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440 melting point • mercury cell
melting point Abbreviation, mp. The temperature
at which a solid starts becoming liquid at a pressure
of one atmosphere. Compare FREEZING
POINT and MIXTURE MELTING POINT.
memory 1. The section of a digital computer that
records and holds data until it is necessary. In
personal computers, the term generally refers to
RANDOM-ACCESS MEMORY and READ-ONLY
MEMORY, contained in integrated circuits (ICs).
Compare STORAGE. 2. See MEMORY DRAIN.
memory address register In computer storage, a
register in which is stored the address of
operands in other locations.
memory area A portion of computer memory reserved
for a specific type of data. Also called area.
memory capacity As a function of the number of
memory locations available, the number of bytes
that can be stored. It is usually specified in kilobytes,
megabytes, or gigabytes. Also see GIGA-
BYTE, KILOBYTE, and MEGABYTE.
memory cycle 1. The period of execution of a sequence
of operations. 2. The complete operational
cycle for inputting data to memory or
retrieving it.
memory dialing In a telephone set, a feature that
allows rapid dialing of stored digits. The simplest
version is the “redial” feature, in which the most
recently dialed number is rapidly dialed at the
touch of a button. Some sets can store several different
numbers, usually including area codes,
and sometimes country codes as well.
memory drain Also called battery memory. A phenomenon
occasionally exhibited by nickel–
cadmium cells and batteries, in which the useful
ampere-hour capacity is reduced even though the
unit is not physically damaged. The depth of the
charge cycle decreases to a fraction of its rated
value. The problem can usually be overcome by
discharging the cell or battery fully, then recharging
fully, and repeating the process several times.
See also NICKEL–CADMIUM.
memory dump In computer operations, to either
print out what is stored in some of or all of the
memory locations or transfer the data from a
bank of memory cells to some external storage
medium.
memory effect See MEMORY DRAIN.
memory guard In a computer, hardware or software
that keeps certain memory locations from
being addressed by a program being run.
memory location In a computer memory, a place
where an information unit (word or character)
can be stored; the stored information can be retrieved
by appropriate addressing instructions.
memory organization packets In artificial intelligence
(AI) and expert systems, a method of arranging
computer memory into general rules or
statements. The statements are used by software
to derive models, forecasts, diagnoses, etc.
memory power Computer memory efficiency in
terms of data processing (cycle) speed.
memory protection A hardware device in a multiple
programming computer that prevents programs
from being altered by other operating
programs in the installation.
memory register In a digital computer, a register
used in all instruction and data transfers between
the memory and other sections of the machine.
memory unit See MEMORY.
mendelevium Symbol, Md. A radioactive element
produced artificially. Atomic number, 101.
Atomic weight, 258 (approx.).
menu In computer operations, a list of commands
for using various functions of the system.
MEP Abbreviation of mean effective pressure.
mercuric iodide Formula, HgI 2 . A compound
whose crystals are useful at room temperature as
detectors in high-resolution gamma-ray spectroscopy.
mercury Symbol, Hg. A metallic element. Atomic
number, 80. Atomic weight, 200.59. The only
metal that is liquid at room temperature. It is
used extensively in switches, certain high-voltage
rectifiers, high-vacuum pumps, and thermometers.
mercury arc The arc discharge occurring in mercury
vapor between solid or liquid (mercury) electrodes.
The discharge emits ultraviolet radiation.
mercury-arc rectifier A heavy-duty rectifier tube
utilizing ionized mercury vapor. The two general
types are MERCURY-VAPOR RECTIFIER and
MERCURY-POOL RECTIFIER.
mercury battery Also called mercuric-oxide battery.
A set of two or more mercury cells stacked
one atop the other, electrically connected in series.
The resulting battery has a cylindrical
shape. A set of four cells provides approximately
5.4 volts under no-load conditions; a battery of
seven cells provides 9.5 volts; a battery of nine
cells provides 12 volts. See MERCURY CELL.
mercury cadmium telluride Formula HgCdTe.
An alloy used as a semiconductor in certain transistors,
integrated circuits, and infrared detectors.
mercury cell Also called mercuric-oxide cell. An
electrochemical cell having a button-like shape,
small enough to fit inside a wristwatch. The
unit is housed in a steel container and has a
mercuric-oxide cathode, amalgamated-zinc anode,
and potassium hydroxide and zinc-oxide electrolyte.
The potential difference under no-load
conditions is 1.35 volts, with a high ratio of
stored energy per unit mass. The cell has a flat
discharge curve; the voltage remains essentially
constant until the charge is almost depleted, and
then the voltage drops rapidly. There has been a
decrease in the use of mercury cells and batteries
in recent years, because mercury is toxic. Unless
these cells are discarded in a special way, the
mercury from them can cause dangerous contamination
of soil and water.