Re: Ohm's Law

[Bernard Cleyet <anngeorg@PACBELL.NET>]

Yes the temp. must be high enuff to disassociate the tungsten halogen gas on the
filament.   At lower temps. the reverse reaction occurs.  It's intended to remove
the W deposited on the envelope.

You high be able to predict the diameter of the filament as the function given by
Levi is:

M* = 0.0069 * D^1.06,  where D is the filament diameter in mm and M* is the
fractional change in brightness (Lumens).  this is for gas filled lamps and 60 Hz
mains.

Tho Levi discusses the tungsten iodine cycle, I doubt the data and other I've
given was obtained from such lamps.  I wouldn't be surprised if it's approximately
correct for them tho.

BTW I've head of 50 Hz mains but not 100!  Also did you find the variation was, as
I predict, asymmetrical?

bc  who thinks maybe you're referring to the "flicker" frequency with 50Hz mains
-- obviously UK speak (Australia too?)


Mark Sylvester wrote:

> 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 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
>
> Mark Sylvester
> UWCAd
> Duino Trieste Italy