Re: Mysterious electric current ! (fwd)

["Mark Sylvester" <msylvest@spin.it>]

On 25 Feb 1997 Donald E. Simanek wrote:

> On Mon, 24 Feb 1997, Mark Sylvester wrote:

> > Then while switching the leads I momentarily had both croc clips 
> > on the same vessel, and there was a good microamp flowing, and the
> > answer was clear before my eyes: thermocouple. My hot hand was
> > setting up a sufficient temperature gradient for the chrome plated
> > croc clips on the copper to generate the current. 
>
> Then you tried it again using cloth gloves on both hands and 
> compared results, right? Then using copper wire and copper cups?
>
> -- Donald

Not entirely sure of the smiley content here, but, no, I didn't do 
all those other experiments. I *did* touch a croc clip directly just 
to check that the deflection changed significantly. It did. The point 
is that when you are looking at a circuit where two croc clips 
are firmly clamped to a lump of copper, and they are making an emf, 
then it can really be nothing else but a thermocouple. All the 
electrochemistry that the original poster was thinking about goes out 
of the window.

Donald did make me think further about where the relevant thrmocouple 
junctions are, and I realised that the copper vessels could be 
disposed of as well. It goes Copper (wire) to whatever the shiny 
plating on the croc clips is and then back to copper on the vessel, 
or the other copper wire, if we simply join the two clips together. 
Joining them in this way gives several microamps when I touch the end 
of a clip where the wire is attached. Thus, with the copper vessels 
colder than the wires, we have two thermocouples back-to-back, with 
the emf dependent on the temp difference between the vessels, while 
we pour water back and forth, clasping them. No wonder it's so hard 
to get any consistent results when you're thinking it's an 
electrochemical phenomenon.

Btw, am I correct in assuming that there's nothing like a Seebeck 
effect in the Cu - electrolyte - Cu part of the circuit?

Mark.


*************************************************
Mark Sylvester, UWCAd, Duino, Trieste, Italy.
msylvest@spin.it    tel: +39 40 3739 255
*************************************************