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Re: [Phys-l] Hewitt's three answers




On 2012, Mar 29, , at 21:49, Ludwik Kowalski wrote:

P.S.

Looking at the Figure 1 again I think that the amplitudes of fluctuations would be much smaller that 22%, if one was measuring temperatures, rather than light intensities. Yes I am thinking about the T^4 law. The tungsten spiral filament probably behaves nearly as if it were an ideal black body.


Actually not -- even tho a spiral is an improvement (like a hole in a cavity), since W is a metal it has a low emissivity. Again Applied optics has much info. on that.

Coil: A difference of 50K between the inside and the outside elements of a coiled filament indicates a brightness increase of 70% of the inside over the outside. .... It also causes an oscillatory change of the average color temperature with viewing angle of a coiled filament.

Tungston ribbon: emissivity at 0.55 micron is abut 5% from 2800K to 1600K the cooler being more emissive (0.45). From 0.3 to 0.7 the change is about 5% for the two temperatures. Curiously in the IR the hot one becomes more emissive. At 2 microns the hot is .25 and the cool about .21

The challenge is how to measure the temperature without depending on radiation. What about plotting the R(t), where R is the resistance = v/i? The dependence of R on temperature is nearly linear, in a narrow range of T.


every thing is linear in a narrow range unless poorly behaved (discontinuous).

However, You [thou] (LK) is quite correct. R^2 for a linear fit of data from table 27 op. cit. is 0.99899! (1k=> 3kK).

The graph is here:

http://www.cleyet.org/Misc._Physics/W(T).tiff

Ludwik
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bc