A hollow
glass ball with an opening at one end sealed with metal, usually about 7 centimetres in diameter, containg a
wire that produces light when
electricity is run through it. Conventional lightbulbs are fairly inefficient and thus also produce a lot of heat.
The metal casing on the non-round end has a spiral groove designed to let the lightbulb lock in to place in a socket in a lamp or a ceiling or wall fixture. Thus, changing a lightbulb is referred to as screwing. This casing is metal to conduct electricity, and connects with two metal leads in the socket to complete a circuit, one touching the tip, one touching the side.
There are many different sizes, wattages, and varieties of lightbulbs. In general, the lower the wattage, the dimmer the light produced, however there are many low-energy fluorescent bulbs which, while expensive, are more efficient and produce a bright light at a great energy savings. Halogen bulbs, which require a special fixture and are tricky to replace, produce the brightest light and are long lasting.
Lightbulbs are a perennial favourite topic of insulting jokes, usually in the form of "How many (members of some group) does it take to change a lightbulb?", playing on the idea that changing a lightbulb is such a simple task that it would take great stupidity to foul up. See: Lightbulb jokes. In practice, changing a lightbulb isn't easy, damnit. Lightbulbs are usually too high to comfortably reach, are very hot to the touch if they've just just burned out, and you have to change them in the dark or holding a flashlight in your teeth (god help you if your flashlight burns out too), and even if you're in a well-lit room, you can't see the socket while you screw the lightbulb in. There's no easy way to test if the connection is completed other than by turning it on, which can leave you with a hot lightbulb in your hand suddenly when the connection closes, and finally, many lamp switches and wall switches leave it ambiguous as to whether they are in the on position or not in the absence of a functionning lightbulb. So what if it takes 17 of you, one to hold the bulb and 16 to turn the house! At least you have moral support! (grin)
Thomas Alva Edison is credited with the invention of the lightbulb.