Connection of Voltage Measuring Instrument

About the connection of voltage measuring instrument

No account so far has been taken of the actual connection between the thermocouple extension leads and the voltage measuring instrument. This introduces two further junctions between dissimilar metals and hence two extra voltage sources E4 and E5, and the total number of voltage sources in the equivalent circuit is increased to five. Provided that these extra junctions are maintained at the same temperature, this is unimportant as E4 and E5 cancel out (E4 = -E5) and the net effect on the temperature indicated by the thermocouple tables is zero. However, in various circumstances, the junctions can be at different temperatures and correction then has to be made for E4 and E5. The junction between a copper connection lead and, say, a chromel extension lead does not involve a standard pair of thermocouple materials and is therefore not covered by thermocouple tables. Use of a thermoelectric law known as the law of intermediate metals has to be used in these circumstances. This states that the e.m.f. generated at the junction between two metals or alloys A and C is equal to the sum of the e.m.f. generated at the junction between metals or alloys A and B and the e.m.f. generated at the junction between metals or alloys B and C, where all junctions are at the same temperature. This can be expressed more simply as:


eAC = eAB + eBC


Suppose that the junction between a copper connection lead and a chromel extension lead is maintained at a temperature of 80C. Equation (above) can be applied by introducing the intermediate alloy, constantan. Then:

ecopper-chromel = ecopper-constantan + econstantan-chromel
  = ecopper-constantan - echromel-constantan


Therefore, from thermocouple tables:

ecopper-chromel = 3.357 4.983 = -1.626 mV

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