Hi strantor,
It's not really the junction of dissimilar metals that messes things up, it is the actual USE of the wrong type of metal that does it. The real problem comes in because a length of wire of the wrong type develops the wrong voltage drop over its length with a given temperature gradient from one end to the other. When thinking about the hot and cold JUNCTIONS try not to think electrical but think thermal. A thermal junction can be one where the wires dont actually touch but simply have the same temperature. So we could have two wires like this:
o--------------------------------o wire 1
o--------------------------------o wire 2
and note that they are not touching at either end yet say on the left side if both ends are close to each other they still form a junction. They dont touch but still form a junction. That's because it is a thermal junction not an electrical junction, and the only requirement for a thermal junction is that both be the same temperature (they could actually have a different voltage though no problem).
Now if the two wires were touching on the right side, they would form an electrical junction but the two on the left side still dont touch. If the right side (the hot junction) is made hotter than the left side then the wires develop a voltage over their lengths and if they are different materials there will be a net voltage across the two ends on the left (the cold junction).
Note the thermal junction on the left is considered to be both wire ends even though the wires themselves do not touch.
Also note that the voltage developed across two different metals that touch is a different effect than that of a voltage that is developed along the length of a wire due to a temperature difference at the ends. The voltage developed across two different metals that touch is called their Anodic Potential found by finding their Anodic Index, while the voltage developed in the thermocouple comes from the Seebeck effect. The anodic reaction is very minor compared to the Seebeck effect in the thermocouple, so the Seebeck effect is the main reason the thermocouple can work.
It's not really the junction of dissimilar metals that messes things up, it is the actual USE of the wrong type of metal that does it. The real problem comes in because a length of wire of the wrong type develops the wrong voltage drop over its length with a given temperature gradient from one end to the other. When thinking about the hot and cold JUNCTIONS try not to think electrical but think thermal. A thermal junction can be one where the wires dont actually touch but simply have the same temperature. So we could have two wires like this:
o--------------------------------o wire 1
o--------------------------------o wire 2
and note that they are not touching at either end yet say on the left side if both ends are close to each other they still form a junction. They dont touch but still form a junction. That's because it is a thermal junction not an electrical junction, and the only requirement for a thermal junction is that both be the same temperature (they could actually have a different voltage though no problem).
Now if the two wires were touching on the right side, they would form an electrical junction but the two on the left side still dont touch. If the right side (the hot junction) is made hotter than the left side then the wires develop a voltage over their lengths and if they are different materials there will be a net voltage across the two ends on the left (the cold junction).
Code:
THERMAL JUNCTION (COLD) ELECTRICAL+THERMAL JUNCTION (HOT)
----------------------- ---------------------------------
25 degrees C 0.150v 100 degrees C
150mv o---------------------------------------------------------------------------+ (0v)
+ |
voltage=50mv |
- |
100mv o---------------------------------------------------------------------------+ (0v)
25 degrees C 0.100 100 degrees C
Wire 1 develops a voltage of 150mv from end to end while wire 2 develops 100mv,
so a voltage of 50mv is measured across the cold junction wire ends.
Note that it is not the touching of the wires that does much, but the length of
the wires and the difference in temperature between one end and the other that
generates the voltage. Since one wire develops a different voltage than the other,
a voltage difference (50mv in this case) is measured across the two ends at the
cold junction.Note the thermal junction on the left is considered to be both wire ends even though the wires themselves do not touch.
Also note that the voltage developed across two different metals that touch is a different effect than that of a voltage that is developed along the length of a wire due to a temperature difference at the ends. The voltage developed across two different metals that touch is called their Anodic Potential found by finding their Anodic Index, while the voltage developed in the thermocouple comes from the Seebeck effect. The anodic reaction is very minor compared to the Seebeck effect in the thermocouple, so the Seebeck effect is the main reason the thermocouple can work.
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