18WuD2x

Maxwell’s equations • measurand 437
predicted the existence of electromagnetic waves,
whose later discovery made radio possible.
Maxwell’s law Also called Maxwell’s rule. Every
part of an electric circuit is acted upon by a force
tending to move it in the direction that results in
the maximum magnetic flux being enclosed.
maxwell-turn A unit of magnetic coupling (linkage)
equal to 1 maxwell per turn of wire in a coil linked
by magnetic flux. Also see MAXWELL.
mayday In radiotelephony, a word spoken as an
international distress signal equivalent to SOS in
radiotelegraphy. The word is the phonetic equivalent
of the French m’aidez (help me).
MB Abbreviation of MIDBAND.
Mb Abbreviation of MEGABAR.
MBB Abbreviation of MAKE BEFORE BREAK.
MBM Abbreviation of magnetic bubble memory.
MBO Abbreviation of MONOSTABLE BLOCKING
OSCILLATOR.
MBS Abbreviation of magnetron beam switching.
mc 1. Symbol for MILLICURIE (mCi is preferred).
2. Symbol for METER-CANDLE.
Mc 1. Symbol for MEGACURIE (MCi is preferred).
2. Obsolete abbreviation of megacycle(s), a term
superseded by MEGAHERTZ.
MCG Abbreviation of MAGNETOCARDIOGRAM.
McLeod gauge An instrument for measuring gas
under low pressure. A measured volume of the
gas under test is first compressed (to a lower
known volume) to a pressure more easily measured
via a mercury manometer and the application
of Boyle’s law.
MCM Abbreviation of MONTE CARLO METHOD.
McProud test A simple test for checking the tracking
efficiency of a phonograph pickup and arm for
microgroove discs.
Mc/s Obsolete abbreviation of megacycle(s) per
second, a term superseded by MEGAHERTZ.
MCS 1. Abbreviation of Master of Computer Science.
2. Abbreviation of missile control system.
MCW Abbreviation of MODULATED CONTINUOUS
WAVE.
Md Symbol for MENDELEVIUM.
MDAS Abbreviation of medical data acquisition system.
m-derived filter A filter whose inductance (L) and
capacitance (C) values are derived by multiplying
those of a constant-k filter by a factor m between
zero and 1. This factor is a function of the ratio
f Ï /f c , where f Ï is the frequency of infinite attenuation,
and f c is the cutoff frequency. This type of filter
exhibits sharper response than the equivalent
constant-k filter.
MDI Abbreviation of MAGNETIC DIRECTION INDI-
CATOR.
M-display See M-SCAN.
MDS Abbreviation of MINIMUM DISCERNIBLE
SIGNAL.
m e Symbol for MASS OF ELECTRON AT REST.
Meacham oscillator A highly stable radiofrequency
oscillator consisting of an amplifier
Crystal
RF
amplifier
Meacham oscillator
RF output
provided with a feedback circuit containing a
four-arm bridge, one arm of which is a quartz
crystal, and another, a tungsten-filament lamp
acting as a nonlinear resistor. Also called bridgestabilized
oscillator.
mean 1. A general term meaning average. 2. See
ARITHMETIC MEAN. 3. See GEOMETRIC MEAN.
mean charge 1. In an object that is nonuniformly
charged, the average charge per unit distance,
area, or volume. 2. In a capacitor carrying a fluctuating
current, the average amount of charge
held by the plates.
mean free path 1. In acoustics, the average distance
that sound waves travel before striking a
barrier or reflecting surface. 2. The average distance
that sound waves travel between reflections
(echoes) in a chamber. 3. In a gas tube, the average
of all the free paths of electrons at a specified
temperature.
mean life 1. Symbol, L. The average life of a radioactive
substance [i.e., the time taken for 1/e
(e = base of natural logarithms) of the substance
to disintegrate]. 2. The time required for excess carriers
injected into a semiconductor to recombine
with carriers of opposite sign. Also called average
life.
mean proportional See GEOMETRIC MEAN.
mean time before failure Abbreviation, MTBF.
The average length of time that a component or
system will perform before the first failure occurs.
It is generally specified in hours.
mean time between failures Abbreviation, MTBF.
The average length of time that a component or
system will perform before failure occurs—either
initially or after repair or replacement. It is generally
specified in hours.
measured A quantity that is presented to an instrument
for measurement.
measured service Any service in which charges
are assessed per unit-time usage block. Online
computer services are a common example. In
some cases, other factors, such as distance, affect
the cost per unit time; most long-distance
telephone services fall into this category.
measurand Any quantity that is measured with an
instrument.