Definitions & qualifiers


The following are definitions of some commonly used terms in thermometry. Further terms can be found in Terminology (see left).


Heat treatment of materials, usually metals and alloys, to remove internal stresses created by prior deformation

Compensating cable

A cable providing a method of extending a thermocouple circuit with wires having similar emf-temperature characteristics as over a limited range. The cable usually comprises wires of dissimilar composition to the thermocouple

Electromotive force

The rate at which energy is drawn from a source that produces a flow of electricity in a circuit

Extension cable

A cable providing a method of extending thermocouple circuits with wires having similar temperature v. emf characteristics to the thermocouple to which it is connected. The cable usually comprises wires of similar composition to the thermocouple.

Fixed points

Fixed points are phase transitions of pure substances whose temperature is fixed and highly reproducible

Heat pipe

A heat pipe is a cylindrical device that is very efficient at transferring heat. Using a metal container (aluminium, copper, etc.) that holds a liquid (water, acetone, etc.) under pressure, the inner surface of the tube is lined with a porous material that acts as a wick. When heat is applied to the outside of the tube, the liquid inside the tube boils and vaporises into a gas that moves through the tube seeking a cooler location where it condenses. Using capillary action, the wick transports the condensed liquid back to the evaporation area

Heat sink

A material that is used to absorb heat. Typically made of aluminium, heat sinks are widely used with amplifiers and other electronic devices that generate heat. Small heat sinks are the most economical method for cooling microprocessors. They are commonly found glued or clipped to the top or side of the element.

Hot junction

The junction of a thermocouple that is exposed to the temperature to be measured. Also known as the measuring junction.

Ice point

The fixed point between ice and air-saturated water under a pressure of one standard atmosphere.

Insulated junction

Thermocouple hot junction, which is electrically insulated from the earth or ground potential: also known as an ungrounded junction.

Insulation resistance

The electrical resistance between the thermocouple and the earth/ground. In a mineral-insulated thermocouple the insulation resistance would be that between each conductor (prior to the formation of the hot junction) and between each thermoelement and the surrounding sheath


The unit of absolute temperature equal to 1/273.16 of the absolute temperature of the triple point of water. One kelvin (1 K) is equal to one degree Celsius (1 C).

Liquid-in-glass thermometer

Liquid-in-glass thermometers make use of the expansion of a liquid (usually mercury or a spirit) inside a glass tube. The temperature is indicated by reading across form the meniscus of the liquid column to the scale engraved on the capillary stem.

Primary standard

A thermometer (eg thermocouple) that has its temperature v. resistance or emf v. temperature relationship established in accordance with the International Temperature Scale.

Protection tube

A tube designed to enclose a temperature sensor, thus protecting it from its environment

Resistance bridge

An instrument or a circuit consisting of four resistors or their equivalent in series, used to determine the value of an unknown resistance when the other three resistances are known. The ability to measure resistance accurately is key to precision thermometry, where the practical temperature scale is implemented using the resistance of a PRT (platinum resistance thermometer). Resistance is measured using a bridge circuit in which the unknown resistance is compared with a known reference resistor. The bridge can be excited using AC (sine wave excitation) or DC, which have their advantages and disadvantages

Resistance thermometer sensor

A temperature responsive device consisting of a sensing resistor within a protective sheath, internal connecting wires and external terminal to permit connection of electrical measuring elements. Specific examples being Platinum Resistance Thermometers (PRT) or Resistance Temperature Devices (RTD)

Reference junction

That junction of a thermocouple that is at a known temperature. Also often known as the cold junction.

Seebeck coefficient

The rate of change in thermal emf with respect to temperature at a given temperature expressed as emf per unit change in temperature

Secondary standard

A thermometer that has had its temperature-output relationship established by reference to a primary standard of temperature

Thermostat system

A thermostat system is a system with a uniform and stable temperature. By bringing two thermometers into thermal equilibrium with using a thermostat, they will both have the same temperature. Commonly used thermostat systems are baths, furnaces and cryostats

Thermoelectric emf

The electromotive force (emf) established in a conductor by virtue of a temperature gradient within it. The thermoelectric emf produced in a conductor per unit of temperature difference is the thermoelectric power or Seebeck Coefficient (see above for definition of Seebeck coefficient).


A thermoelectric device for measuring temperature, consisting of dissimilar conductors (thermoelements) connected together at a measuring junction maintained at the temperature to be measured


The two conductors used in a thermocouple, one of which is designated 'positive' and the other 'negative', according to the polarity of the net emf developed.


A semiconductor that exhibits a large non-linear change in resistance with change in temperature.