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My background: I'm engineer of electronic HW engineering and embedded computer systems. During repair of a vintage digital voltmeter (resulution 1µV) I discovered an effect that probably only a physicist can answer. I isolated the problem and condensed it to a simple experiment:

  1. Take a piece of copper wire (about 20cm) and cut it in two halves.

  2. Join the two halves again by soldering. But don't let the two wires come in direct contact. They should be connected via the solder bead alone.

  3. Measure the voltage across the joint with a 1µV resolving voltmeter.
  4. Apply heat via hot air (about 100 degrees Celsius) to the solder joint.
  5. There is a voltage of about -20µV.
  6. Swap the wire ends at the voltmeter side.
  7. The voltage is about 20µV (to be expected).

My question: I assumed, the thermoelectric voltages from copper to solder and solder to copper would cancel out. But they don't. Why ?

Thanks !

P.S. The solder is regular solder, leaded, out of the pre-lead-free age.

Qmechanic
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Mario Blunk
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2 Answers2

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Mario, welcome to physics SE. You are right, the effects should cancel because of the symmetry of the junctions. In fact, a similar technique is commonly used to null out the effects of ambient temperature at the measuring gauge when a long run of thermocouple wire is used between the junction and the gauge. So you have uncovered a subtle and interesting effect! Here are some thoughts.

Zeroth, try the same measurement with a DIFFERENT voltmeter and leads to see if the meter itself is properly functional.

First, it is always good & prudent to check the null-point calibration of the meter, when making measurements of order ~1 mV- especially when the device has been recently repaired.

Second, how are the meter leads attached to the ends of the copper wire? with toothed clamps? If so, are the clamps metallurgically identical?

Third, how are the meter lead plugs connected to the input jacks on the meter housing? Same commentary applies.

Try these things and then write us back here with your results so we can ponder the imponderables some more, as needed.

Good luck!

niels nielsen
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Interesting.

Assuming there is no "boring" reason (something wrong with the leads or the voltmeter) then there is something about the joint or the wire near it that is not symmetric.

Most likely explanation is that the two copper-solder junctions are not identical because of random variations in the manufacture process. Then they cancel out but not completely. The solder actually forms a thin layer of alloy with copper - the composition changes from that of the wire to that of the solder over a few microns (my guess) of material. But how thick this layer is and how exactly does the composition change across it will likely depend on surface roughness of the copper, time the solder was liquid, temperatures of the copper and solder...

The other option is that the wire itself has some defects that generate a small thermoelectric voltage when there is a temperature gradient along it.

The cheapest thing to do when someone shares an interesting observation is to ask for more experiments so here are a few suggestions:

(0) How big is the voltage produced by a single solder - copper junction? You could attach a long piece of solder to the copper wire and use it as a lead to the voltmeter. You likely will get a lot bigger voltage. If you do it a few times there likely will be a bit of variability between the junctions - if that variability is of the same magnitude as the voltages you get for the "double" junction then the effect is due to uncontrolled differences between the junctions you produce. (what exactly is the nature of the differences would be a whole another story)

(1) If you take your copper wire, not cut it but heat it up in the middle do you get any voltage? There might be defects along the wire that act as thermoelectric junctions. As soon as there is a temperature gradient there can be some voltage. Not very likely but relatively quick to check.

(2) How reproducible is this effect? If you make several junctions the same way do you always get the same voltage with the same polarity? If someone else does it, are the results the same? (there could be something about how exactly you cut the wire or do the soldering that produces the asymmetry in a repeatable way)

(3) Is there a way to control the effect? Let's say after you cut the wire you file one end but leave the other one as is. Does it have change the amplitude or polarity in a predictable way?

Karel
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