The Nature of Temperature Measurement

Most of the measurements discussed in this chapter deal with physical properties, such as length, volume, or weight. Measurement of these properties can be made directly. Temperature is different because it is an energy property, and energy cannot be measured directly. However, we can quantify the effect that one body s energy (in this case heat) has on the physical properties of another body, and we can measure that physical effect. 

Unfortunately, heat energy does not have the same percentage of effect on all materials in the same way. For example, heat makes most materials expand, but few materials, if any, expand the same amount for an equal amount of heat. Thus, the size increase for one material (for a given amount of heat change) is unlikely 

On the other hand, it is possible to obtain the same temperature from two different materials if they are calibrated the same. This operation is done as follows take two different materials and heat them to a specific (and repeatable) temperature. Place a mark on some reference material that has not expanded (or contracted). Then heat the materials to another specific and repeatable temperature and place a new mark as before. Now, if equal divisions are made between those two points, the specific temperature readings along the calibrated region should be the same even if the actual changes in lengths of the materials are different. 

An interesting aspect about temperature measurement is that calibration is consistent across different types of physical phenomena. Thus, once you have calibrated two or more established points for specific temperatures, the various physical phenomena of expansion, resistance, emf, and other variable physical properties of temperature will give the same temperature reading.  

Although not all of these temperature measurement techniques provide a uniform linear measurement, the variations are known and can be calibrated and accounted for. 

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