18WuD2x

242 electromagnetic microphone • electromagnetizer
examples are the dynamic microphone and velocity
microphone.
electromagnetic mirror A reflector of electromagnetic
waves (e.g., antenna elements, ionospheric
layers, buildings, and hills).
electromagnetic momentum The momentum of a
moving electric charge, comparable to that of
matter in motion. Electromagnetic momentum is
the product of electromagnetic mass and charge
velocity.
electromagnetic oscillograph 1. An electromechanical
data recorder for tracing the waveform
or variations of a signal. 2. See ELECTROME-
CHANICAL OSCILLOSCOPE.
electromagnetic oscilloscope 1. An oscilloscope
using electromagnetic deflection. 2. See ELEC-
TROMECHANICAL OSCILLOSCOPE.
electromagnetic pump A device used for moving
conducting or semiconducting fluids. When a
current is passed through the fluid, a force is exerted
on the molecules of the fluid because of the
magnetic field set up by the current.
electromagnetic pulse Abbreviation, EMP. 1. A
broadband electromagnetic field emitted in a
short, intense burst from a lightning stroke or
nuclear explosion. This field can disrupt the operation
of, and (in some cases) cause damage to,
electrical and electronic apparatus. 2. In electromagnetic
induction, the displacement of an electron
in a conductor by the magnetic field.
electromagnetic radiation The propagation of
electromagnetic fields through space; it normally
occurs at approximately 299,792 kilometers per
second or 186,282 miles per second.
electromagnetic reaction The reaction between
magnetic fields. Also see ELECTROMAGNETIC
ATTRACTION and ELECTROMAGNETIC REPUL-
SION.
electromagnetic reconnaissance In military applications,
the use of electromagnetic apparatus
to detect potential enemy activity in a certain geographic
region.
electromagnetic relay See ELECTROMECHANI-
CAL RELAY.
electromagnetic repulsion The repulsion of a pole
of an electromagnet by the pole of another electromagnet
(north pole opposing north pole, south
opposing south). Compare ELECTROMAGNETIC
ATTRACTION.
electromagnetics A branch of physics concerned
with the theory and application of electromagnetism.
electromagnetic screen See ELECTROMAG-
NETIC SHIELD.
electromagnetic shield 1. A partition, can, or box
made of magnetic material (iron, steel, or special
alloy) enclosing a magnetic component. The
magnetic flux generated by the component is confined
by the shield, thus preventing interference
with external components. Likewise, external
magnetic fields are prevented from reaching the
component. 2. A grounded partition, metal sheet,
wire braid, or other barrier that prevents electromagnetic
fields from passing through. Commonly
used in electronic equipment to prevent ELEC-
TROMAGNETIC INTERFERENCE. Also used in
COAXIAL CABLE to confine electromagnetic
fields to the transmission line.
electromagnetic shielding The use of an ELEC-
TROMAGNETIC SHIELD to prevent undesired interaction
among electrical and electronic devices
and systems.
electromagnetic spectrum See ELECTROMAG-
NETIC FREQUENCY SPECTRUM and ELECTRO-
MAGNETIC WAVELENGTH SPECTRUM.
electromagnetic switch 1. A switch actuated by
magnetism produced by control current flowing
through a coil wound on an iron core. 2. See
ELECTROMECHANICAL RELAY.
electromagnetic theory of light The theory that
light consists of electromagnetic waves that are
similar to radio waves, but of shorter wavelength.
electromagnetic tube A cathode-ray tube using
electromagnetic deflection (e.g., a television picture
tube).
electromagnetic unit Abbreviation, emu. A unit
of measure in the electromagnetic system of
CENTIMETER-GRAM-SECOND (cgs) units.
electromagnetic vibrator See INTERRUPTER.
electromagnetic wavelength spectrum The
wavelength range of electromagnetic fields, including
radio waves, infrared, visible light, ultraviolet,
X rays, and gamma rays. It ranges from
many kilometers to a tiny fraction of one millimeter.
electromagnetic-wave polarization The orientation
of the electric flux lines in an electromagnetic
(EM) field, especially a field propagating through
space. The polarization is generally parallel with
the active element of a radio transmitting or receiving
antenna. Thus, a vertical antenna radiates
and receives fields with vertical polarization,
and a horizontal antenna radiates and receives
fields having horizontal polarization. Some antennas
radiate and receive an EM field whose polarization
continually and rapidly rotates. This is
elliptical polarization. If the rate of rotation is constant,
it is circular polarization.
electromagnetic waves Waves produced in a conductor
or in space by the acceleration or oscillation
of electric charge carriers. Such waves have
an electric and a magnetic component acting at
right angles to each other. The waves propagate
at right angles to both the electric and magnetic
flux lines.
electromagnetism 1. Magnetism resulting from
the movement of electric charge carriers (e.g., the
magnetic field surrounding a coil of wire carrying
an electric current). 2. See ELECTROMAGNET-
ICS.
electromagnetizer A magnetizer using continuous
direct current (dc) as the magnetic-field source.