Geoff,

The formulas were derived.  I just assumed a simple linear relation 
between resistance and temperature, which is a decent approximation 
at normal temperatures.  My formula came 
from  (RT2-RT1)=0.00393*(T2-T1).  At very low (cryogenic) 
temperatures, you need a more complex formula.  I should have 
mentioned that the temperatures are Celsius or Kelvin.  As you point 
out, since you are taking a difference, which one you use doesn't matter.

You can find more than you'd ever want to know if you do a Google 
search on     copper temperature coefficient.  That's what I did.

Regards,
Brett

At 10:41 PM 11/16/2012, you wrote:

>mucho gracious, yes, carbon has a reverse coeff.
>too bad they are not identical or you might somehow
>get a hybrid resistor which will stay the same
>as temp changes. i shall assume T = kelvin or rankin
>which are absolute but then as deltas that may not be
>important.
>are these formulas within the crc handbook ?
>geoff

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