited oscillator An oscillator consisting of
an amplifier that supplies its own input signal
through positive feedback, and whose oscillation
frequency depends entirely on circuit constants,