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.

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.

I'm new here and try to keep on the rules set here. So apologize for my mistakes. From 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 degree 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.

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.