Originally Posted by
Falanx
I can get a goodly readable contact potential between copper plate, and nickel particles in a rubber matrix, let alone between to clean pieces of metal, although, yes, a salt bridge would help even moreso :-)
I recommend tomato ketchup/catsup as a contact electrolyte. Or katsu or teriyaki sauces.
The presence of less than ten percent alloying ingredients that in no way alter the crystal structure of the alloy is negligible.
I'll take your word for that but I think the rubber matrix might actually be helping rather than hindering.
In practice, I've never been able to measure contact potential directly with a multimeter and as far as I know it's impossible in theory also.
Here's why: As you know contact potential is dependent on the difference in work functions between the metals. Let's say your multimeter test leads are pure copper. There are then three metal to metal junctions in the circuit: Cu / Ti, Ti / Al and Al / Cu so the net difference in work functions around the circuit is thus Cu - Ti + Ti - Al + Al - Cu which can be rearranged as Cu - Cu + Ti - Ti + Al - Al which is obviously zero.
In the 20s and 30s a lot of work went into measuring the work function of metals because it governs the emission from hot cathodes and I am quite sure no-one was able to do it directly. I have pretty much every important book on electronic theory and practice up until 1960 in my library and they are unanimous on the difficulty of accurately measuring work function.
As always if you can show me where I'm wrong I'm all ears.