Thermometry is the science and practice of temperature measurement. Any measurable change in a thermometric probe can be used to mark temperature levels that should later be calibrated against an internationally agreed unit if the measure is to be related to other thermodynamic variables.

An instrument is measure the temperature of a body is called a thermometer. As the temperature is measured by the value of thermodynamic property of a substance i.e. a property which varies linearly with the temperature, two fixed points are needed to define a temperature scale.

The linear variation in some physical properties of a substance with change of temperature is the basic principle of thermometry and these properties are defined as the thermometric properties (x) of the substance x may be,

(i) Length of liquid in capillary.

(ii) Pressure of gas at constant volume.

(iii) Volume of gas at constant pressure.

(iv) Resistance of a given platinum wire.

In old thermometry, two arbitrarily fixed points, ice and steam point (freezing point and boiling point at 1 atm), are taken to define the temperature scale.

In Celsius scale, freezing point of water is assumed to be 0°C while boiling is taken as 100°C and the temperature interval between these is divided into 100 equal parts.

So, if the thermometric properties at temperature 0°C, 100°C and TC°C are x₀, x₁₀₀ and x respectively. Then, by linear variation (y = mx + c) we can say that:

0 = mx₀ + C

100 = mx₀ + C

TC = mx₀ + C

From these equations:


∴ \({{T}_{C}}=\frac{x-{{x}_{0}}}{{{x}_{100}}-{{x}_{0}}}\times {{100}^{0}}C\).