A question regarding thermocouples
Different metals A and B (at the same temperature) have a different density of electrons. When they are brought into contact electrons will flow from the metal with the higher density to the metal with the lower density. The metal that loses electrons will become more positive the other more negative. The resulting field will move electrons in the opposite direction and a thermodynamic equilibrium is possible, which results in a certain voltage between the two metals. 

In electronic devices a lot of metal connections are present. Pins, solder, copper, connectors, etc. Therefore a lot of voltages exist, but luckily this is not a problem.

When there is a connection like ABA, the resulting potential between A and B appears twice from A to B and from B to A but with the opposite sign, because the direction is different! Therefore the voltage cancels out and the metal B drops out of the equation. It is possible to go on and put as many metals in between, like ABA, ABCA, ABCDA, ... and so on. As long as the first and the last metal is the same the net voltage is zero. 

For this reason the voltage between AB can't be measured, because we would need to make contact again with probes of, say, metal M and we would have MABM.

For your case with contacts AC and BC (brought into contact again) the same reasoning leads to the result that only the outermost metals are relevant and ACBC becomes AC.

The voltage between two metals depends on the temperature so if we have ABA and one junction is at a different temperature, the net voltage is no longer zero and we can measure this difference.

So, a thermocouple can't measure the temperature, but only the difference of the temperature between two junctions. A thermometer based on thermocouples therefore has to have a temperature sensor built in to determine the absolute temperature of the thermocouple, because the voltage difference only represents the temperature difference between the thermocouple and the meter.