18WuD2x

thermal switch • thermoelectric junction 681
thermal switch A switch actuated by a temperature
change. Types vary from the simple thermostat
to complex servosystem switches with a
temperature-transducer input.
thermal time-delay relay A delay relay utilizing
the slow-heating and slow-cooling property of one
of its components.
thermion An ion or electron emitted by a hot body,
such as the heated cathode of a vacuum tube.
thermionic Pertaining to thermions and their applications.
thermionic cathode A heated cathode, in contrast
to a cold cathode, used as an emitter of electrons
or ions. Also see THERMION.
thermionic current Current caused by thermionic
emission—especially in an electron tube.
thermionic detector A vacuum-tube detector.
Also see THERMION and THERMIONIC EMIS-
SION.
thermionic diode A hot-cathode diode tube.
thermionic emission The emission of electrons by
a hot body, such as the filament or cathode of a
vacuum tube. Also see THERMION and HOT
CATHODE.
thermionics The study of electron emission from
objects or materials at high temperature.
thermionic tube An electron tube (i.e., one in
which electrons or ions are emitted by a heated
cathode). Also see THERMION, THERMIONIC
CURRENT, and THERMIONIC EMISSION.
thermionic work function The energy required to
force an electron from inside a heated cathode
into the surrounding space (in thermionic emission).
Also see WORK FUNCTION.
thermistor A temperature-sensitive resistor, usually
made from specially processed oxides of
cobalt, magnesium, manganese, nickel, uranium,
or mixtures of such substances. Thermistors are
available with either a positive or negative temperature
coefficient of resistance. The name is a
contraction of thermally sensitive resistor.
Semiconductor
layer
thermistor
Metal
electrodes
thermistor bridge A four-arm bridge (see BRIDGE,
1) in which one arm is a thermistor and, therefore,
is temperature sensitive.
thermistor power meter A radio-frequency
power-measuring instrument based on a thermistor
bridge.
thermistor probe A temperature probe containing
a thermistor as the sensing element.
thermistor thermometer An electronic thermometer
in which the temperature-sensitive element
is a thermistor.
thermoammeter See THERMOCOUPLE-TYPE
METER.
thermocouple A device consisting essentially of a
bond between two wires or strips of dissimilar
metals (such as antimony and bismuth). When
the bond is heated, a direct-current voltage appears
across it.
Electrodes
thermocouple
Glass envelope
thermocouple bridge A four-arm bridge (see
BRIDGE, 1) in which one arm is a thermocouple;
therefore, it is temperature sensitive, as well as
being voltage-productive.
thermocouple meter See THERMOCOUPLE-TYPE
METER.
thermocouple-type meter A radio-frequency (RF)
meter consisting of a thermocouple and directcurrent
(dc) ammeter, milliammeter, or microammeter,
connected in series. The thermocouple
is heated, directly or indirectly, by an applied RF
current, and the resulting dc output deflects the
meter pointer.
thermodynamics The science dealing with the relationships
between heat and mechanical energy
and their interconversion. Also see CARNOT
THEOREM, FIRST LAW OF THERMODYNAMICS,
SECOND LAW OF THERMODYNAMICS, and
THIRD LAW OF THERMODYNAMICS.
thermoelectric cooler A cooling device based on
the Peltier effect.
thermoelectric effect The production of thermoelectricity
by certain materials.
thermoelectricity Heat-produced electricity, as in
thermocouple operation.
thermoelectric junction A junction between two
conductors that exhibits variable characteristics
under conditions of changing temperature.