Alpha particles cause the oxygen and nitrogen molecules in air to ionize:
O2   →   O2+ + e-
Positively charged particles, alpha particles and O2+, move to the negative electrode.
    Negatively charged particles, electrons, move to the positive electrode.
The flow of charged particles between the electrodes causes a current to flow.
When smoke is present, alpha particles attach to the smoke particles so air
    molecules are no longer ionized, the current drops, causing an alarm to sound.
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