To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature gradient. Otherwise, if the same material is used for measurement, the voltage generated by the measuring conductor would simply cancel that of the first conductor. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. It is thus clear that, based on Seebeck's principle; thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings.
The principle behind it states that