11 May
2010
11 May
'10
2:06 p.m.
On 11 May 2010 at 15:49, Brad Parker wrote:
I've seen this too - also helping my son :-)
My guess is the resistance changes as the filament begins to "glow".
Or at least that's what I remember thinking at the time. caveat:
there may have been gin envolved - as I recall it was the weekend.
I'd be curious to know the real answer.
Well, tungsten-filament incandescents have a strong positive
temperature coefficient to their resistance, so it's entirely likely
that the lamp with the highest cold resistance tends to glow much
more brightly when in series.
Cold resistance of a typical tungsten filament is around 1/10 that of
the filament when the lamp is at full brightness.
This is one of contributing factors to lamps going "poof" most often
when the current is first applied. One used to be able to buy
graphite "buttons" to insert in a lamp socket to ameliorate the
effect. (Carbon has a higher cold resistance than hot resistance).
Old 1950's era TV sets sometimes had an inrush current limiter to
extend the life of the tube heaters.
To extend your son's experiment a bit, it might be interesting to
devise a switched setup where lamps can be switched quickly from
parallel to series operation. (a DPDT switch should do the trick) to
see if pre-heating the filaments has an effect.
In other words, don't stop at the "book" experiment, but attempt to
find a cause. That's what *real* learning is about, after all.
--Chuck