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

[Leigh Palmer <palmer@SFU.CA>]

> 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.

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. The oscillation is readily seen because the nodes glow
more brightly than the loops. Bu varying the tension in the wire
(pulling on the poles that support the wire above the floor) one can
vary the number of nodes seen.

I didn't point out the reason for this demonstration. When one gets the
wire red hot and not vibrating, turning off the current allows the wire
to cool uniformly. At a critical temperature the wire undergoes a
crystalographic transformation from fcc to bcc, with a corresponding
*expansion* on cooling. I hang a small weight from the center of the
wire to demonstrate this. The demonstration is not origibally mine. I
saw it first in a solid state class at Berkeley taught by Art Kip. I
use piano wire instead of pure iron wire.

Leigh