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Re: Optical pyrometers, was R = V/I ?



At 11.21 08/05/00 -0700, Leigh Palmer wrote:
At 10:17 AM -0700 5/8/00, Mark Sylvester wrote:

>At 10.52 07/05/00 -0700, Leigh Palmer wrote:
>
> >Well, if one wishes to call the ratio V/I the resistance of a light
> >bulb at a specific operating point one may certainly do so. It is a
> >matter of some concern to me that this resistance can't be used to
> >calculate anything else. It does not condense the information in a
> >way that will simplify a later calculation - operating voltage or
> >current must still be specified along with this resistance, so how
> >is that an improvement over specifying two other numbers, say
> >voltage and power?
>
>We do a lab where students measure I and V for a range of values, using a
>12V bulb. They calculate R at each point in order to find the temperature
>of the filament (and then go on to plot power vs temperature in pursuit of
>the 4th power radiation law). I don't think this is a counter-example to
>your comment, but it does show that the concept (of resistance) is useful
>even when limited in the sense that you point out.

You make my point; thank you. You have your students plot temperature
vs. power (I assume). If the student has measured I and V, can he not
calculate power without first calculating this questionable resistance
parameter? Of what value, then, is V/I?

It's used to determine the temperature of the filament, from R = Ro(1 +
alpha*theta). For Ro we use the smallest I and V that we can measure to 2
SD, using the best meters we have. We use the tcr of tungsten as found in
the data book.


On a related issue: do your students use an optical pyrometer to
measure the filament temperature? I think this instrument (which may
be a tad expensive) is a marvelous one to introduce students to, and
it fits well into the course when one is studying Stefan's law as
your students are doing...

I've never used one of these instruments. We recently acquired a Pasco
bolometer, which I'm planning to marry to this experiment.

Mark




_____________________________________
Mark Sylvester
United World College of the Adriatic,
34013 Duino TS, Italy.
_____________________________________