Thermocouple
It is difficult to measure the precise voltage that a thermocouple is producing when heat is applied to its junction. This is because any voltmeter wires that are connected to the thermocouple will probably be made of a different material than the wires of a thermocouple. This means that the connection between the voltmeter and the thermocouple is itself another thermocouple. Engineers designing microchips face the problem that virtually every electrical connection on a microchip is a thermocouple whether they want it to be or not.