18WuD2x

meteorology • metrology 443
meteorology The science of the atmosphere, especially
the study of weather and climate. (Not to be
confused with METROLOGY.)
meteor-scatter propagation The reflection of radio
signals from the ionized trails produced by meteors
as they pass through the upper atmosphere.
This can result in over-the-horizon radio communication
or reception. A meteor produces a trail
that persists for a few tenths of a second up to several
seconds, depending on the size of the meteor,
its speed, and the angle at which it enters the atmosphere.
This is not sufficient time for the transmission
of very much information, but during a
meteor shower, ionization can be almost continuous.
Meteor-scatter propagation has been observed
at frequencies considerably above 30 MHz.
meteor-trail reflections Momentary reflection of
signals by the ionized trails of meteors passing
through a signal path.
F layer
E layer
Ionized
trails
meteor-trail reflections
Meteor
paths
meter 1. An instrument for measuring and indicating
the value of a particular quantity. See, for
example, CURRENT METER and VOLTMETER.
2. Abbreviation, m. A unit of linear measure
and of electrical wavelength, equivalent to
1.65076373 × 10 6 wavelengths (in a vacuum) of the
radiation corresponding to the transition between
the two levels of the krypton-86 atom, and
approximately equal to 39.37 inches. 3. To supply
in specific increments or by a governed amount.
meter alignment See VISUAL ALIGNMENT.
meter-ampere A unit of transmitted radio signal
intensity. Determined by multiplying the antenna
current (in amperes) by the height (in meters) of
the antenna above ground.
meter-candle Abbreviation, mc. A metric unit of illuminance,
equivalent to the illumination on a
surface 1 meter from a light source of 1 candle
power. Compare FOOT-CANDLE and LUX.
meter equivalent The number of meters equal to a
given English measure of length (e.g., the meter
equivalent of 3 feet is approximately 0.9144).
meter-kilogram-second Abbreviation, mks. The
system of units in which the meter is the standard
unit of length, the kilogram is the standard
unit of mass, and the second is the standard unit
of time. Compare CENTIMETER-GRAM-SECOND
and INTERNATIONAL SYSTEM OF UNITS.
meter multiplier See MULTIPLIER RESISTOR.
meter protector A nonlinear resistor, such as a
varistor or semiconductor diode, used to prevent
overswing in an electric meter by limiting the current
flowing through it.
meter rating The maximum reading on a meter, at
or below which the accuracy is within a specified
limit, but above which the error might exceed
that limit.
meter rectifier A light-duty semiconductor diode
or bridge circuit, used to change alternating current
(ac) to direct current (dc) for deflection of a
D’Arsonval-type dc milliammeter or microammeter.
meter relay A sensitive relay that is essentially a
moving-coil meter, whose pointer closes against a
stationary contact mounted at some point along
the scale.
meter resistance Symbol, R m . The internal resistance
of an electric meter. In a simple D’Arsonval
meter, it is the resistance of the movable coil. In
more-complicated meter circuits, it is the resistance
of the parallel combination of the coil and
METER SHUNT.
meter scale factor See SCALE FACTOR, 1.
meter sensitivity See VOLTMETER SENSITIVITY.
meter shunt A resistor connected in parallel with
an ammeter, milliammeter, or microammeter to
increase the range of currents that the device can
measure.
meter torque See DEFLECTING TORQUE.
meter-type relay See METER RELAY.
methyl methacrylate resin Also known by the
trade name Lucite. A plastic insulating material.
Dielectric constant, 2.8 to 3.3. Dielectric
strength, 20 kV/mm.
metre Abbreviation, m. Alternate spelling of meter
when used to specify displacement or wavelength.
See METER, 2.
metric system The decimal system of weights and
measures based on the meter, kilogram, and second.
Also see CENTIMETER-GRAM-SECOND,
METER-KILOGRAM-SECOND, and INTERNA-
TIONAL SYSTEM OF UNITS.
metric ton Abbreviation, MT. A metric unit of
weight equal to 1000 kilograms or 1.1023 English
tons.
metric waves British designation for electromagnetic
energy having wavelengths ranging from 10
meters down to 1 meter, corresponding to frequencies
from 30 MHz up to 300 MHz.
metrology The science of weights and measures,
including electrical standards and electronic instruments
and measurements. (Not to be confused
with METEOROLOGY.)