Why go the trouble of surrounding a filament with vacuum surounded by a thin glqass barrier? Glass is fragile enough by itself, but why confound the problem by making an odd shape out of it only a few millimeters thick?
Why not just surround the wire with a glass cladding and be done with it? It would be stronger, cheaper top make, and could even be made so as not to shatter into the horrible jaggies.
Perhaps
Glass is an insulator, but vacuum is the best insulator. Of heat I mean. Thusly, a greater deal of heat would be conducted away from the filament, thus requiring a greater amount of electricity to continue heating the tungsten up to the proper temperature. The light bulb would be 1% efficient instead of 5%. Or some absurdly small number like that.
Again, glass is a worse insulator than vacuum. Glass might melt att the temperatures needed to make tungsten glow white, and would not lose that heat easily. This would be a dangerous situation.
Heating expansion problems. Glass is brittle, and might crack or explode if heated suddenly by a high wattage incandescent light.
I believe that they are actually filled with an inert gas at low pressure; atoms of metal from a hot filament in a vacuum can evaporate off, but apparently when there’s gas present an evaporated atom stands a fair chance of being ‘bounced’ back onto the filament, prolonging the life.
Welcome to the SDMB! Per Mangetout your assumptions about the final construction (ie “evacuated”) of light bulbs is incorrect. Also FWIW a light bulb’s glass shell is considerably thinner than “a few millimeters”. How things work
Additionally, before the bulb is assembled, the filament is dipped in something called “getting.” Upon the initial in-factory lighting, this getting combusts, consuming any residual oxygen left in the bulb.
Another point is that if you have a hard vacuum inside the bulb, then the pressure outside is more likely to force air (including oxygen) through any microscopic leaks or imperfections (possibly diffusion through the glass itself - can anyone confirm this or otherwise?) - oxygen inside the enclosure is bad because the filament will burn.
I once worked in the “sealed-beam” headlights section of our plant. The tungsten wire came to us in the right diameter, and our machines wound it into filaments. The filaments were clamped into slots in the support rods. After the lens was welded on top, the air was sucked out and replaced with nitrogen. Then the little tube on the back was heated until soft and nipped shut. The lamp was test-lit, and if it still had air in it, the filament would burn, filling the lamp with thick yellow smoke. The whole process was much more complicated than that, and it would take quite a while to describe.
By the way, here’s an answer. If the filament were simply encased in solid glass, the glass would absorb the filament’s heat. It would take an unacceptably long time to get hot enough to light up.
cdhostage, I think all 4 of your reasons are valid. But there are also more. Note that the bulbs in overhead projectors are quite small and of course generate a lot of light. The glass of these bulbs are quite pure and transparent. Also, you are NOT supposed to touch your finger to the bulb while installing. The grease from your fingers will cause a slight variation in heating of the bulb and it will break. Note also the bulb gets so hot that you would burn yourself touching it but it is safely enclosed when running. (And with a fan.) Imagine a 100w bulb’s heat and now concentrate it in 1/10th the surface area. Ouch.
Presumably, in mass production, the glass of regular light bulbs is really cheap stuff and you can’t expect Joe Consumer to understand “don’t ever touch the bulb”, hot or cold.
As far as solidly encasing it. Everything would crack. The bulb, the filament, the connection with the base.
The shape itself is pretty much legacy. Folks will buy the kind that fit the lamps they have, and folks will make the type of lamp that most people want, and most people want lamps that they can get bulbs for easily.
The shape of the bulb hasn’t changed all that much since Edison.
And red-hot glass is a conductor (white hot glass being even more conductive.)
Old physics classroom demo: a glass rod is wrapped with two wires with a couple of cm between the two wrappings. Connect the two wires to a high voltage power supply. Heat up the exposed glass with a torch. Suddenly the red-hot glass spontaneously glows yellow, then incandescent white, then it dribbles away and breaks the circuit. It became conductive and temporarily formed a high-power resistor.
Molten glass, being full of mobile ions, is actually an electrolyte. Hardened glass has just as many ions, but they cannot flow along as an electric current.
However, vacuum tubes, believe it or not, do have a near-vacuum inside. Vacuum tubes are what we used to use instead of transistors for electric amplification. Also, the computer screen that you’re looking at right now is one. They need to have a vacuum because the stream of electrons would ionize any gas inside screw up what it’s trying to do.
I’ve seen CRT manufacturing, and they use “getters.” A getter is a little pot of some dark metallic material, located inside the tube, facing the glass wall. After the tube is sealed, the getter is heated using electrical induction from the outside, causing the material to boil off. The material then hits the glass wall and sticks. But on its way there, it will carry with it any air molecules it encounters, making a more perfect vacuum inside. After that’s done, the tube glass near the getter will be mirrored in that spot because it has metal on its backside. If you ever see a bare CRT, you’ll see mirrored spots on it.
A light bulb filament couldn’t be encased in glass because the glass would be molten at that temperature. Glass gets runny as it gets hot - it’s made when it’s just barely orange-hot. At white-hot it would be much runnier. And white-hot glass is the same temperature as white-hot tungsten, by the way. You can tell the temperature of a material by the EM radiation coming off it without knowing what the material is.