Types of thermometer
There are different kinds of thermometers.
1. Liquid Thermometers
These thermometers are based on the principle of change in volume of a liquid with change in temperature. Mercury and alcohol thermometers are based on this principle.
2. Gas Thermometer
These are based on the principle of change in pressure or volume with change in temperature, e.g., Callendar’s constant pressure thermometer, constant volume hydrogen thermometer, etc.
3. Resistance Thermometers
These are based on the principle of change in resistance with a change in temperature, e.g., platinum resistance thermometer.
4. Thermoelectric Thermometers
These are based on the principle of thermoelectricity, i.e., the production of thermo-E.M.F. in a thermo-couple when the two junctions are at different temperatures.
The various thermometers commonly used are:
- Copper and constantan
- Iror and constantan
- Chromel and constantan
- Chromel and alumel
- Platinum and Rhodium
5. Radiation Thermometers
These are based on the number of heat radiations emitted by a body e.g., furnaces. These instruments are known as pyrometers.
6. Vapour Pressure Thermometers
These are based on the principle of change of vapor pressure with change in temperature. These are used to measure low temperatures. e.g., helium vapor pressure thermometer, etc.
7. Bimetallic Thermometers
These thermometers are based on the principle of expansion of solids. A bimetallic strip is taken in the form of a spiral. Its one end is fixed and the other end is attached to a long pointer. The pointer moves on a scale, calibrated in degrees. These thermometers are used in meteorology for recording the changes in temperature during the day. They are also used to measure temperatures at high altitudes.
8. Magnetic Thermometers
These thermometers are based on the principle of change in the susceptibility of a substance with temperature. These thermometers are useful for measuring low temperatures near the absolute zero temperature.
