5 ms·

As I understand it the issue with ordinary incandescents is that the alternating heat and cold from being used eventually causes the glue attaching the glass to

As I understand it the issue with ordinary incandescents is that the alternating heat and cold from being used eventually causes the glue attaching the glass to the metal part to crack, which lets air into the bulb (ordinarily a vacuum/inert gas). The next time you turn it on, the air causes the filament to burn, causing the characteristic flare-up and scorch.

MisterTea 5y ago
Er, that glue only holds the glass bulb in the socket. The atmosphere contained within is completely glass sealed. What happens is the filament sputters away (like evaporation) and becomes thinner over time. It eventually becomes so thin it breaks from thermal shock when turned on. The dark spot on dead bulbs in the evaporated tungsten. Fun fact: In theater a blown bulb can ruin a production so some of the more critical lamps are dimmed instead of power cycled to avoid blowing a lamp.
- devindotcom 5y ago
  Really? I was under the impression that the glass isn't a fully enclosed "bulb" so to speak, but is cut off at one end to accommodate the rest of the necessary hardware, and glue completes the seal. Perhaps you're right. I can't seem to find a diagram or anything that shows this, though.
  - xenadu02 5y ago
    The leads are inserted through a glob of hot glass, along with a glass tube. The bulb's open base is heated to make it soft, the blob containing the leads with attached filament are inserted, then the base is welded to the lead blob. Last the tube is used to evacuate the bulb and/or inject inert gasses. While under this vacuum the tube is melted and smashed closed, permanently sealing the bulb. Attachment of the base comes later.
  - MisterTea 5y ago
    If you examine a clear incandescent bulb, you will see that the support for the filament is hollow and has a tube inside extending down. This little "straw" is the point of connection for evacuation where the air is removed and then back-filled with argon. The argon coaxes the tungsten back onto the filament preventing rapid sputtering which would happen in a pure vacuum. After the argon the little tube is heated until it hits melting point and crimped off for a near perfect hermetic seal. The argon pressure is still well below atmosphere so the bulb is still considered under "vacuum" compared to atmosphere.

MisterTea 5y ago

Er, that glue only holds the glass bulb in the socket. The atmosphere contained within is completely glass sealed. What happens is the filament sputters away (like evaporation) and becomes thinner over time. It eventually becomes so thin it breaks from thermal shock when turned on. The dark spot on dead bulbs in the evaporated tungsten. Fun fact: In theater a blown bulb can ruin a production so some of the more critical lamps are dimmed instead of power cycled to avoid blowing a lamp.

devindotcom 5y ago
Really? I was under the impression that the glass isn't a fully enclosed "bulb" so to speak, but is cut off at one end to accommodate the rest of the necessary hardware, and glue completes the seal. Perhaps you're right. I can't seem to find a diagram or anything that shows this, though.
- xenadu02 5y ago
  The leads are inserted through a glob of hot glass, along with a glass tube. The bulb's open base is heated to make it soft, the blob containing the leads with attached filament are inserted, then the base is welded to the lead blob. Last the tube is used to evacuate the bulb and/or inject inert gasses. While under this vacuum the tube is melted and smashed closed, permanently sealing the bulb. Attachment of the base comes later.
- MisterTea 5y ago
  If you examine a clear incandescent bulb, you will see that the support for the filament is hollow and has a tube inside extending down. This little "straw" is the point of connection for evacuation where the air is removed and then back-filled with argon. The argon coaxes the tungsten back onto the filament preventing rapid sputtering which would happen in a pure vacuum. After the argon the little tube is heated until it hits melting point and crimped off for a near perfect hermetic seal. The argon pressure is still well below atmosphere so the bulb is still considered under "vacuum" compared to atmosphere.