5. Secondary Reference Temperature Standards (See figure 14).
a. The secondary reference standards for temperature indicating devices are classified
into three types. The first is the thermocouple. As early as the 19th century an
experimenter named Seeback discovered the thermal-electric effect. He twisted two
copper wires and one iron wire together, placing one junction in ice water and heating the
other, while connecting the other ends to an indicating device. He found this caused a
current and called it the Seeback effect. The principle of operation for all thermocouple
devices is based on the differences in junction temperature between any two dissimilar
metals, thus producing an EMF.