The most commonly known method of measuring temperature is with the liquid-filled glass thermometers that are attached to buildings to measure air temperature or inserted in your mouth to measure internal body temperature. These are based on the principle that the liquid will expand or contract inside the glass tube depending on the external temperature. It is understood that physical temperature affects all matter by increasing its volume when heated and decreasing volume when cooled. This is not the only way that temperature affects the physical world. Temperature has an effect on many different material properties that can be recorded and measured.
Besides physical volume, temperature affects electrical properties as well. A change in temperature affects the conductivity or resistance of any material. In the case of conductors, a change in temperature will actually create an electrical voltage. In the case of insulators, a change in temperature will affect the level of resistance of the material.
Temperature measurement devices or sensors can be separated into two categories: Contact and Non-Contact. Contact sensors are the most popular and inexpensive. There is a broad variety of contact sensors. Non-contact sensors are newer technology that are more expensive, but usually more accurate.
Below is a list of the different types of temperature sensors that are currently available.
Gas and Vapor Thermometers
Filled System Thermometers
Semiconductor Thermometer Devices
Phase Change Thermometers