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In this video:

At their core, smoke alarms are very simple devices that need only two functions: a way of detecting smoke and a way of alerting people to the problem. From new technology involving lasers to old world technology that relies on a lonely individual sitting in a tower on the side of a mountain just waiting to see smoke, all do the same thing in different ways. The two most commonly used smoke alarms (mainly because they are inexpensive) are photoelectric and ionization detector alarms.

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Resources/Further Reading:

http://home.howstuffworks.com/homeim...
http://chemistry.about.com/cs/howthin...
http://www.epa.gov/radiation/radionuc...
http://www.systemsensor.com/lifesafet...
http://www.nfpa.org/categoryList.asp?...
http://en.wikipedia.org/wiki/File:Fir...

Image Credit (Thank you)

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