The
thermometer is a device that measures
temperature or
temperature gradient using a variety of different principles; it comes from the
Greek roots
thermo,
heat , and
meter, to measure. A thermometer has two important elements: the temperature sensor (e.g. the bulb on a
mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer). Industrial thermometers commonly use electronic means to provide a digital display or input to a computer.
Thermometers can be divided into two groups according to the level of knowledge about the physical basis of the underlying
thermodynamic laws and quantities. For
primary thermometers the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the
velocity of sound in a gas, on the thermal noise (see
Johnson–Nyquist noise )
voltage or
current of an electrical resistor, and on the angular
anisotropy of
gamma ray emission of certain
radioactive nuclei in a
magnetic field .
Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example,
triple points and
superconducting transitions, occur reproducibly at the same temperature.
Internationally agreed temperature scales are based on fixed points and interpolating thermometers. The most recent official temperature scale is the
International Temperature Scale of 1990 . It extends from 0.65
K (−272.5 °C/−458.5 °F) to approximately 1,358 K (1,085 °C/1,985 °F).
