Background

A smoke detector is a device that senses the presence of smoke in a building and warns the occupants, enabling them to escape a fire before succumbing to smoke inhalation or burns. Equipping a home with at least one smoke detector cuts in half the chances that the residents will die in a fire. In 1992 the readers of R&D Magazine selected home smoke alarms as one of the "30 Products that Changed Our Lives." Smoke detectors became widely available and affordable in the early 1970s. Prior to that date, fatalities from fires in the home averaged 10,000 per year, but by the early 1990s the figure dropped to fewer than 6,000 per year.

Two basic types of smoke detectors are currently manufactured for residential use. The photoelectric smoke detector uses an optical beam to search for smoke. When smoke particles cloud the beam, a photoelectric cell senses the decrease in light intensity and triggers an alarm. This type of detector reacts most quickly to smoldering fires that release relatively large amounts of smoke.

The second type of smoke detector, known as an ionization chamber smoke detector (ICSD), is quicker at sensing flaming fires that produce little smoke. It employs a radioactive material to ionize the air in a sensing chamber; the presence of smoke affects the flow of the ions between a pair of electrodes, which triggers the alarm. Between 80 and 90% of the smoke detectors in American homes are of this type. Although most residential models are self-contained units that operate on a 9-volt battery, construction codes in some parts of the country now require installations in new homes to be connected to the house wiring, with a battery backup in case of a power failure.

The typical ICSD radiation source emits alpha particles that strip electrons from the air molecules, creating positive oxygen and nitrogen ions. In the process, the electrons attach themselves to other air molecules, forming negative oxygen and nitrogen ions. Two oppositely charged electrodes within the sensing chamber attract the positive and negative ions, setting up a small flow of current in the air space between the electrodes. When smoke particles enter the chamber, they attract some of the ions, disrupting the current flow. A similar reference chamber is constructed so that no smoke particles can enter. The smoke detector constantly compares the current flow in the sensing chamber to the flow in the reference chamber; if a significant difference develops, an alarm is triggered.

History

The development of these life-saving appliances began in 1939 when Ermst Meili, a Swiss physicist, devised an ionization chamber device capable of detecting combustible gases in mines. The real breakthrough was Meili's invention of a cold-cathode tube that could amplify the small electronic signal generated by the detection mechanism to a strength sufficient to activate an alarm.

Although ionization chamber smoke detectors have been available in the United States since 1951, they were initially used only in factories, warehouses, and public buildings because they were expensive. By 1971 residential ICSDs were commercially available; they cost about $125 per detector and sold at a rate of a few hundred thousand per year.

A fluny of new technological developments occurred over the next five years, reducing the cost of the detectors by 80% and boosting sales to 8 million in 1976 and 12 million in 1977. By this time, solid-state circuitry had replaced the earlier cold-cathode tube, significantly reducing the size of the detectors as well as their cost. Design refinements, including more energy-efficient alarm horns, enabled the use of commonly available sizes of batteries rather than the hard-to-find specialty batteries that had previously been required. Improvements in the circuitry made it possible to monitor both the decrease in voltage and the build-up of internal resistance in the battery, either of which would trigger a signal to replace the power source. The new generation of detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned for more effective operation.

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