For the measurement of temperatures, there are three basic types of electrical thermometers.
1. Resistance thermometers operate on the principle that the resistance of a metal varies
in direct proportion to its temperature. They are normally used for temperatures up to approximately 1500°F.
2. Thermocouples operate on the principle that a difference in temperature in different metals generates a voltage, and are used for measuring temperatures up to about 3000°F.
3. Radiation pyrometers and optical pyrometers are generally used for temperatures above 3000°F. They combine the principle of the thermocouple with the effect of radiation of
heat and light.