How Smoke alarms work

Households have had the benefit of fire detection since as early as the 1870s, when inventor William B. Watkins’ heat-sensing alarm system made it possible to automatically send a telegraph to the fire station when the temperature in a home exceeded a specified threshold. Later, fire detection capabilities improved with the development of commercial smoke detectors in the 1960s.

Today, smoke detectors are the fastest and most effective means of fire detection we have. The importance of filling ones home with smoke detectors and the importance of changing each unit’s batteries regularly are well-known, but the inner-workings of these important systems often go unexplained.

There are two main types of smoke detectors: photoelectric detectors and ionization detectors. For the curious, here’s a brief explanation of each:

Photoelectric detectors: The inside of a photoelectric detector contains a light source and a sensor. The light source is positioned so that it projects a beam of light that goes past the sensor without activating it. However, if smoke enters the chamber, the light drifts so that it hits the sensor and activates the detector’s electronic alarm. Photoelectric detectors are good at detecting things like burning fabric, which produce more smoke than flames.

Ionization Detectors: Generally more sensitive than their photoelectric counterparts, ionization detectors contain two magnets and a tiny amount of a radioactive material called americium. The radiation from the americium, which isn’t even powerful enough to penetrate a single sheet of paper, breaks electrons off the oxygen atoms in the detector’s chamber. These electrons in conjunction with the chamber’s magnets generate a small electrical current. The splitting of an electron from its atom is called ionization, so this chamber is known as the ionization chamber. If smoke enters the chamber, the ionization process is disrupted. Electronics in the detector then sense the electrical current being broken and sound the alarm.