measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used. A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees. The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water. On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0 degrees Celsius and the boiling point as 100 degrees Celsius, and the difference between them is divided into 100 degrees. On the Fahrenheit temperature scale, the melting point is taken as 32 degrees Fahrenheit and the boiling point as 212 degrees Fahrenheit, with the difference between them equal to 180 degrees. The Réaumur scale, used in some parts of Europe, also sets the melting point at zero, but it has an 80-degree temperature difference between 0 degrees R,aumur and the boiling point at 80 degrees R,aumur. The temperature of a substance does not measure its heat content but rather the average kinetic energy of its molecules resulting from their motions. A one-pound block of iron and a two-pound block of iron at the same temperature do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the two-pound block has more molecules than the one-pound block and thus has greater heat energy. A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy (see gas laws). Such a point is called absolute zero, and such a scale is known as an absolute temperature scale. The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale; the Rankine temperature scale is an absolute scale having degrees the same size as those of the Fahrenheit temperature scale. The relationship between absolute temperature and average molecular kinetic energy is one result of the kinetic-molecular theory of gases. See heat; thermodynamics.

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