There are two empirical laws of thermocouples that permit the analysis practical thermocouple circuits. These can be stated as follows
1. The output voltage from a thermocouple is unaffected by the temperature of the wire between thermocouple junction and the reference junction. This principle permits the wires between the t junction and the reference junction to be routed throughout various temperature regions of a plant without affecting the o voltage
2. A third metal may be connected to either side of a thermocouple without af fecting the output voltage so long as both junctions of the third metal are at the same temperature. This principle allows a measuring de such as a voltmeter with copper terminals and wire to be inserted into an iron-constantan thermocouple circuit for measuring the voltage without affecting the voltage. This also says that the thermocou- ple junction itself can be either soldered with a third metal or welded and the output voltage will not be affected