How do smoke alarms work asked Sabrinah

A. In a smoke alarm there are two small and parallel plates, about one centimetre apart, wired up to a battery. Radioactive americium 241 is in the centre of one of the plates, emitting alpha particles, which collide with air molecules. During the collisions, the air molecules are ionised to form positive and negative ions. The voltage from the battery makes one plate positive and the other negative, so each plate attracts ions of opposite charges, creating a current in the circuit attached to the plates. The electronics in the smoke alarm sense the small amount of electrical current that these electrons and ions moving toward the plates represent. When smoke enters the detector, its particles attach themselves to the ions and neutralise them, disrupting the current. The smoke alarm senses the drop in current between the plates and sets off the alarm.

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Q. As smoke alarms contain radiation are they dangerous

A. No, because the amount of radiation in a smoke alarm is extremely small. It is also predominantly alpha radiation, which can't even penetrate a sheet of paper. However as americium can be dangerous if inhaled you must never poke a smoke alarm, which could release americium into the air.

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Q.�Why do you have to replace smoke alarms

A.�Because the electronic alarm inside might go wrong. Electronic gadgetry is prone to failure over a period of time, just like your television may break down after several years. But because your smoke alarm could save your life manufacturers advice you to replace it about every five years.

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Q.� When I check my smoke alarm using the 'test' button does this mean it's working completely

A.� Pressing the 'test' button only tests the sounder on most smoke alarms. The proper way to test the alarm is to direct steam or smoke into it to be completely sure.

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by Lisa Cardy

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