The branch of physics (or heat) relating to the measurement of temperature of a body is called thermometry. Thermometer is an instrument used to measures the temperature of a body.
The physical properties of some substances play an important role in the construction of a thermometer as well as to measure the temperature. The physical properties of the materials which changes uniformly with temperature and used for measuring the temperature are called thermometric properties and the materials used for making thermometers are called thermometric substances.
For example, in a mercury thermometer, the principle of expansion of mercury with rise in temperature is used. The platinum resistance thermometer is based on the principle of the change in resistance with change in temperature.