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A radioactive isotope of Americium231 is used in smoke alarms because they emit alpha particles. The alpha particles ionise the air molecules between the charged plates. A current can now flow between the charged plates. A current detector measures the current, and if it’s high enough, the alarm doesn’t sound.
When smoke fills the smoke detector, it fills the gap between the charged plates.
This slows the ions between the charged plates, which reduces the flow of current in the circuit. The current detector detects a lower current, which causes the alarm to sound. When the smoke is cleared from between the charged plates, the ions move faster between the charged plates, and the current increases, which causes the smoke alarm to turn off.

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