Temperature and temperature scales

Temperature is an indirect measure of the kinetic energy of particles composing matter. The SI unit for temperature is the kelvin (K). There is no degree sign associated with the kelvin.

Kinetic-molecular theory asserts that temperature is a property of matter that results from molecule motion (kinetic energy) and/or atomic vibration. A common misconception is that at absolute zero (0 Kelvin), atomic and molecular motion ceases. In reality, although absolute zero represents the absence of kinetic energy, it represents only the absolute minimal state of molecular or atomic vibration (i.e., electrons still "orbit" the nucleus and nuclear processes including transformations are possible). Temperature is the size-independent quantity that indirectly relates the average kinetic energy of all the particles within a body. Its size-independence stems from the fact that two objects made of the same matter at the same temperature (i.e., objects in thermal equilibrium) will have the same average kinetic energy of constituent particles.

Temperature is commonly measured with a thermometer—a device designed to relate the expansion of liquids (e.g., the rising of mercury in a tube) to changes in temperature. One of the first attempts at articulating a universal temperature scale was made by the Greek scientist and physician Galen (ca. a.d.170). Galen based his scale on comparative temperatures with an equal mixture of ice and water assuming the center of a four point scale. By the mid-seventeenth century, Italian scientists and builders fashioned crude alcohol-in-glass thermometers and the English scientist Robert Hooke utilized an alcohol-in-glass thermometer with zero assigned to the freezing point of water in meteorological experiments.

During the early eighteenth century, Danish astronomer Ole Roemer advanced a temperature scale based on two points, an assigned temperature of crushed ice and the boiling point of water.

German-born physicist (born in what is now Danzig or Gdansk, Poland) Daniel Gabriel Fahrenheit (1686–1736) began creating thermometers containing mercury. Fahrenheit utilized mercury's ability to be easily visualized in glass tubing and its ability to remain a liquid over a wide range of normal atmospheric temperatures. Fahrenheit eventually designated the boiling point of water to be 212 degrees. Later he measured the freezing point of water to be 32 degrees, 180 degrees below the boiling point of water. The deviations on the scale were later named after its creator, and the scale is read in degrees Fahrenheit. The Fahrenheit scale still exists today, but is primarily used in the United States for reporting the weather . The Celsius and Kelvin temperature scales are more commonly used in scientific investigation.

In 1745, Swedish naturalist Carl von Linné (also known as Carl Linnaeus) (1707–1778) devised a centigrade (Latin for "one hundred steps") scale to measure temperature. He began his scale with the freezing point of water at zero degrees and set the boiling point of water at 100 degrees. Andrew Celsius used the same number of deviations in his scale, but he instead reversed the order to where the boiling point was zero and the freezing point was 100.

Subsequently, the International Committee of Paris adopted measurements of the freezing point and of the boiling point of water as fundamental markers for temperature scales. The Celsius scale was reversed, and in 1948 was revised to set the triple point of water (that temperature where solid, liquid and gas phases exist in equilibrium) at 0.01°C, and the boiling point of water at 99.975°C. The Celsius scale is used primarily in scientific investigation worldwide and in weather reporting for daily atmospheric temperatures everywhere but the United States.

In order to convert temperature from Celsius to Fahrenheit, the following formula is used:

°F = 1.8 (°C) + 32.

In order to convert temperature from Fahrenheit to Celsius, the following formula is used:

°C = 5/9 (°F − 32).

The necessity for an absolute temperature scale emerged from the advancement of kinetic-molecular theory. The thermodynamic temperature scale—incorporating the concept of absolute zero—evolved and is now accepted as the fundamental measure of temperature. In 1933, the International Committee of Weights and Measures adopted the triple point, or freezing point, of water as 273.16 Kelvin, named after Scottish physicist William Thomson (Lord Kelvin) (1824–1907).

In order to convert temperature to Kelvin from Celsius, the following formula is used:

K = C + 273.15

The absolute, or Kelvin, scale is used primarily in conjunction with the Celsius scale because the deviations are equal in magnitude.

In addition to thermometers, other devices can be used measure temperature. Changes in gas volume (e.g., as used in a constant-pressure gas thermometer), electrical resistance to current passage, and thermocouple voltage generation can also be calibrated to changes in temperature.

Another absolute temperature scale (i.e., a scale that incorporates absolute zero) still cited in literature is the Rankine scale. The Rankine is an absolute temperature scale where degree increments are the same magnitude as Fahrenheit degree increments. On the Rankine scale, the freezing point of water at standard temperature and pressure is 491.67°R and the normal boiling point is 671.67°R.

See also Atmospheric chemistry; Atmospheric inversion layers; Atmospheric lapse rate; Atomic theory; Chemistry; Energy transformations; Freezing and melting; Geothermal energy; Geothermal gradient; Quantum theory and mechanics