Measurement of temperature by electrical means

The measurement of a physical quantity implies, generally, the numerical comparison of the quantity with a certain selected quantity of the same kind taken as a unit. Temperature, however, can not be treated as a quantity in the same sense. It is rather to be considered as a state in which matter is found, and all temperature measurements are made by comparing the changes produced by heat in some form of matter. As shown by Lord Kelvin as early as 1848, temperature may be expressed on a scale which is independent of any particular form of matter, but this thermodynamic scale can not be used in actual temperature measurements, which, in practice, consist in comparing the changes in some particular forms of matter produced by heat.