Gareth Magennis wrote in message
...

"DaveM" wrote in message
...
"Gareth Magennis" wrote in message
...

"N_Cook" wrote in message
...
For a given length of fine copper wire of diameter 0.072 mm (2.9 mil)
=
0.004 sq mm,
if it is squashed to cross-section dimensions of 0.02 * 0.2 mm (2 * 20
mil)
proportionally how much does the resistance change ?
and then to 0.01 * 0.4mm (1 * 40 mil) ?





AFAIK the resistance of wire is proportional to its Cross Sectional
Area.
Period. If this remains unchanged, so does the resistance.



Gareth.



That is correct, but the length also has to remain unchanged The
formula
for the resistance of a conductor is
R=r*L/A
where R= Resistance
r=Resistivity of the conductor (1.7x10^-8 for copper)
L=Length
A=cross section area

As you can see, the resistance remains constant as long as L and A
remain
the same, or change in a manner that produces the same ratio.

--


So that begs the question, how much can a piece of copper wire be
compressed? If you do squash it into a different shape, does or can its
volume change significantly?


Gareth.



So it may be an effect of work hardening , relative increase in the effect
of imperfections/micro fractures or some other metallurgical effect.
Mackie speaker voice coil failures due to this flattening/ribboning process
to make the tails to the outside world.
Previous failure at the juncture of round to flat (0.07mm round to about
0.02 x 0.2mm) so at the peak stress point.
This one along the length of the ribbon section, but the whole 50mm or so
run was brittleised and disintegrated on touch, not the slightest sign of
overheating on the remaining 25 turns of round wire.
broken end marked B on this pic
http://home.graffiti.net/diverse:gra...ckie_horn1.jpg
http://home.graffiti.net/diverse:gra...ckie_horn2.jpg
Cannot expore the metallurgy as that curve of "wire" as totally
disintegrated to dust.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/