Date: Fri, 22 Mar 2002 04:32:38 -0600

Author: (Karl Trappe)

Subject: Re: Non-linear Lamp


Geoff: Is the curve a parabola? I would have guessed a 4th power, as well.
What did I overlook? Karl

>"George M. Caplan" wrote:
>> I connect a #47 pilot bulb to a variable power supply.
>> I make a table of V vs. I.
>> I plot the graph. Clearly it is not linear.
>> (This is especially obvious if I include the point [0,0].)
>> The curve is fit very well by a parabola.
>> Can someone please tell me where I read an accurate
>> but understandable explanation of why this is true.
>Sure. As the filament gets hot, its resistance gets larger. A
>significant part of the resistance at room temperature comes from the
>electrons scattering off thermal vibrations (phonons). As it gets
>hotter, there are more phonons to scatter off of, so the resistance is
>You can use this effect as a thermometer to measure the temperature of
>the filament and make a very nice lab demonstrating Stefan's law (power
>radiated is proportional to T^4). Interested parties can contact me for
>-Geoff Nunes

Dr. Karl I. Trappe Desk (512) 471-4152
Lecture Demonstration Office Office (512) 471-5411
Physics Department, Mail Stop C-1600 Home (512) 264-1616
The University of Texas at Austin
Austin, Texas 78712-1081