A thermometer is a device that registers the temperature of a substance relative to some agreed upon standard. Thermometers use changes in the physical or electronic properties of the device to detect temperature variations. For example, the most common thermometer consists of some sort of liquid sealed into a narrow tube, or capillary, with a calibrated scale attached. The liquid, typically mercury or alcohol, has a high coefficient of thermal expansion, that is to say the volume changes significantly with changes in temperature. Combined with the narrowness of the tube, this means that the height of the column of liquid changes significantly with small temperature variations.
The oldest thermometers were not sealed, which means that air pressure caused inaccurate readings. The first sealed thermometers were manufactured in the seventeenth century. A further improvement took place in 1714, when the German physicist Daniel Fahrenheit (1686-1736) started using mercury instead of alcohol as the measuring liquid. The Fahrenheit thermometer set a standard for accuracy that was accepted by scientists.
All material exhibits a certain resistance to electric current that changes as a function of temperature; this is the basis of both the resistance thermometer and the thermistor. The resistance thermometer consists of fine wire wrapped around an insulator. With a change in temperature, the resistance of the wire changes. This can be detected electronically and used to calculate temperature change from some reference resistance/temperature. Thermistors are semiconductor devices that operate on the same principle.
A thermocouple is another temperature sensor based on electrical properties. When two wires of different materials are connected, a small voltage is established that varies as a function of temperature. Two junctions are used in a typical thermocouple.
One junction is the measurement junction, the other is the reference junction kept at some constant temperature. The voltage generated by the temperature difference is detected by a meter connected to the system, and as with the thermistor, this information is converted to temperature.
A pyrometer is a temperature sensor that detects visible and infrared radiation and converts it to temperature. There is a direct relation between the color of light emitted by a hot body and its temperature; it's no accident that we speak of things as "red hot" or "white hot." All surfaces (including animals) emit or reflect radiation whose wavelength is proportional to their temperature. Pyrometers essentially compare the brightness and color of a reference filament to the radiation being emitted or reflected by the surface under test. They are excellent devices for non-contact measurements.
A wide variety of devices exist for measuring temperature; it is up to the user to choose the best thermometer for the job. For contact measurements requiring only moderate accuracy, a capillary thermometer is appropriate. Thermocouples measure temperature over a very wide range with good precision. A more accurate thermal sensor is a thermistor, which boasts the added advantages of being easy to use and inexpensive. Extremely precise contact measurements can be made with a resistance thermometer, but the devices are costly, and pyrometers are useful for non-contact measurements.