When the temperature of a body increases, it increases the kinetic energy of its particles. These fast-moving particles can cause variations in different physical properties (volume, pressure, change in colour, electrical resistance, etc.) in a predictable way. These variations may be used to measure the temperature of a body. The physical properties that a thermometer relies on are called its thermometric properties.
Most liquids expand upon heating. Liquids that expand uniformly on heating can be used as thermometric materials. This property is used in liquid-in-glass thermometers, which use mercury or alcohol as thermometric materials. When the thermometer is touched by a hot body, it absorbs heat and causes the mercury to expand. This uniform expansion varies linearly with temperature, providing a reading on the thermometer scale.
When a gas is heated, its volume and pressure may change. These variations are used in gas thermometers to measure temperature. There are two types of gas thermometers: constant pressure gas thermometers and constant volume gas thermometers.
A constant volume gas thermometer uses the principle that the pressure of a gas is directly proportional to the temperature, while the volume is kept constant (Gay-Lussac's law). As the temperature of the thermometer bulb increases, the kinetic energy of gas particles increases, causing a rise in pressure. This pressure variation is used to measure temperature.
A constant pressure gas thermometer operates on the principle that the volume of a gas is directly proportional to the temperature, while the pressure is kept constant (Charles's law). The gas in the bulb expands and pushes a piston according to the increase in volume, which is used to measure temperature.
Liquid crystals are materials that change colour with a change in temperature. These crystals are packed inside a plastic strip, and when touched with a body, their temperature change causes a colour shift. By matching the colour to a temperature scale, we can determine the temperature. Such thermometers are often used as fever thermometers and for aquariums and baby bottles.
Resistance thermometers measure temperature by detecting changes in electrical resistance due to variations in temperature.
Bimetallic strip thermometers measure temperature through the variation in volume expansion of two different metals that are bonded together.
Thermocouples measure temperature by detecting changes in electromotive force (emf) due to temperature variations.
| Thermometer Type | Thermometric Property |
|---|---|
| Constant pressure gas thermometer | Volume expansion of a gas |
| Laboratory or clinical thermometer | Volume expansion of a liquid |
| Bimetallic strip thermometer | Volume expansion of a solid |
| Constant volume gas thermometer | Pressure change of a fixed mass of gas |
| Thermocouple | Changes in e.m.f |
| Resistance thermometer or thermistor | Changes in electrical resistance |