0 Members and 2 Guests are viewing this topic.
I have measured the resistance of many light bulbs at different temperatures - it's something GCSE Science students all have to do. The resistance more or less doubles as the temperature changes from 300K to about 6000K. This is a rate of about 0.03% per degree C. You would need a pretty big temperature difference to produce a significant/measurable change in resistance. A 'bridge' circuit might help but, still.How cold could we expect the filament of the light bulb to get?It would have to rely on cooling by radiation.If you could rely on significant good cooling with this arrangement there would surely be a refrigerator in every home using the principle. Or even Stirling Engines to tap this free source of energy. I did hear, once, that the Arabs used to produce ice for their drinks centuries ago using the radiation cooling from large, shallow, ponds at night in the desert. However, the desert sky is very clear - very little pollution or water vapor and a lot of the heat loss could have been through evaporation.You would have conduction down the connecting wires and some convection through the argon gas in the glass envelope.. Also, the filament is not totally screened from radiation from the Earth. I bet you'd only get a very few degrees of lowering of temperature. at most.A possible experiment might be to use a large , deep, parabolic reflector and a sensitive thermistor.
They used to make car headlamps about 20cm diameter with an integral parabolic reflector and tungsten filament.I have often wondered if one measured the resistance of the filament and pointed it to either the 300°K Earth or the 2.74°K clear sky one could notice a difference in the resistance
I think Sophiecentaur has underrated the Resistance change of the Tungsten filament between 300°K and its maximum operating temperature of about 3400°K I believe it is about 20 times.I realise that a headlamp makes a pretty crude radiometer but I just wondered if anyone had tried it.
If you were really worried by heating effects, couldn't you build a system that applied a current, and measured the resulting voltage and alter the current so as the product of voltage and current was a constant. You could then measure the resistance by dividing the measured voltage by the applied current.