Smoke alarms

[sabrinah]

How do smoke alarms detect smoke?

[Sensei]

There is a small radioactive component within the detector. When smoke passes throught the radioactive field, the flow of the radiation is interrupted (sort of like a rock in a stream). The detector detects the change in the radiation, not the smoke itself. I believe there is actually a warning on the detector about radiation.

[Henryn]

There's an indepth article on smoke alarms in this weeks How it works section.