This is a close-up view of a ceiling-mounted fire sprinkler. The exposed part (what you can see on the ceiling) of this sprinkler measures 1 5/8" long and 1 1/4" wide. Many residential fire sprinklers are semi-recessed into the ceiling so that as little as 1/2" is visible. Some models can be completely recessed and covered with flat plates that match the ceiling.
The threaded end screws into a water pipe in the ceiling and is not visible from below.
The cap seals the opening and prevents water from flowing out. The glass bulb holds the cap/seal in place.
The glass bulb is filled with liquid.
Heat from a fire expands the liquid, which breaks the bulb. The cap/seal falls away and water will stream out.
The water stream hits the deflector, which breaks it into a spray of tiny droplets.
