HP 34970A User Manual page 344

Chapter 8 Tutorial
Measurement Fundamentals
An ice bath is used to create a known reference temperature (0 C).
Once the reference temperature and thermocouple type are known,
the temperature of the measurement thermocouple can be calculated.
Internal DMM
Ice Bath
The T-type thermocouple is a unique case since one of the conductors
(copper) is the same metal as the internal DMM 's input terminals.
If another type of thermocouple is used, two additional thermocouples
are created. For example, take a look at the connections with a J-type
thermocouple (iron and constantan):
Internal DMM
Ice Bath
Two additional thermocouples have been created where the iron (Fe)
lead connects to the internal DMM 's copper (Cu) input terminals.
Since these two junctions will generate opposing voltages, their effect
will be to cancel each other. However, if the input terminals are not at
the same temperature, an error will be created in the measurement.
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