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In the narrow sense of the word, a high temperature is any temperature that exceeds room temperature and is attained by some means of heating. There are various methods of obtaining high temperatures. For example, the heating of metal conductors by an electric current makes it possible to reach several thousand degrees; heating in a flame, approximately 5,000 degrees; heating by electrical discharges in gases, from tens of thousands to millions of degrees; heating with a laser beam, up to a few million degrees. The temperature in a thermonuclear reaction zone may reach a hundred million degrees.
In the broad sense of the word, high temperatures are temperatures that exceed some characteristic temperature at which a qualitative change in the properties of matter occurs. Strictly speaking, no unique boundary exists between low and high temperatures. Thus, for every substance the Debye temperature θDdetermines the temperature limit above which quantum effects do not come into play. In this case, temperatures T ≫ 0D should be considered high; for most substances θD lies in the range 100–500°K. The melting point divides the regions of the solid and liquid states of substances. The critical temperature determines the upper limit for the coexistence of vapor and liquid. Other examples of characteristic temperatures are the temperatures at which the dissociation of molecules (~103°K), the ionization of atoms (~104°K), and thermonuclear reactions (~107°K) begin.
E. I. ASINOVSKII