After some more research, I think I've answered some of my own questions -
Tellurium copper is used for binding posts, not because it has any
special thermal or EMF mojo, but because it machines much better than
pure copper. And, I suppose, because it sounds like it's extra special.
The Seebeck coefficients (uV/C, relative to Cu) of some relevant materials:
Cu 0.0
Ag .2
Au .5
Yellow brass 1.5
Phosphor bronze 2.0
63/37 solder 3.0
Sn 3.1
Stainless steel 3.1
Beryllium copper 5.0
Fe -12.3
Ni 22.3
Te -49.25
Based on the extreme Seebeck coefficient of pure tellurium vs. copper,
I'd expect that there might be some coefficient between Cu and CuTe
(0.5% Te), but I could find no reference. The relatively large number
for CuBe is interesting, since that's a common material for banana plug
springs, where one might expect the greatest temperature differential to
occur in such a connection (between the thermal masses of the binding
post/jack and the bulk of the banana plug). Heat has to flow a
considerable distance through the springs, very much more than when it
flows through a surface plating.
The Pomona (Fluke) EM5295-48-0# uses CuBe (gold plated) for the spring
contacts. It seems there might be an improvement to be had by using the
older style pin plugs, where a solid pin was partially sliced into 4
sections which were then spread apart a bit to create tension. That
could eliminate relatively large thermocouples at a thermal gradient,
and might also be expected to have less thermal resistance, allowing the
connection to settle quicker.
But maybe not - I'm still not clear on how plated conductors behave in
this situation. For a high impedance voltage measurement where almost no
current flows, the gold plating may carry the signal, so there is no
real thermocouple (or more correctly, it's entirely contained within the
connector). But if that's the case, why fool around with special copper
connectors when common brass ones would be easier/cheaper? For current
or resistance, the signal would also flow through the base metal, so
does this have an effect (especially for tinned copper test leads, where
there may be a larger temperature difference between the ends???
Nickle is avoided as a contact material largely because it is subject to
fretting corrosion. Tests done by AMP
(http://www.te.com/documentation/whitepapers/pdf/p154-74.pdf) show that
a Ni to Ni contact can increase from 8 mOhm to 5 Ohms (sic!) in a short
time due to this, while Ag and Au plated contacts exhibit negligible
changes.
Cu (with Be for better machining) seems to be used as the base material
for jacks/plugs to get thermal EMF cancellation to the wiring on both
sides (i.e. use copper everywhere except where there is a minimal
thermal gradient, like platings).
--
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