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40 arm • arsenic
arm 1. Any of the distinct branches of a circuit or
network. Also called leg. 2. A movable element in a
device, usually containing a contact for switching.
armature 1. The rotating member of a motor. 2.
The rotating member of some types of electro-mechanical
generator. 3. The movable member of a
relay, bell, buzzer, or gong. 4. The movable member
of an actuator. 5. The soft-iron keeper placed
across the poles of a permanent magnet to conserve
power.
armature coil A coil of insulated wire wound on a
ferromagnetic core to provide the electromagnetic
properties of an armature. In a motor or generator,
the armature coil is distinguished from the
FIELD COIL.
armature core The ferromagnetic core upon which
the armature coil of a motor or generator is
wound.
armature gap 1. In a motor or generator, the space
between an armature core and the pole of a field
magnet. 2. In a relay, the space between the armature
and the relay-coil core.
armature hesitation A momentary delay in the
movement of a relay.
armature-hesitation contact chatter Undesired
(usually rapid, repetitive) making and breaking of
relay contacts. Generally caused by armature
hesitation.
armature-impact contact chatter Undesired
(usually rapid, repetitive) making and breaking of
relay contacts, caused by contact bounce when
the armature strikes the relay core (closure) or
backstop (opening).
armature relay A relay that uses an electromagnet
to pull a lever toward or away from a set of fixed
contacts.
armature travel The distance traveled by an armature
during relay operation.
armor A protective metal cable covering.
Armstrong FM system (Edwin H. Armstrong,
1890 –1954). A phase-shift method of frequency
modulation. See PHASE MODULATION.
armature voltage control A means of controlling
motor speed by changing the applied armature
winding voltage.
armchair copy An amateur radio term for reception
of exceptionally clear signals.
arming the oscilloscope sweep Enabling an oscilloscope
to trigger on the next pulse by closing a
switch.
Armstrong oscillator (Edwin H. Armstrong,
1890–1954). An oscillator circuit that uses inductive
feedback between the output and input.
Either the output coil or the input coil can be
tuned to set the oscillator frequency. The amount
of positive feedback is controlled by varying the
coupling between the coils.
Armstrong superheterodyne circuit See SUPER-
HETERODYNE CIRCUIT.
Armstrong superregenerative circuit See SU-
PERREGENERATIVE CIRCUIT.
ARPA Acronym for Advanced Research Projects
Agency, a subsidiary of the U.S. Department of
Defense.
array 1. A directive antenna that consists of an assembly
of properly dimensioned and spaced elements,
such as radiators, directors, and
reflectors. 2. A coordinated group or matrix of
components, such as diodes, resistors, memory
cells, etc., often enclosed in one capsule. 3. Subscripted
variables representing data arranged so
that a program can examine the array and extract
data relevant to a particular subscript.
array device A group of similar or identical components
that are connected together in a certain
fashion, to perform a specific task.
arrester 1. A device used to protect an installation
from lightning. It consists of a varistor or an air
gap connected between an antenna or power line
and an earth ground. The device passes little or
no current under ordinary conditions, but passes
heavy current to ground during a lightning
stroke. Also called LIGHTNING ARRESTER. 2. A
self-restoring protective device used to reduce
voltage surges on power lines.
ARRL Abbreviation for American Radio Relay
League.
arrowhead A wideband, log-periodic antenna with
linear polarization.
ARS Abbreviation of Amateur Radio Service.
arsenic Symbol, As. A metalloidal element. Atomic
number, 33. Atomic weight, 74.91. Arsenic is familiar
as an n-type dopant in semiconductor processing.