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atomic reactor • attracted-disk electrometer 45
atomic reactor See REACTOR, 2.
atomic theory The scientific theory that all matter is
composed ultimately of atoms, which are the smallest
particles retaining the identity of an element.
Atoms combine to form molecules, the smallest
particles that retain the identity of a compound.
Atoms themselves contain minute subatomic particles,
some of which carry electric charges. See
BOHR ATOM and RUTHERFORD ATOM.
atomic time 1. A means of time determination
that makes use of the resonant vibrations of certain
substances, such as cesium. 2. Synchronized
astronomical time, as determined by an
ATOMIC CLOCK.
atomic unit of energy In a hydrogen atom, the potential
energy of the electron in the lowest-energy
shell, as averaged over a certain length of time.
The shell represents the mean energy of the electron.
atomic weight 1. The mass of a particular atom in
ATOMIC MASS UNITS (amu). 2. A number characterizing
the average mass of individual atoms
for a specific isotope of an element. Thus, carbon
12 (C12) has an atomic weight of 12, oxygen 16
(O16) has an atomic weight of approximately 16,
and uranium 238 (U238) has an atomic weight of
about 238.
atomistics The science of the atom and atomic energy.
Also called atomics.
attack 1. The rise of a pulse from zero to maximum
amplitude. 2. The time required for a pulse to rise
from zero to maximum amplitude. 3. The initialization
of a circuit voltage or current for a certain
purpose, such as an automatic gain control. 4.
The rise of a musical note from zero to full volume.
attenuate To reduce in amplitude.
attenuation A reduction of signal amplitude.
attenuation characteristic Also called attenuation
constant. 1. In an amplifier, network, or component,
the decrease in signal amplitude as a
function of frequency, usually expressed in decibels
per octave. 2. In a transmission line, the decrease
in signal amplitude per unit length.
Usually expressed in decibels per 100 feet, decibels
per mile, or decibels per kilometer.
attenuation constant See ATTENUATION
CHARACTERISTIC.
attenuation distortion A type of distortion characterized
by variation of attenuation with frequency
within a given frequency range.
attenuation equalizer An equalizer that stabilizes
the transfer impedance between two ports at all
frequencies within a specified frequency band.
attenuation-frequency distortion Distortion
characterized by the attenuation of the frequency
components in a complex waveform. Frequencysensitive
RC networks (such as a Wien bridge) exhibit
this type of distortion when they attenuate a
fundamental and each harmonic unequally.
attenuation network A combination of components
(R, C, or L singly or in any necessary
combination) that provide constant signal
attenuation with negligible phase shift throughout
a frequency band.
attenuation ratio The ratio indicating a relative
current, voltage, power or energy decrease. For
example, for voltage, E input /E output = 6/2 = 3:1 = 3.
attenuator A device for reducing signal amplitude
in precise, predetermined steps, or smoothly over
a continuous range. A network of resistors, capacitors,
or both. The simplest attenuator consists
of one or more noninductive resistors.
attack time The time required for an applied signal
that suddenly increases in amplitude to reach
63.2 percent of its final, stable value.
attemperator An automatic temperature-controlling
device; a thermostat.
attention display A computer-generated chart or
graph, displayed as an alert signal concerning a
particular situation.
attitude The position of an aircraft or space vehicle
relative to a (usually terrestrial) reference point,
often determined with electronic instruments.
atto- Abbreviated, a. A prefix meaning 10 –18 or
multiplication by 10 –18 .
attofarad Abbreviation, aF. An extremely small
unit of low capacitance; 1 aF equals 10 –18 F.
attracted-disk electrometer A device to measure
potential difference consisting of two parallel
metal disks—one of which is connected to a tension
spring. The force between the disks indicates
the magnitude of the electric field.