[volt-nuts] 732A and Prologix received

[Mike S]

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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