Answer
$\alpha-$particle emitted by $Am^{241}$ ionizes the air molecules between the plates. The battery voltage makes one plate positive and the other negative, so each plate attracts ions of opposite charge. This creates a current in the circuit. The presence of smoke particles in between the plates reduced the current as the ions collide with smoke particles and are usually absorbed. This drop in current triggers alarms.
Work Step by Step
$\alpha-$particle emitted by $Am^{241}$ ionizes the air molecules between the plates. The battery voltage makes one plate positive and the other negative, so each plate attracts ions of opposite charge. This creates a current in the circuit. The presence of smoke particles in between the plates reduced the current as the ions collide with smoke particles and are usually absorbed. This drop in current triggers alarms.
