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Although there is some variation in the defined temperature range involved, cryogenics (kri-o-jen-iks) usually indicates concern with temperatures in the liquid oxygen range (about 90 K or —183 C) or lower. This temperature range will be discussed primarily in this chapter. Since a triple point (of water) or ice bath reference junction often is used, additional comments and values will be given for the entire subzero (0 C) range. Most aspects of cryogenic thermometry are similar to those applicable at room or high temperatures. In particular, the measurement systems and thermoelectric theory are nearly identical. However, there are significant differences with respect to some materials, techniques or assembly and fabrication, calibration schemes, and methods of practical usage. Fortunately the added difficulties with some details are offset by the removal of a few problems peculiar to high-temperature thermometry: chemical transformations are insignificant; oxidation, reduction, and impurity migration do not occur because of the low temperatures. Annealing of physical imperfections is also absent for the same reasons. Maintenance of fixed points and techniques of calibration are usually considerably easier and sometimes much more accurate. Thermal radiation is usually not important, at least if simple precautions are taken to account for it. Several books have been written on thermometry and on the experimental techniques necessary for cryogenic research by Scott  on cryogenic engineering, and by White  and by Rose-Innes  on smaller, scientific systems.