861. How does a smoke alarm work? — GL, San Leandro, CA
The most common type of smoke detector uses a tiny amount of a radioactive element called americium to inject electric charges into the air. In the absence of smoke, these electric charges attach themselves to individual air molecules and the resulting ions (electrically charged molecules) move rather easily through the air. However, if there is smoke present in the air, the electric charges attached themselves to the smoke particles and the resulting charged smoke particles don't move easily through the air. The smoke detector measures how easily the charged items it produces move through the air. If these items move easily, then the air is clean. However if they don't move easily, the air contains smoke and the detector signals danger.