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The variation is certainly noticeable when one uses the Pasco "light
sensor" with a datalogging interface. We were using a halogen lamp
on 12V
ac as the source in a polarisation experiment some time back. The
measurements drifted up and down in a regular way that turned out to
be a
slow beat between the 100 Hz of the mains and the sampling
frequency, not
quite 20 Hz. We learnt that we had to use well-smoothed DC. We also
learnt,
btw, that halogen lamps die very quickly when run at below their
rated voltage.
Mark
At 14:28 07/03/03 -0800, Bernard Cleyet wrote:
"There is some hysteresis caused by the time lag of thetemperature
response tothese
the heating/cooling cycles, but the difference in resistance due to
temperature excursions isn't a great fraction of the overallaverage
resistancedisk
as a function of time."
A 71/2 W lamp works well as a strobe for setting the speed of a
turntable. Knowing this prompted me to check in my favo. referencebook,
Levi's Applied Optics. Sure nuff, lotsa data.and total
It gives heating ( brightness 0=>90% and cooling 100=>10%) times
variation in brightness for various lamps powered by 50 and 60 Hz.determine the
They also have extensive tables on W props from which one may
temperature and resistance. More easily, one (bc hopes to do) mayuse a dual
beam (dual trace) o'scope directly. Or more hi-tech, digitize andplot the
product.from mean,
Here's some, 115V 60 Hz, extracted data:
gas filled first:
W M*% t(h) (msec) t(c) (M* % variation
total;of
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
brightness variation (M*) with wavelength for a 120 V lamp (wattagenot given
-- is this like voltage?). The variation is 10% to 4%, 0.4 micronto 1.2
(respectively)values of M*.
The large values of t(h) and t(c) paradoxically belie the large
The paradox is resolved by examining the graphs of heating andcooling. The
curves are approximately exponential, so, for example, a 40 W (gasfilled)
lamp's brightness (lumens) drops from 100% to less than 35% in onlyten
milliseconds. the eye's is a log detector, so not so obvious.Despite W lamp
resistance is ~ linear WRT the applied voltage (quasi DC), Isuspect its
variation should be quite noticeable.its
bc
David Bowman wrote:
Regarding Mark's observation:
Interesting to see the evolution of consensus on this topic. In
brought itprevious incarnation (inverbation?) on this list (when I
up) thethe
view was that the filament lamp is a non-ohmic *device* even if
justtungsten wire is an ohmic conductor.
Mark
I suspect that this phenomenon may be a function of the set of
Maybewhich list members happened to respond in these two cases.
timelast time more of the non-ohmites answered the call, and this
heating &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
America) ofcooling cycles (1/100 sec in Europe & 1/120 sec in North
thethe applied AC waveform. To a semi-decent approximation, when
formslamp is operating the instantaneous voltage and current wave
heating/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
averagecooling cycles, but the difference in resistance due to these
temperature excursions isn't a great fraction of the overall
ACresistance as a function of time.
Of course if the lamp was operated with a much higher frequency
hystereticpower source, then even these tiny (so-called non-ohmic)
*such* aeffects would vanish. We just must not operate the lamp at
ahigh frequency that the filament's inductive reactance becomes
ansignificant fraction of its resistance (otherwise it would be
wideinductive load rather than a resistive one). But there is a
range of frequencies where this is not a problem and still the
lamp acts fully "ohmically".
David Bowman
Mark Sylvester
UWCAd
Duino Trieste Italy