Thermometer, an instrument for measuring temperature. The measurement is always indirect—that is, it is a measurement not of temperature itself but of the effect of temperature on some physical property of a material used in the thermometer. In response to changes in temperature, for example, solids, liquids, and gases expand and contract; the pressure of a gas that is kept at a constant volume changes; and the electrical resistance of a metal or alloy increases and decreases.
Most thermometers have scales marked off in degrees, the units in which temperature is measured. The most common scales are: