How much of a vacuum do lightblubs have

[john_g]

I came across the item below from 1902, which raised in my mind 2 questions. If a failing bulb is hot (mine are always hot), does that mean with a hard vacuum the bulb should be cool? Therefore are our incandescent bulbs (if you can find them in the UK) made to a lower quality than they should be? So instead of an inert gas should they just have a hard vacuum?

[rickoff]

Reply to John:

Hi John,

Light bulbs are typically made with a very thin shell of glass, and would not withstand atmospheric pressure, of about 14 psi, without breaking unless the chamber is filled with something that counters the external pressure. Thus, the use of an inert gas. The shell could, of couse, be made of glass thick enough to withstand atmospheric pressure with a total vacuum inside, but this would also make manufacture and purchase of the bulbs more costly.

A bulb filament produces light because it is heated to a high temperature due to its highly resistive properties. In atmosphere, the same filament would burn out immediately. An inert gas is by definition one that does not promote combustion, so there should be no reason why a filament surrounded by inert gas would burn hotter than a filament surrounded by vacuum, unless of course pressure vs non-pressure is a consideration. That being said, Irving Langmuir discovered in 1913 that filling a bulb with inert gas resulted in doubling the light output over that of a vacuum filled bulb. Since 90% of the power used by an incandescent light bulb is converted to heat, it would seem reasonable to assume that any bulb which burns brighter is burning hotter. So a bulb with internal vacuum would burn cooler, but produce just half the light output.

Best regards,

Rick