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Measurement of the hotness of a body relative to a standard scale. The fundamental scale of temperature is the thermodynamic scale, which can be derived from any equation expressing the second law of thermodynamics. Efforts to approximate the thermodynamic scale as closely as possible depend on relating measurements of temperature-dependent physical properties of systems to thermodynamic relations expressed by statistical thermodynamic equations, thus in general linking temperature to the average kinetic energy of the measured system. Temperature-measuring devices, thermometers, are systems with properties that change with temperature in a simple, predictable, reproducible manner. See Temperature, Thermodynamic principles
In the establishment of a useful standard scale, assigned temperature values of thermodynamic equilibrium fixed points are agreed upon by an international body (General Conference of Weights and Measures), which updates the scale about once every 20 years. Thermometers for interpolating between fixed points and methods for realizing the fixed points are prescribed, providing a scheme for calibrating thermometers used in science and industry.
The scale now in use is the International Temperature Scale of 1990 (ITS-90). Its unit is the kelvin, K, arbitrarily defined as 1/273.16 of the thermodynamic temperature T of the triple point of water (where liquid, solid, and vapor coexist). For temperatures above 273.15 K, it is common to use International Celsius Temperatures, t90 (rather than International Kelvin Temperatures, T90), having the unit degree Celsius, with symbol °C. The degree Celsius has the same magnitude as the kelvin. Temperatures, t90, are defined as t90/°C = T90/K - 273.15, that is, as differences from the ice-point temperature at 273.15 K. The ice point is the state in which the liquid and solid phases of water coexist at a pressure of 1 atm (101,325 pascals). [The Fahrenheit scale, with symbol °F, still in common use in the United States, is given by tF/°F = (t90/°C × 1.8) + 32, or tF/°F = (T90/K × 1.8) - 459.67.] The ITS-90 is defined by 17 fixed points.
Primary thermometers are devices which relate the thermodynamic temperature to statistical mechanical formulation. The fixed points of ITS-90 are all based on one or more types of gas thermometry or on spectral radiation pyrometry referenced to gas thermometry. Secondary thermometers are used as reference standards in the laboratory because primary thermometers are often too cumbersome. It is necessary to establish standard secondary thermometers referenced to one or more fixed points for interpolation between fixed points. Lower-order thermometers are used for most practical purposes and, when high accuracy is required, can usually be calibrated against reference standards maintained at laboratories, such as the U.S. National Institute of Standards and Technology, or against portable reference devices (sealed boiling or melting point cells). See Gas thermometry, Low-temperature thermometry, Pyrometer, Thermistor, Thermocouple, Thermometer