Any physical property that depends consistently and reproducibly on temperature can be used as the basis of a thermometer. For example, volume increases with temperature for most substances. This property is the basis for the common alcohol thermometer and the original mercury thermometers. Other properties used to measure temperature include electrical resistance, color, and the emission of infrared radiation.

As many physical properties depend on temperature, the variety of thermometers is remarkable. In the most common type of thermometer, the alcohol, containing a red dye, expands more rapidly than the glass encasing it. When the thermometer’s temperature increases, the liquid from the bulb is forced into a narrow tube, producing a large change in the length of the column for a small change in temperature. Another common type of thermometer is the pyrometer. A pyrometer measures the infrared radiation (whose emission varies with temperature) from the source and quickly produces a temperature readout. Infrared thermometers are also frequently used to measure body temperature by gently placing them in the ear canal. Such thermometers are more accurate than alcohol thermometers placed under the tongue or under the armpit.

Thermometers measure temperature according to well-defined scales of measurement. The three most common temperature scales are Fahrenheit, Celsius, and Kelvin. Temperature scales are created by identifying two reproducible temperatures, most commonly the freezing and boiling temperatures of water at standard atmospheric pressure.