Right, I understood it was a resonanace, what I meant by some special
way was that I didn't know which resonance it was looking for, though
the lowest mode probably makes most sense. The adjustment did change
the free length as you suggested, but I didn't make that very clear.

On Apr 6, 2005 12:45 PM, Barry Gershenfeld <barry_g@zmicro.com> wrote:
> The special way that piano wire vibrates is called "resonance".  And the
> adjustment likely changed the free length of the wire, until it
> resonated.   This suggests that the "other" way of measuring the frequency
> (RPM) can be had by low-pass filtering and looking for the largest
> (zero-crossing) signal...effectively what that gauge did.   Interesting
> gadget, though.  I'll watch for one at swap meets.
> 
> Barry
> 
> >Such a mechanical device exists. I'm not sure what they're called, but
> >my grandpa who was a mechanic had this little round thing that was
> >magnetic. You stuck it to an engine and turned the dial until the
> >little steel piano wire vibrated some special way (probably so that
> >the tip was stationary) and then you could read the engine speed from
> >the dial. Not as fancy as an FFT but it seemed to work.
> >
> >On Apr 5, 2005 1:36 PM, Barry Gershenfeld <barry_g@zmicro.com> wrote:
> > > I had a (strange / PICList-suitable) idea last week, along similar lines,
> > > so I guess this would be a good time to fling it out there.  Same basic
> > > idea, but build an audio tachometer.
> 
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