On Fri, 05 Jun 2009 16:00:04 -0700, Winston
wrote:

Ned Simmons wrote:
On Fri, 05 Jun 2009 13:01:16 -0700, Winston
wrote:

Ned Simmons wrote:

(...)

That's a pretty remarkable claim -- that, at least in the range where
a normal lamp operates, the current flowing thru a filament is
completely independent of the voltage applied to it.
Your model shows a net reduction of voltage available to the bulb.

Not my model. It's the one you proposed in your previous post ("Model
it as two PTC resistors in series, one of which is about a percent of
the value of the second one"), except I moved one resistance outside
the lamp's envelope and turned a knob rather than tweaking it with a
laser.

But first, you changed R1 to a linear resistance.

But the voltmeter was connected only across the lamp, the point being
to determine how an unmodified filament behaves when the voltage
across it varies in small increments. And for that purpose, as long as
the measurements are taken at equilibrium, it doesn't matter whether
R1 is temperature dependent or not.

So we know that small changes in the voltage across a lamp result in
proportionally smaller, but measurable, changes in current. Now
replace R1 (my variable resistor) with a second lamp. Apply the
appropriate voltage to the string and note the current. Thin the
filament in the second lamp with a laser, or a genie with an angle
grinder. As we agreed before, the network will reach a new
equilibrium, with the voltage divided according to the new ratio of
the filament resistances, such that the voltage across the unmolested
lamp is slightly higher than it was before. Consequently, the current
in the circuit will have increased a small, but detectable amount, and
so has the sum of the power consumed by the two lamps.

Which is contrary to what was reported in the article. In other words,
a localized thinning of the filament can't explain an increase in
brightness without an increase in power.

--
Ned Simmons