Re: wind instrument +- filtering of broadband noise /long/

[brian whatcott <inet@INTELLISYS.NET>]

Starling & Woodall wrote:
> > A stretched horizontal wire heated by an alternating current can be set
> > into vibration if its natural frequency is equal to that of the supply. The
> > rate of supply of heat to the wire is a maximum twice in each cycle of the
> > electrical supply, and the temperature of the wire therefore also
> > fluctuates at this double frequency.  Owing to thermal expansion, the wire
> > is thus slightly longer twice per cycle. and is therefore able to vibrate
> > in its fundamental mode.
At 16:02 12/10/00 -0800, Leigh wrote:
> This phenomenon is seen when I perform the thermal expansion of an iron
> wire demo I mentioned in the discussion of Variacs. The wire oscillates
> at 60 Hz due to the force acting on the current in Earth's magnetic
> field. I'm sure the thermal expansion phenomenon mentioned here is a
> far smaller effect, and I suggest that the book's explanation is
> probably wrong.   ////
> Leigh


Anyone who also runs a vibrating hot wire demonstration could easily
confirm Leigh's contention of a magnetomotive cause of the vibration by
aligning the hot wire along the direction and dip of the geomagnetic field.

If some doubt persisted, a Helmoltz coil arrangement would surely set
the matter at rest.   Leigh mentions a fundamental at the line frequency
and Starling & Woodall mention twice line frequency. Which *is* it?

(But really - doubting Starling & Woodall, it's  -ah- incedible!)

:-)



brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!