To understand the thermopile, it is necessary to review the basic theory of the thermocouple.

If two different metals are connected at any temperature above absolute zero, a potential difference (thermoelectric EMF or contact potential) will be generated between the two metals, which is a function of the junction temperature (see the thermoelectric EMF circuit in Figure 2).

If two wires are connected at two points, two nodes are formed (see the thermocouple connected to the load in Figure 2). If the temperatures at the two nodes are different, a net EMF is generated in the circuit and current flows through it. The current is determined by the EMF and the total resistance of the circuit (see Figure 2). If one of the wires breaks, the voltage at the break point is equal to the net thermoelectric EMF of the circuit. And if this voltage can be measured, it can be used to calculate the temperature difference between the two nodes (see the thermocouple voltage measurement in Figure 2). Remember, a thermocouple measures the temperature difference between two nodes, not the absolute temperature at one node. The temperature at the measurement node can only be measured when another node (commonly known as the reference node or cold node) is known.

However, it is very difficult to measure the voltage generated by a thermocouple. Suppose the voltmeter is connected to the first thermocouple measurement circuit (refer to the actual thermocouple voltage measurement of the cold junction shown in Figure 2). The wires connecting the voltmeter form more thermocouples at the connection points. If the temperatures of these additional nodes are the same (regardless of the temperature), the intermediate metal rule indicates that they make no net contribution to the total EMF of the system. If their temperatures are different, errors will occur. Because each pair of different contact metals generates thermoelectric EMF - including copper sheets/solder joints, kovar/copper sheets (Kovar is an alloy used for IC lead frames), and aluminum/kovar (for soldering within ics) - the problem becomes more complex in actual circuits. It is necessary to ensure with extreme caution that the temperatures of all node pairs in the peripheral circuit of the thermocouple (except for the measurement node and the reference node itself) are the same.