Science Puzzle
How Does a Smoke Alarm Smell Smoke?
Burn the toast and the smoke alarm on the ceiling shrieks within seconds. Many alarms contain a speck of a radioactive metal called americium, about a five-thousandth of a gram, inside a small chamber with two metal plates and a battery.
How does a smoke alarm detect smoke, and what is the radioactive speck for?
The Answer
The americium makes the air inside the chamber carry a tiny electric current. Smoke gets in the way of that current, and when the current drops, the alarm sounds.
Air doesn't normally conduct electricity, because its molecules carry no electric charge. The americium gives off alpha particles, which crash into the air molecules in the chamber and knock electrons off them, leaving charged molecules called ions. The battery pulls the ions towards the two plates, so a tiny, steady current flows across the air between them.
When smoke drifts in, the ions stick to the smoke particles and get carried off or cancelled out. Fewer ions reach the plates, the current falls, and a circuit that's watching the current sets off the alarm.
The radiation stays inside. Alpha particles can't get through a sheet of paper, let alone the alarm's plastic case, so an alarm on the ceiling is harmless; just don't take it apart. Americium-241 has a half-life of 432 years, so the speck will far outlast the alarm. Optical alarms work another way: they shine a light across a dark chamber, and smoke scatters some of it onto a sensor. They're better at catching slow, smouldering fires, while the ionisation kind reacts faster to fast, flaming ones.
The principle: Ionisation. Alpha particles knock electrons off air molecules so the air carries a small current; smoke mops up the charged particles, the current falls and the alarm sounds.