When incandescent light bulbs are manufactured, they are filled with an inert gas to insulate the filament and prevent its sublimation. The pressure inside a freshly manufactured light bulb is something like 80% of the normal atmospheric pressure. The gases normally used are argon and a little bit of nitrogen.

Why not fill them with gas at normal atmospheric pressure?

  • $\begingroup$ I suspect it's due to the cost/benefit relationship of using more gas vs a longer lasting filament. $\endgroup$ – jm22b Nov 12 '16 at 12:22
  • $\begingroup$ @Jacobadtr NOTE: Both gases, which are argon and nitrogen are relatively inexpensive. Cost is not therefore a determining factor. $\endgroup$ – Utsav Nov 12 '16 at 12:42
  • $\begingroup$ en.wikipedia.org/wiki/Incandescent_light_bulb#Gas_fill This suggests that a low pressure is used to reduce heat loss via conduction and convection. $\endgroup$ – jm22b Nov 12 '16 at 12:50

Light filaments are mainly tungsten, which oxygen corrodes, so reducing the lifetime of the bulb. Argon reduces this corrosion and basically, you only need a small amount, so it keeps the cost down by using the minimum amount.

This duplicate Pressure inside a light bulb seems to answer the rest of your question.

You want to have the bulb at atmospheric pressure when it's on, and hot, so having it at 80 percent when it's off, means a bulb turned on will then go to normal atmospheric pressure.

If you filled them at normal pressure when cold, they are more likely to fail or explode when heated up.

  • $\begingroup$ Cost cannot be the only reason for this cause now can it? I was looking for an answer that has something to do with air pressure maybe $\endgroup$ – Utsav Nov 12 '16 at 13:06
  • $\begingroup$ Hi, I have summarized the link in my post, but the full article does go into the subject in excruciating detail ,:) it's interesting though, especially when you think that making the perfect lightbulb brought us quantum mechanics, indirectly. $\endgroup$ – user108787 Nov 12 '16 at 13:29
  • $\begingroup$ Sorry about that, smartphones are terrible at displaying and writing on this website, regards. $\endgroup$ – user108787 Nov 12 '16 at 13:53

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