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(kăl'ərĭm`ətrē), measurement of heatheat,
nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system. Measures of Heat
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 and the determination of heat capacityheat capacity
or thermal capacity,
ratio of the change in heat energy of a unit mass of a substance to the change in temperature of the substance; like its melting point or boiling point, the heat capacity is a characteristic of a substance.
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. Heat is evolved in exothermic processes and absorbed in endothermic processes; such processes include chemical reactions, transitions between the states of matter, and the mixing of two substances to form a solution (see thermodynamicsthermodynamics,
branch of science concerned with the nature of heat and its conversion to mechanical, electric, and chemical energy. Historically, it grew out of efforts to construct more efficient heat engines—devices for extracting useful work from expanding hot gases.
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). A number of different units are used in heat measurement, e.g., the caloriecalorie,
abbr. cal, unit of heat energy in the metric system. The measurement of heat is called calorimetry. The calorie, or gram calorie, is the quantity of heat required to raise the temperature of 1 gram of pure water 1°C;.
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, the British thermal unitBritish thermal unit,
abbr. Btu, unit for measuring heat quantity in the customary system of English units of measurement, equal to the amount of heat required to raise the temperature of one pound of water at its maximum density [which occurs at a temperature of 39.
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 (Btu), and the joulejoule
, abbr. J, unit of work or energy in the mks system of units, which is based on the metric system; it is the work done or energy expended by a force of 1 newton acting through a distance of 1 meter. The joule is named for James P. Joule.
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. The apparatus used in heat measurement is called a calorimeter. The measurement given by the most common type of calorimeter depends upon the temperature change in a fixed quantity of water (or some other liquid whose heat capacity is known) when heat is transferred between the water and an exothermic or endothermic process. If the temperature change is not too large, then the heat transferred is equal to the heat capacity of the water times the mass of the water times the change in temperature. The accuracy of this method of heat measurement depends on the assumption that all the heat transferred in the process passes into or out of the water in which the temperature change is measured, no heat being lost to the environment and none being absorbed by the walls of the container. The amount of heat given off by the combustion of a fuel can be determined very accurately in the so-called bomb calorimeter, which consists of a combustion chamber (the "bomb") set in another chamber filled with water. Heat generated by combustion of the fuel is transmitted to the water, raising its temperature. The calorie content of food is tested this way. Calorimeters are also employed to measure the energies of elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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the totality of methods for determining the heat effects (quantities of heat) accompanying various physical, chemical, and biological processes. Calorimetric methods are used to determine heat capacities of materials; heats of phase transitions (melting, boiling, and other transitions); heat effects of magnetization, electrification, solution, sorption, chemical reactions (for example, combustion), and metabolic processes in living organisms; and in a number of cases, the energy of electromagnetic radiation and that of nuclear processes.

The apparatus used in calorimetric measurements are called calorimeters. Their design is determined by the conditions of measurement (primarily by the temperature interval) and by the required accuracy. Calorimeters for use above 400°K (arbitrary limit) are called high-temperature calorimeters, whereas those for use in the temperature range corresponding to liquid nitrogen, hydrogen, and helium are called low-temperature calorimeters.

The results of calorimetric measurements are widely used in heat engineering, metallurgy, and chemical technology. They are used to calculate the quantities of heat required to heat, melt, or vaporize materials in various engineering processes and to calculate the time limits of chemical reactions and the conditions for carrying out these reactions. Thus, the temperature and pressure range in which synthetic diamonds are obtainable from graphite were determined by calculations based to a large extent on calorimetric determinations of heat capacities and heats of combustion of these materials. Calorimetric measurements make it possible to determine the regions of stability of various minerals and to elucidate the conditions for their simultaneous presence in rocks. Low-temperature calorimetric data are being widely used in studies of mechanical, magnetic, and electrical effects in solids and liquids at low temperatures as well as in calculations of thermodynamic functions (for example, the entropy of substances).


In biology, calorimeters are used for determining the heat effects accompanying the processes of life. Two types of chemical processes are continuously occurring in organisms: endothermic processes (with heat absorption) and exothermic processes (with heat evolution), the latter type predominating. Calorimetry has shown, for example, that a coliform bacterium evolves 4 × 10-9 joules (J) (10-9 cal) of heat per hour; a mouse evolves 420 J (100 cal); and a human evolves 2 × 105 J, or ∽ 5 × 104 cal [specific heat evolution presents a completely different picture: 1, 050 J/(g-hr), 21 J/(g-hr), and 4 J/(g-hr), respectively]. The organisms are usually placed into a calorimeter for measurement of their heat production. When direct calorimetry is difficult, indirect methods are used. Indirect determination of the heat production by an organism may be performed, for example, on the basis of the intensity of its gas metabolism. In this case, the quantities of oxygen (O2) absorbed by the organism per unit time and the quantities of carbon dioxide (CO2) liberated per unit time are measured. The ratio of these quantities (respiratory coefficient) yields the quantity of O2 expended separately for the oxidation of proteins, fats, and carbohydrates. The heat content of these reactions is known, which makes it possible to calculate the total heat production of the organism.



The measurement of the quantity of heat involved in various processes, such as chemical reactions, changes of state, and formations of solutions, or in the determination of the heat capacities of substances; fundamental unit of measurement is the joule or the calorie (4.184 joules).
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