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"There is some hysteresis caused by the time lag of the temperature
response to
the heating/cooling cycles, but the difference in resistance due to these
temperature excursions isn't a great fraction of the overall average
resistance
as a function of time."
A 71/2 W lamp works well as a strobe for setting the speed of a disk
turntable. Knowing this prompted me to check in my favo. reference book,
Levi's Applied Optics. Sure nuff, lotsa data.
It gives heating ( brightness 0=>90% and cooling 100=>10%) times and total
variation in brightness for various lamps powered by 50 and 60 Hz.
They also have extensive tables on W props from which one may determine the
temperature and resistance. More easily, one (bc hopes to do) may use a dual
beam (dual trace) o'scope directly. Or more hi-tech, digitize and plot the
product.
Here's some, 115V 60 Hz, extracted data:
gas filled first:
W M*% t(h) (msec) t(c) (M* % variation from mean,
total;
40 27 65 26
100 13 125 59
500 4.5 380 190
vacuum
6W 74! 39 12
40 14 128 58
Another, rather interesting, but conforms to intuition, is a graph of
brightness variation (M*) with wavelength for a 120 V lamp (wattage not given
-- is this like voltage?). The variation is 10% to 4%, 0.4 micron to 1.2
(respectively)
The large values of t(h) and t(c) paradoxically belie the large values of M*.
The paradox is resolved by examining the graphs of heating and cooling. The
curves are approximately exponential, so, for example, a 40 W (gas filled)
lamp's brightness (lumens) drops from 100% to less than 35% in only ten
milliseconds. the eye's is a log detector, so not so obvious. Despite W lamp
resistance is ~ linear WRT the applied voltage (quasi DC), I suspect its
variation should be quite noticeable.
bc
David Bowman wrote:
> Regarding Mark's observation:
>
> >Interesting to see the evolution of consensus on this topic. In its
> >previous incarnation (inverbation?) on this list (when I brought it
up) the
> >view was that the filament lamp is a non-ohmic *device* even if the
> >tungsten wire is an ohmic conductor.
> >
> >Mark
>
> I suspect that this phenomenon may be a function of the set of just
> which list members happened to respond in these two cases. Maybe
> last time more of the non-ohmites answered the call, and this time
> the ohm-ites responded.
>
> The thing is that an incandescent lamp plugged in to the local
> electric utility will behave, to a pretty decent approximation,
> 'ohmically' because the filament doesn't change its absolute
> temperature by a very great fraction over the time between heating &
> cooling cycles (1/100 sec in Europe & 1/120 sec in North America) of
> the applied AC waveform. To a semi-decent approximation, when the
> lamp is operating the instantaneous voltage and current wave forms
> obey V(t) = R*I(t) throughout all phases of the applied AC wave
> where R is almost time independent. There is some hysteresis
> caused by the time lag of the temperature response to the heating/
> cooling cycles, but the difference in resistance due to these
> temperature excursions isn't a great fraction of the overall average
> resistance as a function of time.
>
> Of course if the lamp was operated with a much higher frequency AC
> power source, then even these tiny (so-called non-ohmic) hysteretic
> effects would vanish. We just must not operate the lamp at *such* a
> high frequency that the filament's inductive reactance becomes a
> significant fraction of its resistance (otherwise it would be an
> inductive load rather than a resistive one). But there is a wide
> range of frequencies where this is not a problem and still the
> lamp acts fully "ohmically".
>
> David Bowman