Thermometric Property
An instrument, a thermometer, is required to measure temperature objectively. The thermometer makes use of a physical property of a thermometric substance which changes continuously with temperature. The physical property is referred to as thermometric property.
Thermometric Properties Used In Various Thermometers
The following table shows some of the thermometric properties of matter that are used in the various thermometers:
A good thermometric property of matter should vary:
- continuously with temperature
- uniquely over the the range of temperature to be measured
- its variation should be measurable.
Volume As Thermometric Property
Most solids or liquids or gases expand and contract their volume when the temperature around them changes.
Mercury is the only elemental metal that is liquid at ordinary temperatures. Mercury is silvery white and freezes into a soft solid like tin or lead at about -39 °C. Mercury does not wet glass or cling to it, and this property, coupled with its uniform volume expansion throughout its liquid range, makes it useful in liquid-in-glass thermometers.
Gases such as nitrogen or simply air are also suitable substances for use in volumetric gas thermometers. Typically, these gas thermometers work best at measuring very low temperatures.
Solid strips of different materials such as copper and aluminum can be made to stick together (bi-metallic strip). When the temperature of its surroundings changes, the different materials expand at different amounts, causing the bimetallic strip to bend, providing an indication to the change in temperature.
