Page 30 - Electrical Engineering Dictionary

(2) factor describing the fractional atten- rameter are closest to the parameters of an
uation of light with distance traversed in a ideal capacitor. Hence, not only a capaci-
medium, generally expressed as an exponen- tance is measured in terms of capacitance (in
tial factor, such as k in the function e −kx , resistive ratio arms bridges), but the induc-
with units of (length)-1. Also called attenu- tance as well is measured in terms of capac-
ation coefficient. itance (Hay and Owen bridges).
The AC bridges with ratio arms that are
absorption cross section energy ab- tightly coupled inductances allow measure-
sorbed by the scattering medium, normal- ment of a very small difference between cur-
ized to the wavenumber. It has dimensions rents in these inductances, and this fact is
of area. used in very sensitive capacitance transduc-
ers.
absorption edge the optical wavelength
or photon energy corresponding to the sep- ACcircuit electricalnetworkinwhichthe
aration of valence and conduction bands in voltagepolarityanddirectionsofcurrentflow
solids; at shorter wavelengths, or higher pho- change continuously, and often periodically.
ton energies than the absorption edge, the ab- Thus, such networks contain alternating cur-
sorption increases strongly. rents as opposed to direct currents, thereby
giving rise to the term.
absorption grating (1) a diffraction
grating where alternate grating periods are
AC coupling a method of connecting two
opaque.
circuits that allows displacement current to
(2) an optical grating characterized by
flow while preventing conductive currents.
spatially periodic variation in the absorption
Reactive impedance devices (e.g., capacitors
of light. Absorption gratings are generally
and inductive transformers) are used to pro-
less efficient than phase gratings.
vide continuity of alternating current flow
between two circuits while simultaneously
absorption optical fiber the amount of
blocking the flow of direct current.
optical power in an optical fiber captured
by defect and impurity centers in the energy
AC motor an electromechanical sys-
bandgap of the fiber material and lost in the
tem that converts alternating current electri-
form of longwave infrared radiation.
cal power into mechanical power.
AC See alternating current.
AC plasma display a display that em-
ploys an internal capacitive dielectric layer
AC bridge one of a wide group of
to limit the gas discharge current.
bridge circuits used for measurements of re-
sistances, inductances, and capacitances, and
AC steady-state power the average
toprovideACsignalinthebridgetransducers
power delivered by a sinusoidal source to a
includingresistors, inductors, andcapacitors.
network, expressed as
The Wheatstone bridge can be used with
a sinusoidal power supply, and with an AC
detector (headphones, oscilloscope), one can P =| V |·| I | cos(θ)
use essentially the same procedure for mea-
√ √
surement of resistors as in DC applications. where 2·| V | and 2·| I | are the peak
Only a small number of other AC bridges are values, respectively, of the AC steady-state
used in modern electric and electronic equip- voltage and current at the terminals. θ rep-
ment. A strong selection factor was the fact resents the phase angle by which the voltage
that in a standard capacitor the electrical pa- leads the current.

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2000 by CRC Press LLC