Thermometers are familiar devices used to measure the temperature of an object or a system. Thermometers use the principle of thermal equilibrium, also known as the zeroth law of thermodynamics to find the temperature of an object. Thermometers may be low-cost models that may be found in a medicine cabinet, or high-tech models used in research and engineering. Different types of thermometers must be used over different temperature ranges.

To define a thermometer, the concept of temperature must also be precisely defined. Temperature is an indirect quantitative measure of the average kinetic energy in a body or system resulting from the movement or agitation of particles within the body or system. Temperature is usually expressed in units related to a particular scale (e.g. Celsius, Kelvin, etc.) In thermodynamic terms, temperature determines the direction of heat flow between systems that are not in thermal equilibrium. Heat "flows" (Note...