18WuD2x

198 directivity index • directrix
where E 1 is the average intensity over an entire
sphere surrounding the transducer, and E 2 is the
intensity on the acoustic axis. 2. For an acoustic
pickup transducer, the ratio, in decibels, of E 1 to
E 2 , where E 1 is the average response over an entire
sphere surrounding the transducer, and E 2 is
the response on the acoustic axis.
directivity of antenna For a beam antenna, the
ratio E max /E avg , where E max is the maximum field
intensity at a selected distance from the antenna
and E avg is the average field intensity at the same
distance.
directivity of directional coupler The ratio, in
decibels, of P 1 to P 2 , where P 1 is the power at the
forward wave-sampling terminals (measured with
a forward wave in the transmission line) and P 2 is
the power at the terminals when the wave is reversed
in direction.
directivity pattern The calculated or measured radiation
or response pattern (transmission or reception)
of an antenna, microphone, loudspeaker,
or similar device, with particular attention to the
directional features of the pattern.
directivity signal A spurious output signal resulting
from finite directivity in a coupler.
direct light Light rays traveling directly from a
source to a receptor or target without reflection.
directly grounded Connected to earth or to the
lowest-potential point in a circuit, without any intervening
resistance or reactance.
directly heated cathode A vacuum-tube filament.
It is so called because, when heated, it becomes
the cathode of the tube (i.e., the emitter of electrons).
directly heated thermistor A thermistor whose
temperature changes with the surrounding temperature,
and also as a result of power dissipation
in the device itself. Compare INDIRECTLY
HEATED THERMISTOR.
directly heated thermocouple A meter thermocouple
heated directly by signal currents passing
through it. Compare INDIRECTLY HEATED
THERMOCOUPLE.
direct measurement Immediate measurement of
a quantity, rather than determining the value of
the quantity through adjustments of a measuring
device (e.g., measuring capacitance with a capacitance
meter, rather than with a bridge). Compare
INDIRECT MEASUREMENT.
direct memory access Abbreviation, DMA. The
transfer of data from a computer memory to some
other location, without the intervention of the
central processing unit (CPU).
direct numerical control In a computer or data
system, the capability for distributing information
among numerically controlled machines
whenever desired.
director
In a multielement directional antenna, an
element that is usually mounted in front of the
radiator element, and that is phased and spaced
to direct the radiation forward. The director functions
in conjunction with the reflector element,
which is usually mounted behind the radiator.
directory See DICTIONARY.
direct pickup The broadcasting, especially in television,
of events at the same time as they occur
(e.g., without recording/reproduction).
direct piezoelectricity The production of a piezoelectric
voltage by mechanically stressing a suitable
crystal.
direct playback In audio or video recording, the
reproduction of a recording without additional
processing (e.g., the playing of an original
recorded tape, rather than a tape that has been
mass produced).
direct-point repeater A relay-operated telegraph
repeater. The received signals actuate the relay,
which switches the second line.
direct-radiator loudspeaker A loudspeaker whose
cone or diaphragm is directly coupled to the air.
direct ray An electromagnetic ray (wave) that
reaches a receiver without reflection or refraction,
and without encountering obstructions.
direct recording 1. A record produced by a
graphic recorder. 2. The technique of producing
such a record.
direct-recording instrument A device, such as a
graphic recorder, that directly produces a permanent
record (such as an inked trace) of the variations
of a quantity.
direct resistance coupling A form of coupling in
which the output of the first amplifying device is
connected through a resistor directly to the input
of the second device. The resistance value can
vary; sometimes the connection is a short circuit.
directrix A fixed line to which a curve is referred
(e.g., the axis of a parabola).
+
direct resistance coupling