heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. Note: This heat transmission rate is also called the overall coefficient of heat transfer.

quantity or rate of heat energy or equivalent in work or electrical energy. See Btu; therm; ton-hour; watt; joule.

thermoelectrical element in which the electrical resistance falls appreciably with a rise in temperature; often used as a thermometer. See also anemometer (thermal).

layer of fluid in which the temperature and density gradient is greater than, and which separates, the cooler fluid below it and the warmer fluid above it.

junction of two wires of dissimilar materials, not necessarily metal, with the property of generating an emf related to the temperature of their junction. Compare thermopile.

combination of one or more thermocouples in a circuit with switching and measuring devices to enable temperatures to be indicated or recorded continuously or intermittently.

equilibrium in a system when the physical variables have uniform values that do not change in time. Furthermore, if the system is not an isolated one, these variables should have the same values for both the system and its surroundings.

basic qualities used in defining the condition of a substance, such as temperature, pressure, volume, enthalpy, entropy, etc.

implosive impact in liquid, caused by sudden condensation of vapor into its subcooled liquid. This phenomenon can occur when the liquid is about 60 to 85°F (33 to 47°C) cooler than the saturation temperature of the contact vapor. It creates a loud sound and can cause severe local pressure stresses in the container.

science of the relation of heat to other forms of energy. See also thermodynamics laws.