Next to be discussed are thermal electric harvesters. The basic construction unit of a thermal harvester is a thermocouple. This thermocouple is composed of an ‘N’ type material electrically in series with the ‘P’ type material. Heat flows from top to bottom. The energy from the applied heat allows the free electrons and holes to move and form an electric potential and current flow if there is a closed circuit. In order to create more power, an array of thermocouples is placed in a series electrically and in parallel thermally. This array is a thermopile. The electrical model is simply a voltage source which is dependent on the temperature difference multiplied by the differential Seebeck coefficients of the materials. Practical Seebeck coefficients are in the order of 100 µV per Kelvin, so many thermocouples need to be serialized. Even so, electronics which can harvest energy from small voltages are important. To extract the maximum power requires controlling the load to the generator. The electrical impedance of the generator needs to be matched.