18WuD2x

Z meter • zwitterion 771
(3) an impedance bridge for evaluating the reactive
and resistive components of an unknown
impedance; (4) a section of transmission line
used with a signal source and voltmeter for measuring
impedance in terms of a standard resistor
and/or standing waves.
ZMODEM
In data communications, an error-correction
mode similar to XMODEM, except that
when an error is found during transmission of a
block of data, the source retransmits only that
portion of the block following the error. This improves
data transmission speed because, when
an error occurs, the number of bytes retransmitted
is generally fewer than the 128K block size
standard in XMODEM. Compare XMODEM and
YMODEM.
Zn 1. Symbol for ZINC. 2. Symbol for AZIMUTH.
Z 0 Symbol for CHARACTERISTIC IMPEDANCE.
zone 1. On a magnetic disk, a group of tracks
whose associated transfer rate is not the same as
that for the rest of the disk. 2. In computer operations,
the area of a storage medium containing
digits. 3. In a computer system, a main memory
area set aside for a particular purpose. 4. In a security
system, a specified area or region under
surveillance.
zone blanking In a radar system, a method of extinguishing
the cathode-ray-tube beam during a
portion of the antenna sweep.
zone candle power In a given zone, the luminous
flux per steradian, emitted by a light source under
test.
zoned circuit In a security system, a circuit in
which some areas are protected at all times, while
the protection in other areas can be temporarily
disabled for entry or exit.
zone leveling See ZONE REFINING.
zone marker See Z MARKER.
zone melting See ZONE REFINING.
zone of silence The region between alternate reflections
of a radio wave, in which no signal is detectable.
Also called skip zone.
zone plate antenna A rapid-scan radar antenna
having a reflector composed of confocal parabolas
arranged in a circle.
zone-position indicator A radar that reveals the
position of an object to a second radar having a
restricted field.
zone purification See ZONE REFINING.
zone refining A method of purifying semiconductor
materials (such as germanium and silicon) in
which a molten zone in an ingot of the material
moves along the length of the ingot, dissolving
impurities as it travels, eventually depositing
them at the end of the ingot, which is sawed off.
This concentrated and segregated melting is accomplished
by means of radio-frequency heating.
zone time In a given time zone, standard time in
terms of the number of hours that must be added
to local time to equal the time at the zero (Greenwich)
meridian.
zoning 1. A method of fabricating a microwave
reflector in concentric, flat regions, producing
the same practical results as a continuous
paraboloid. 2. In a communications system, the
division of the coverage area into different geographic
regions for a specific purpose.
zoom 1. To rapidly change the focal length of a
television or motion-picture camera lens so that
the object seems to advance toward or recede
from the viewer, remaining in focus as it does so.
2. See ZOOM LENS. 3. To magnify the image in a
computer graphical user interface. A user can enlarge
a specific portion of the display, at the expense
of other portions. It is generally measured
in percent (e.g., 200% zoom represents a magnification
factor of 2).
zoom lens A continuously adjustable lens system
that allows zoom effects to be obtained with a
television or motion-picture camera, or a similar
arrangement for still cameras that obviates the
need for lens interchange when different focal
lengths are needed. The lens system, which can
be operated electronically, allows either narrowor
wide-angle views to be obtained without losing
focus at any time.
Z parameters Device or network parameters expressed
as impedances.
ZPI Abbreviation of ZONE-POSITION INDICATOR.
Z-plunger In a waveguide, a combination choke
and bucket plunger for radiation leakage reduction.
Zr Symbol for ZIRCONIUM.
Z signals A collection of letter groups, each starting
with the letter Z, used for simplified telegraphy
and radiotelegraphy by the military services.
Zulu Phonetic alphabet communications code
word for the letter Z.
Zulu time Greenwich mean time. Also see ZEBRA
TIME.
zwitterion An ion that carries both a positive and
a negative charge.