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An online electrical engineering study site. Today's topic is Laws of Thermocouples
Let us take two dissimilar metals M1 and M2. One strip of metal M1 is shaped as you see here
Two smaller strips of metal M2 which are shaped as you see here
Now connect these three strips in the way as shown here. There are two bimetallic junctions formed
One is this one, and another is this one. Now this junction is heated by this candle flame
And this junction is immersed in this ice water. Now we will connect one sensitive multimeter, as shown here
You see, this meter is giving some reading. This is nothing but the voltage between these hot and cold junctions
Now if we increase the temperature of the hot junction, by increasing the intensity
of the flame, we will see the reading of the multimeter also increases
This proves that the voltage between these two junctions is the function of temperature
difference between them. A German physicist named Thomas Johann Seebeck has discovered this effect
His discovery is called the Seebeck effect, and it is the basis of all thermocouples
We have three different laws of thermoelectric circuits. These laws are known as Law of Homogeneous Circuit, Law of Intermediate Metals and Law
of Intermediate Temperatures. The first law, homogeneous circuit, states that in a circuit composed of a single homogeneous
metal, cannot generate an electric potential by the application of even sufficient temperature
difference between two different points of the circuit. Here the temperature of this hot junction is T1 and temperature of cold junction is T2
There is a temperature difference between these two junction, and that is T1 minus T2
but still the multimeter gives zero reading, as the junctions are made of same metals
The second law is also known as thermocouple law of intermediate metals, states that the
net EMF in a circuit consisting of interconnections of a number of unlike metals, maintained at
the same temperature, is zero. That means, if other metal conductors are inserted in the thermocouple circuit and the
junctions of these interconnected metal conductors are in same temperature, then output EMF will
not be affected and remains same as if it was made of only two metals
Then come to this circuit, here T1 and T2 is the temperature of hot and cold junction respectively
Now take the reading of the multimeter. Here this third metal is inserted here in the circuit
This creates new two junctions in the circuit. If T3 is the temperature of both new junctions and, T1 and T2 is the temperature of hot and
cold junction respectively. Now again take the reading of the multimeter. In these two cases you will see the multimeter reading is same, which proves the statement
thermocouple law of intermediate metals. The third law, successive or intermediate temperatures, where EMF E1 is generated when
two dissimilar metals have junctions at temperatures T1 and T2 and EMF E2 results for temperatures
T2 and T3. It follows that an EMF E3 results at output terminals when the junctions are at temperatures
T1 and T3. Here, E1 plus E2 equals E3. Thank you. Hope, you got a clear and basic idea
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