Definition of filament:
Filament is the curved metal wire inside the light bulb, the light doesn't light up it does. Filament is the continued version of the original circuit.
When the circuit comes into the light bulb, the energy gets converted into light, and lights up the bulb.

Filament is very thin electric wire that goes half-way up the bulb. It is slightly twisted and quite sharp. At the center, the
filament curves back down to the bottom of the bulb. It is flexible and threadlike and is made of electro metal. Filament can also
get scorching hot, up to 3600-4500 degrees F!


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Close up of filament starting to light.

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Whole light bulb.


Pros and Cons about Filament:
(Positives and Negatives)
  • Great light color
  • Filament light bulbs are much safer than bulbs that contain mercury
  • Easy to fix/replace
  • Expensive - $10 to $20 per bulb
  • Don't last long
  • Energy wasting (LED bulbs save energy)

All pictures from Google, hover over the picture the address/credits will be there.
Mercury information (2nd pro) gotten from Wikipedia.