Imparting heat to the junction of two dissimilar metals causes a small continuous electromotive force (EMF) to be generated. One of the simplest of all temperature sensors, the thermocouple (TC) depends upon the principle known as the Seebeck Effect. T.J. Seebeck discovered this phenomenon in 1821, and in the ensuing years the thermocouple has become the most widely used electrical temperature sensor. The word is a combination of thermo for the heat requirement and couple denoting two junctions. A TC is an assembly of two wires of unlike metals joined at one end, designated as the hot end. At the other end, referred to as the cold junction, the open circuit voltage or Seebeck voltage is measured. This voltage (EMF) depends on the temperature difference between the hot and the cold junctions and on the Seebeck coefficients of the two metal wires. THEORY OF OPERATION An ordinary TC consists of two different kinds of wires, each of which must be made of a homogeneous metal or alloy. The wires are fastened together at one end to form a measuring junction, normally referred to as the hot junction, since a majority of the measurements are made above ambient temperatures. The free ends of the two wires are connected to the measuring instrument to form a closed path in which current can flow. After the TC wires connect to the measuring instrument, the junction inside is designated as reference junction, or the cold junction (see Figure 4.13a) The EMF developed at wire junctions is a manifestation of the Peltier Effect and occurs at every junction of dissimilar metals within the measuring system. This effect involves the liberation or absorption of heat at the junction when a current flows across it. The resultant heating or cooling depends upon the direction of current flow. Applications of this principle are becoming increasingly useful in electric heating and refrigeration. A second EMF develops along the temperature gradient of a single homogeneous wire. This is the Thomson Effect. It is most important that each section of wire in a given circuit be homogeneous. This is because if there is no change in the composition or physical properties along its length, the circuit EMF depends only upon the metals employed and the temperature of their junction. Therefore, the circuit EMFs are independent of both length and diameter of wires. Another reason for requiring homogeneous wire is that thermal EMFs within a single strand passing from a warmer to a cooler area, or vice versa, will cancel each other. Further, if both junctions of a homogeneous metal are held at the same temperature, the metal does not contribute to the net EMF of a circuit. Since some TCs are made of expensive metals, this fact can be used to cut costs by supplying copper extension wire for long runs. It follows, then, that by holding temperatures constant at all junctions except one within