Most smoke detectors are of the so-called ionizing type. They contain a tiny source of radioactive energy, equal to 0.2 micrograms of the element americium, which is number 95 in the periodic table.
This radioactive material continuously emits a stream of alpha particles that strip electrons from oxygen and nitrogen in the air between two metal plates.
It gives the air between the plates an electrical charge.
The battery in the smoke detector ensures that there is a weak current between the metal plates.
If smoke passes between the discs, the smoke particles neutralize oxygen and nitrogen ions. Then the current drops and the sensor emits a powerful sound that can wake up a sleeping person.
Smoke detectors are cheap and have saved many lives. Most people do not die in the fire but from smoke inhalation.
