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: There is no tangible substance termed heat, it is a property of as body or system.) from a region of higher temperature to system or body of lower temperature.
A thermometer measures a body or system by itself becoming in thermal equilibrium with that body or system. The exact mechanism that is used to read the thermometer varies, but all thermometers are calibrated (with another thermometer, for example) so that the temperature can be read directly from the thermometer.
An important consideration when making a thermometer is to minimize the thermometer's ability to change the temperature of the object being measured. Because of this, thermometers should be small enough that they do not absorb much energy from the object being measured.
The simplest kind of thermometer is the common mercury or alcohol thermometer. This thermometer consists of a glass tube with a small amount of fluid inside. When the temperature of the thermometer increases or decreases, the fluid expands or contracts, respectively. Markings on the outside of the thermometer allow measurement of the length of the column of fluid. Essentially mechanical expansion and contraction resulting from thermal energy changes is thereby translated into a quantitative measure of temperature.
A different type of thermometer is called a thermocouple. The thermocouple is a resistor, which is sensitive to temperature changes. A resistance meter continuously measures the resistance. Increases or decreases in resistance correspond to increases or decreases in temperature, respectively. The thermocouple is used to measure very low temperatures, such as temperatures of objects submersed in liquid nitrogen (about 77K). A mercury or alcohol thermometer is not useful at this temperature because the fluid inside the thermometer would freeze.
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