18WuD2x

light load • light sensor 407
light load A load that is a fraction of the usual
value for a given application. That is, its resistance
or impedance is several times higher than
normal.
light meter An electronic instrument for measuring
the intensity of light. It generally consists of
a photodiode, a battery, and a direct-current microammeter
connected in series. A directcurrent
amplifier can be used to increase the
sensitivity.
light microsecond A unit of electrical distance;
the distance that light, or any electromagnetic
disturbance, travels in free space in 1 microsecond.
Approximately equal to 300 meters.
light modulation Variation of the instantaneous
brightness of a visible light beam in synchronization
with the instantaneous amplitude of a modulating
signal. Also see LIGHT MODULATOR.
light modulator A device with which a beam of
light can be modulated by an electrical signal.
light negative Pertaining to negative photoconductivity,
the decrease in conductivity of a photosensitive
material under illumination. Compare
LIGHT POSITIVE.
lightning The discharge that occurs between positive
and negative poles in the atmosphere. Common
in and near areas where heavy rainfall is
occurring. It also can occur in snow storms, in
sand storms, and over erupting volcanoes. Generally,
the negative pole is in a cloud and the positive
pole is at the surface of the earth, resulting
in a flow of electrons from cloud to ground. Some
lightning occurs as a flow of electrons from
ground to cloud, or between two clouds. Such
discharges sometimes attain current levels of
more than 1,000,000 amperes.
lightning arrester A device that bypasses highvoltage
pulses from most nearby lightning discharges
to the earth, helping to protect electronic
equipment connected to an outdoor antenna or
power line. Is not a total guarantee of protection,
however.
lightning conductor 1. A system for protecting
buildings from lightning strikes. A common system
includes a lightning rod, heavy conductor,
and ground rod. The ground rod is placed at least
six feet from the base of a building and is at least
eight feet long. 2. The conductor between the
lightning rod and ground rod in a system, as defined
in 1.
lightning detector See KERAUNOGRAPH and
KERAUNOPHONE.
lightning rod A protective device mounted on the
outside of structures, consisting of a pointed,
grounded metal rod that will conduct a lightning
discharge to earth.
lightning strike A direct hit of an object by a lightning
stroke. It usually causes extensive damage
to electrical and electronic equipment through
which the discharge passes.
Lightning
rod
Cone of
protection 45° 45°
lightning rod
lightning stroke The discharge that occurs with
lightning. The peak current is typically several
tens of thousands of amperes, but in some cases
can exceed 1,000,000 amperes. A stroke can consist
of one discharge or several individual discharges
in rapid succession.
lightning switch See GROUND SWITCH.
light-operated relay See PHOTOELECTRIC RE-
LAY.
light-operated switch A PHOTOELECTRIC RE-
LAY, or a switch operated by such a relay.
light pen A probe containing a tiny photosensor in
its tip. The tip of the light pen is touched to the
screen of a cathode-ray tube to sense the beam
when it passes the spot of contact. It is used as
an input device in some computers and terminals.
light pipe 1. An OPTICAL FIBER. 2. A cable consisting
of numerous optical fibers in a bundle.
See FIBEROPTICS, 1.
light positive Pertaining to positive photoconductivity,
when the conductivity of a photosensitive
material increases under illumination. Compare
LIGHT NEGATIVE.
light quantum See PHOTON.
light ray A thin beam of light. Theoretically, a ray
emerges from a point source (i.e., it has no width).
light receiver See LIGHT-BEAM RECEIVER.
light relay A photoelectric device that operates a
relay, according to fluctuations in the intensity of
a light beam.
light-sensitive cathode Also called photocathode.
A cathode that emits electrons when exposed to
light.
light-sensitive diode A semiconductor diode usable
as a photoconductive cell. Such diodes are
available as both junction and point-contact
types.
light-sensitive material A photoconductive or
photoemissive substance.
light-sensitive resistor See PHOTOCONDUCTIVE
CELL.
light sensor 1. A light-sensitive device, such as
a photocell, photodiode, phototransistor, or