So it appears the thermal voltages i actual had in my setup (220 and 250 nV) are a good result even for a copper-to-copper contact.
I get around 3 uV with Pomona 5291A cables (supposedly CuTe bananas with CuBe springs) stuck into a 34401A (Cu sockets). This is when heating the cable end that sticks into the meter with my fingers. I also see the opposite polarity when heating the other cable, and the offsets take several minutes to decay - which is to be expected.
I guess they are called "low emf" not "zero emf" for a reason
I get about the same behavior with Multi-Contact Multilam bananas (brass body, CuTe spring) and Multi-Contact LS4 (CuBe hollow bananas).
Edit: for comparison, I get around 9 uV for a Pomona 5291A stuck into a Pomona 3770 binding post (also supposedly CuTe), and on the order of 100 uV for a copper-Kovar junction (made with a pin from a hermetic transistor package -- like the LTZ1000 uses
). That is certainly not "low emf". So your binding posts are certainly not steel.
Can anyone explain why Pomona and others use Cu-Te? Tellurium is listed with 500 uV/°C, so i'd guess even a small fraction of Te could distort everything.
It's been discussed on this forum before, apparently the reason is mechanical. Pure Cu is hard to machine, CuTe is apparently much easier, while still retaining "low emf" characteristics.