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  Category: Articles » Home & Family » Home Security » Article

How Smoke Alarms Work

By James Gregory

Ever looked up at the ceiling, saw your smoke alarm and thought, "I wonder how that works?" Well, the first thing to understand is that there are two basic smoke alarm designs. The first and the most commonly used design is nuclear powered and is known as an ionisation smoke detector, these require little space are cheap and are more common than the other type of detector, which is the photoelectric smoke detector.

Ionisation Smoke Alarms

At the heart of an ionisation smoke detector is a small amount of radioactive material called Americium 241, along with two plates with opposite electrical polarity. This sensor arrangement is held in an air flowing metal chamber and along with its support electronics is powered by a long life battery, usually 9-volt. The metal chamber acts as the negative plate; a positive plate covers the Americium 241.

So how does an ionisation smoke alarm work? Well, alpha particles are generated by the Americium element, these alpha particles ionise the nitrogen and oxygen atoms that circulate through the smoke detector. This simply means that electrons are stripped from both the nitrogen and oxygen elements, when this happens the "air" inside the smoke detector becomes positively charged. These "air" atoms are drawn to the negative (metal chamber) plate, while the negatively charged electrons are drawn to the positive plate. As long as this process continues the smoke alarm will not activate. However, when smoke enters the device it interferes with the ionisation process and the alarm is triggered. Ionisation smoke alarm detectors are considered sensitive enough to do their job correctly and robust enough to last several years under normal operating conditions.

Should you worry about radioactive Americium 241?

Being radioactive does mean that it produces radiation, however the amount is very small. Moreover, the type of radiation emitted is in the form of alpha particles, which are the weakest type of radiation. A sheet of paper can stop alpha particles. The metal canister that holds the Americium is more that capable of containing any harmful radiation. You should however make sure that any discarded ionisation detectors are properly disposed of, check with your local authority or council to find out the preferred method in your area.

Photoelectric Smoke Alarms

These are probably the least popular type of smoke detector; this is down to the fact that they're not as sensitive as an ionisation detector. They function by employing two channels; one emits light, while the other channel detects the light. If smoke finds it way into the channels it will interfere with the light and trigger the alarm.

The type of smoke alarm you choose really depends on the application. While ionisation detectors are seen as the preferred choice, they perform best in detecting fast flaming fires with small smoke particles. Optical sensor smoke alarms on the other hand perform best at detecting slow, smouldering fires with large particles. It's recommended that you fit a combination of the two types of smoke alarm to your home, preferably interconnected. It's possible to string together several smoke detectors to form an impressive smoke alert system.

What ever you do install some smoke alarms today. They're cheap, easy to fit and could save you and your family's life.
About the Author
James Gregory reviews and writes about security products for To see smoke alarms like the ones mentioned in this article please visit

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