chapter  65
Temperature Measurements, Scales, and Calibrations
Pages 18

Temperature cannot be measured in the same way as other fundamental quantities, for example, length. œe unit size of the degree having been deŸned, it cannot subsequently be labeled “unit interval” and used to measure temperature in the same way as the meter in length measurements. œat is, an additive procedure cannot be used for temperature, by which its value is determined from the number of the “unit intervals” contained in it.Temperature values, instead, can only be determined by comparing two temperatures, one of which is the reference, and by observing whether they are equal-or which one is higher-by observing, in accordance with the zeroth law of thermodynamics, whether there is-or not-a heat žow, and by noting its direction. To assign a numerical value to each temperature, one has to Ÿrst “order” the measured temperatures, that is, to establish a scale in which the heat žows always in the same direction, and then assign a sign to the žow. Accordingly, it can be said that {… T1 > T2 > T3…}: temperature is a simply ordered manifold. By this procedure, one cannot yet assign a numerical value to di¥erent temperatures, but only give them a serial number that will be altered by the arbitrary addition of any new measurement. Nor can one say yet that the value of any T2 is closer to that of T1 than to that of T3 (in fact, one cannot assign a distance between two temperatures), even if he or she should decide to take the interval, say {T1, T2}, as the unit interval, because neither intervals nor ratios can be compared by means of heat žow measurements.