18WuD2x

334 headset • heat therapy
headset An assembly consisting of one or two earphones,
a headband, and a flexible cord. Also see
HEADPHONE and RECEIVER, 2.
head stack In magnetic recording, an assembly of
two or more heads for multitrack service. Also see
HEAD, 2.
head station See BASE STATION.
head-to-tape contact In magnetic-tape recording
or playback, physical contact between the tape
and the head.
hearing aid A miniature audio-frequency device
that amplifies sound for people with impaired
hearing. It consists of a microphone, a high-gain
amplifier, and an earphone or bone-conduction
transducer.
hearing-aid battery A physically small battery designed
for use with hearing aids. Such a battery
is usually of the lithium type, or some other type
that has long life under conditions of low current
drain.
hearing loss A measure of hearing impairment.
Generally expressed as the ratio, in decibels, of
an individual’s threshold of hearing to the normal
threshold of hearing. Also see AUDIOLOGIST,
AUDIOMETER, and AUDIOMETRIST.
heart fibrillation A condition in which the heart
muscle twitches at random, rather than pumping
blood normally. This can be caused by an electric
shock through the heart of 100 mA to 300 mA. If
normal heart function is not restored, death will
follow.
heart pattern See CARDIOID PATTERN.
heart telemetry See ECG TELEMETRY.
heat A form of energy transferred by conduction,
convection, or radiation between two bodies having
different temperatures. The amount of heat is
expressed in degrees, British thermal units, calories,
joules, or kelvins.
heat aging 1. The degeneration of a substance, aggravated
by high temperatures. 2. A test that
indicates the immunity of a substance to
degeneration because of high temperatures.
heat coil A device that disconnects a circuit when
the temperature reaches a certain minimum
level.
heat detector A sensor of heat. See, for example,
BOLOMETER, INFRARED DETECTOR, MICRO-
RADIOMETER, RADIOMETER, THERMISTOR,
THERMOCOUPLE, and THERMOPILE.
heated-pen recorder See THERMAL RECORDER.
heated-stylus recorder See THERMAL RE-
CORDER.
heated-wire flowmeter See HOT-WIRE FLOWME-
TER.
heated-wire sensor A hot wire used to discriminate
between substances, according to how they
affect its heating. See, for example, GAS DETEC-
TOR (also usable as a vacuum gauge), HOT-WIRE
ANEMOMETER, and HOT-WIRE MICROPHONE.
heat engine A machine that converts heat energy
into mechanical energy.
heater 1. The filament of an indirectly heated vacuum
tube. 2. The filament in an indirectly heated
thermistor.
heater-voltage coefficient The amount of frequency
change per volt of fluctuation in the filament
voltage of a Klystron.
heat exchanger A device or system that removes
heat from a hot body and transfers it to another
body or to the surrounding air.
heat-eye tube An infrared-sensitive device used
for the purpose of locating objects in visible darkness.
The tube consists of a cathode-ray device
that is sensitive to infrared radiation.
heat gradient The temperature difference between
two points on a body, divided by the distance between
the two points.
heating depth See DEPTH OF HEATING.
heating effect The production of heat (power loss)
by electric current flowing in a conductor.
heating element 1. See HEATER. 2. The resistance
element (such as a strip or coil) that generates
heat in an electric-heating device.
heat loss 1. Heat emitted by conduction, convection,
or radiation from a body at a relatively high
temperature. 2. Power loss as a result of the heating
effect of an electric current.
heat of fusion The amount of heat required to melt
a unit mass of a solid that has reached its melting
point.
heat of radioactivity Heat generated during the
process of radioactive disintegration.
heat of reaction In a chemical or electrochemical
reaction, the heat (in calories) absorbed or released.
heat of vaporization The amount of heat required
to convert 1 gram of a liquid to a vapor without
raising its temperature.
heat radiator See HEATSINK.
heat rays See INFRARED RAYS.
heat remover 1. See HEATSINK. 2. A forced-air or
forced-liquid cooling system.
heat-resistant glass See PYREX.
heat-sensitive resistor See THERMISTOR.
heat-sensitive switch A make-and-break device,
such as a thermostat, that is actuated by a
change in temperature.
heat-shrink tubing An insulated flexible sleeving
made from a plastic that shrinks permanently for
a tight fit when heated; it is commonly used at the
joint between a cable and connector.
heatsink A heat exchanger in the form of a heavy,
metallic mounting base or a set of radiating fins.
It conducts heat away from such devices as
power transistors, heavy-duty resistors, or power
tubes, and dissipates the heat into the surrounding
environment via convection and radiation.
heatsink resistance The opposition offered by a
heatsink to the flow of heat.
heat therapy 1. The use of radio-frequency heating
for therapeutic purposes. Also see DIATHERMY. 2.
The use of infrared rays for therapeutic purposes.