I've been wondering about this for a while and couldn't find an answer on the internet, it's probably a simple question, but still... why do some light bulbs have a hole in their bases? I'm talking about the kind the manufacturer would put there, I've seen this in a few bulbs I have and wondered, the only thing in common all of them have is that they're incandescent (both normal and halogen), other than that it's different shapes, manufacturers, wattages, etc.

Not all of them have it of course, I've encountered some over the years and they work just fine. The design does not allow air to go inside the glass envelope, yes I've checked by cutting the thread using a rotary tool ^^ (after the filament was burnt)

Attached an example picture, zoom in and you'll see the hole in the thread.

I've noticed this on a few of my lamps, I always thought that it was due to something in the manufacturing process where they join the base to the glass and the arm? thing they might have used punches a small hole while holding the base for gluing.

Maybe to avoid pressure buildup on the most sensitive part of the bulb?

I've noticed this on a few of my lamps, I always thought that it was due to something in the manufacturing process where they join the base to the glass and the arm? thing they might have used punches a small hole while holding the base for gluing.

like a bit of metal that's part of the same mould and then gets cut?

Maybe to avoid pressure buildup on the most sensitive part of the bulb?

I'm the one asking if that was the case all of them would have a hole. Thinking.

Maybe it depends on the cement material - if it seals it completely (then a hole needs to be artificially made) or allows gasses to "breathe through" (then no hole is needed after the thing cures uo)...
I would guess the critical step would be when the bulb is inserted into the blob of cement - the hole makes sure the air from within the cap can escape and is not pushing the cement out of the joint.
Some makers form that hole around the place where the lead wire is soldered to the screw cap and after assembly it gets covered by the blob of solder, so it is not visible anymore.
Or other possibility: It is an artifact from the way how the components are being handled by the machinery during some stages of production (e.g. hold the metal cup and prevent its rotation when the shape is formed,...).
Then someone using different techniques or machines may need a different hole or no hole at all.
The exact reason for the hole presence/absence or its exact design may also vary, depending on how is the lamp assembly exactly designed and manufactured.

Caps with a hole in the side are a cheap solution used by some manufacturers to improve safety at end if life.

When a gasfilled incandescent lamp fails, an arc can be formed inside the bulb which draws several hundred amps.  It can cause the lamp to explode, or damage its lampholder or the house wiring.  For this reason, internal fuses are compulsory in most countries.  But even then, when the fuse fails a secondary arc can form inside the cap.  The suddenly vaporised copper in the cap creates a pressure pulse that may blast the glass part of the lamp out if its cap, resulting in injury or burns - and the difficulty of then getting the remaining cap out of the lampholder. 

Good quality lamps avoid this by building various arc-quenching mechanisms into the fuses or the cap area, and using better quality cements that hold the bulb firmly fixed to its cap even after a thousand hours of very hot service.  Cheap lamps employ no such protection.  They simply punch a small hole in the cap shell that lets the metal vapour from the fuse escape, with less of a pressure pulse and reduced chance of explosion.

Further details at http://www.lamptech.co.uk/Documents/IN%20Fusing.htm

Thank you for the enlightening read, James! Your site never ceases to amaze me in the knowledge and thoroughness which it possesses. Thank you for creating such a valuable resource, and for making it available to all. 

Great info. First time I read it, I'll bookmark the site to browse a bit more later on

Ahh, so that might explain why i had a dollarstore C12 lamp attempt to take flight a few months ago.

Ahh, so that might explain why i had a dollarstore C12 lamp attempt to take flight a few months ago.

Odd, never had one fail that way, they usually die with a small flicker when switching them on