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

[Leigh Palmer <palmer@SFU.CA>]

> 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.

I will bet that this is a wildly inaccurate way to determine temperature.
Why do you expect the resistance to vary linearly with temperature over
a range of nearly a factor of ten when you already know its resistivity
varies by more than factor of ten? You need a thermometer, and the optical
pyrometer is the thermometer of choice for this purpose.

On a related issue, how do your students determine the effective area of
the radiating filament? You should examine a filament with a hand lens
sometime, and you should recognize that its dimension will change
significantly between cold and hot, though not nearly as much as the
resistivity will change.

Leigh