Ask A Scientist© Environmental Earth Science Archive

name         Ellen
status       student
location     OR

Question -   I wanted to know how long the average light bulb filament
is. My Current Science Magazine said that the filament was much longer
than the science room. (pretty big!)
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Not my subject, but I wonder about the length. The fact the bulb is sealed
with a gas inside, leads me to think it works on an atomic level. The current
through the filament exciting atoms and electrons.  The ultimate product
light. I do know something ticker takes longer to react, so a thin filament,
probably of some exotic  material works best.  I recall Edison trying many
things for the filament on the first bulb. So I can see thin, but my guess
would be a smaller amount of feet, than room size.  Even 4 feet of a finer
than hair like wire, when wound is a lot of work and not that small.  What
kind of filament would be in a Christmas tree light bulb?  Just the logic 
of cost.  In the dollar store you
can get two or three bulbs for a dollar.  If there were filaments of 30 to 40
feet, that would not seem consistent with cost, especially if the filament is
special.  I might even try an experiment to take one apart, you may not nee
to do unwinding  just look closely with a good magnifying glass.

James Przewoznik
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That sounds slightly exaggerated, but the filaments in 120vac bulbs are 
much longer
than they look inside the bulb.
They have to be, to have the high resistance needed to draw the right 
amount of power at 120v.
(remember Power = Volts^2 / Resistance, P=V^2/R.)

If you get a 12v tungsten-halogen bulb, the kind made entirely of clear 
glass and a wire,
you can see the real filament, and it is sometimes a simple spiral of 
visibly thick shiny wire.

For 120v operation, they need to make the wire much thinner and longer, 
and pack it into the same bulb,
and preferably keep it all close together for efficiency and longer life.
So they double- spiral the long, tiny wire.
First it is coiled very tightly, around a diameter almost the same as the 
thicker wire in a 12v bulb.
then they spiral _that_
into the large visible spiral
which you may be able to see if the bulb's glass is clear.
Then it is bent into the overall U shape which points its ends into the 
glass base.

12v is low voltage and it takes a largish 4 amps to make 50watts.
    12v/4 amps is only 3 ohms.  Just a longish piece of medium-thin wire, hot.

120v is high voltage and it takes less, 0.4 amps, to make 50watts.
    120v / 0.4 amps is 300 ohms, which does take many feet of medium-thin 
 wire,
             or a few feet of thinner-than a hair wire.

Jim Swenson
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