The thermocouple was discovered in 1821 by the Russian-born German physicist, J. Seebeck . He discovered that at the junction of two dissimilar conductors a voltage could be generated by a change in temperature (see Figure 5.1). Using this effect, Melloni produced the rst bismuthcooper thermocouple detector in 1833 , to investigate the infrared spectrum. The small output voltage of thermocouples, of the order of some µV/K for metal thermocouples, prevented the measurements of very small temperature differences. Connecting several thermocouples in series, for the rst time by Nobili in 1829, generates a higher and therefore measurable voltage.