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.

In the experimenter's case, the amount of voltage to the bulb remained
the same but it was distributed differently in the filament eg. a 'hot spot'
area dissipating significantly more power than it had before it was thinned.

And in my experiment, the voltage across the two resistors was also
constant.


I said that the positive temperature coefficient of the filament would tend
to limit the change of the current through the bulb as the power distribution
in the bulb was changed by thinning some portion of the filament.

What you said (in order to explain the experimenters' observation that
no change in power accompanied the increase in light output) was that
the PTC of the untreated section of the filament would cause the
current to restabilize at a point "very close, if not identical to the
pre-modification current." For that to happen, the current passing
thru the untreated filament must be independent of the applied
voltage, and that's clearly not the case.


Your numbers show that the variation in current through your bulb was
about half the variation in voltage across the bulb. A one volt change
across the bulb caused a 2 milliamp change in current. This is the
nonlinear positive temperature coefficient variation I was on about.

Had that PTC effect not been in place, we would expect to see a 5 milliamp
change in current for a 1 volt change. A new current equilibrium was
established that was within 0.35% of the pre-modification current, under
a voltage change of 0.82%.

Exactly. If treating a spot on the filament changed its resistance (as
I changed the resistance in series with my filament), the resistance
of the adjacent untreated filament would change in the opposite
direction, but the magnitude of change would be smaller. In other
words, there'd be a net change in the lamp's resistance, which would
show up as a change in power.

--
Ned Simmons