Different measurement systems depend on the application or whether very high or shallow values are to be measured. However, the best known and used tool is the thermometer.
To determine the temperature of a system, it must be in thermodynamic equilibrium. Variations in a body’s thermal state cause changes in physical properties such as dilatation, the evolution of electrical resistance, the creation of electromotive forces, pressure changes, volume changes in gas, etc.
Consequently, variations in these properties allow them to be used for the construction of instruments that detect temperature variations.
Previously, the mercury thermometer was used to measure body temperature. It is currently in disuse due to its dangerousness.