Ned Simmons wrote:
On Wed, 03 Jun 2009 20:49:44 -0700, Winston
wrote:

Ned Simmons wrote:

(...)

If the filament has a thin section the power density at that spot will
indeed be higher, but the overall consumption of the lamp will be
lower. (P=V^2/R) The article says, "... we could actually see this one
patch was clearly brighter than the rest of the filament, but there
was no change in the bulb's energy usage."
Yes, but bulb life is significantly reduced.

Can we really call it a "better light bulb" if we have to replace it
~twice as often? (I don't think so.)

http://en.wikipedia.org/wiki/Incande...b#cite_note-24

"Small variations in resistivity along the filament cause "hot spots" to form
at points of higher resistivity; a variation of diameter of only 1% will cause
a 25% reduction in service life. The hot spots evaporate faster than the
rest of the filament, increasing resistance at that point—a positive feedback
which ends in the familiar tiny gap in an otherwise healthy-looking filament."

As I said above, if the experiment created a hot spot by thinning the
filament the resistance of the filament, and the power consumed, would
change. The experimenters reported there was *no change* in the power
consumption.

In *net* consumption, no change was detected. I get that.
I hypothesize that the local power consumption at the thinned area of the
filament did increase, because it's increased resistance dropped more voltage
across the thinned area for a given amount of current. The increased resistance
of the filament as a whole would have decreased the filament current a tiny
amount, cancelling the effect of the local power consumption increase.

Model it as two PTC resistors in series, one of which is about a percent of the
value of the second one. Triple the value of the smaller resistance and it's
power consumption triples. The net resistance of the network as a whole is
decreased by a couple percent, causing the second resistor to decrease in value.
As a result, current increases once again and a new equilbrium is established
which is very close, if not identical to the pre-modification current.
The bulb lifetime is significantly decreased but the study did not encompass
bulb lifetime, just hotspots and spectral shift.

The relationship between power density, temperature, efficiency and
life is well understood -- you can find it in old texts that date back
to the early days of electric lighting. It's hard to believe that
these guys, whose previous work involved fiddling with the emissivity
of metals, are unaware of that relationship and have misinterpreted
the results of their experiment.

They didn't misinterpret, as far as I can see.
They didn't say a word about the post-modification reliability of the bulb.
You could argue that they committed an error of omission about that, if you
were feeling very charitable.


--Winston

--

We now return you to the economic collapse, already in progress.