Russell McMahon wrote:

> EVERY inter-metallic junction is a thermocouple. If you extend it with
> wire of the appropriate identical type in each leg then they are
> zero-potential thermocouples and will not add extra potentials. If you
> join to eg copper in each leg you will have different couples in each
> leg and a potential difference will be caused. If the joins are at a
> point of intermediate temperature and/or uncontroled temperature them
> you will have a random addition to your signal. If you temperature
> controlled the junction point or if you measured the junction
> temperature you could account for the extra potential but in about
> 99.99% of cases it's easier just to use the "right" wire. It's readily
> available and quite cheap. 

Basically it boils down to where you place the cold junction temperature
compensation sensor -- this is where you should have your cold junction.
The "cold junction" is where both thermocouple metals are connected,
usually through a third metal (copper) that then completes the loop. 

Gerhard

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