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One advantage of basing everything on thermodynamics is that there is no need to make assumptions about mercury in a mercury thermometer, platinum in a resis­ tance thermometer or any other substance. Each of these, if assumed to vary linearly with temperature, would define its own temperature scale. This is an altogether dif­ ferent thing from the Celsius and Fahrenheit scales; these just involve shifting the zero of the absolute scale or changing the temperature interval belonging to one degree or both.