why did this create a spark?

I recently cut some piano wire under tension and it produced a spark. The same wire cut without tension did not. What caused the spark? I
repeated this hundreds of times (or as many as a piano has) and it was very consistent.
thanks
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Very interesting. How much tension was it under?
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On 22 Nov 2004 11:42:01 -0800, snipped-for-privacy@juno.com (Lawrence) wrote:

How was the tension generated? Could be that sudden relaxation of the tension frame generated piezoelectric or triboelectric jolt that traveled down the cut wire and jumped the gap.
--

Boris Mohar

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(Lawrence) wrote:

I don't know how much tension the wires had. I assume it varied some. They were installed in a piano.
I don't think the spark was electrical in nature. I'd rather believe it was a hot piece of metal since I observed it some distance away from the wire and traveling away from the side cutter. Its color was brilliant orange. What seemed so odd was that I could repeatedly cut the same un-tensioned wire and not generate any spark yet most tensioned wires did spark.
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Lawrence wrote:

Any possibility it was a visual afterimage, not a spark at all? The shiny metal could leave behind an afterimage as it moves away. You could test this by covering the wire with black soot from a candle flame, to see if still "sparks" under the same conditions.
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I'm quite sure it could not have been an afterimage. The wires were quite old and rusty appearance. The spark I observed did not travel parallel to the wire but in some other direction (probably random). Unfortunately I can't repeat the experiment at this time.
As I recall the wires were easier to cut while under tension. Perhaps just a wire nick caused the wire to part. Could this localized stretching and failure caused by the tension have caused enough heating to create the spark?
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snipped-for-privacy@juno.com (Lawrence) wrote ....

It is plausible that (given sufficient tension) local stress relaxation effects might generate temperatures high enough to initiate oxidation of small bits of metal. Like the sparks generated during grinding.
I've observed glass fibers under high tensile stress completely (like a million fiber diameters) exploding to very fine powder on failure. I never thought to measure the associated temperature increase. Darn.
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