A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors/alarms are usually housed in plastic enclosures, typically shaped like a disk about 125 millimetres (5 in) in diameter and 25 millimetres (1 in) thick, but shape and size vary. Smoke can be detected either optically (photoelectric) or by physical process (ionization). Detectors may use one or both sensing methods. Sensitive detectors can be used to detect and deter smoking in banned areas. Smoke detectors in large commercial and industrial buildings are usually connected to a central fire alarm system. Household smoke detectors, also known as smoke alarms, generally issue an audible or visual alarm from the detector itself or several detectors if there are multiple devices interconnected. Household smoke detectors range from individual battery-powered units to several interlinked units with battery backup. With interlinked units, if any unit detects smoke, alarms will trigger all of the units. This happens even if household power has gone out. Residential smoke alarms are usually powered with a 9-volt battery, a non-removable "10 Year Sealed" battery, or by mains electricity. Some smoke alarms use a combination of the two, usually using a battery as an extra power source should loss of mains power occur. Commercial smoke detectors issue a signal to a fire alarm control panel as part of a fire alarm system. Usually, an individual commercial smoke detector unit does not issue an alarm; some, however, have built-in sounders. The risk of dying in a residential fire is cut in half in houses with working smoke detectors. The US National Fire Protection Association reports 0.53 deaths per 100 fires in homes with working smoke detectors compared to 1.18 deaths without (2009–2013). Smoke detectors are not suitable for every location in a building, for instance in a kitchen of a domestic property, where a heat detector would be more suitable instead.
History
The first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton, an associate of Thomas Edison. In 1902, George Andrew Darby patented the first European electrical heat detector in Birmingham, England. In the late 1930s, Swiss physicist Walter Jaeger attempted to invent a sensor for poison gas. He expected the gas entering the sensor to bind to ionized air molecules and thereby alter an electric current in a circuit of the instrument. However, his device did not achieve its purpose as small concentrations of gas did not affect the sensor's conductivity. Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current. Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit's current. Jaeger's experiment was one of the developments that paved the way for the modern smoke detector. In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines. He also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism so that it was strong enough to activate an alarm. In 1951, ionization smoke detectors were first sold in the United States. In the following years, they were used only in major commercial and industrial facilities due to their large size and high cost. In 1955, simple "fire detectors" for homes were developed, which detected high temperatures. In 1963, The United States Atomic Energy Commission (USAEC) granted the first license to distribute smoke detectors that used radioactive material. In 1965, the first low-cost smoke detector for domestic use was developed by Duane D. Pearsall and Stanley Bennett Peterson. It was an individual, replaceable, battery-powered unit that could be easily installed. The "SmokeGard 700" was beehive-shaped, fire-resistant, and made of steel. The company began mass-producing these units in 1975. Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors. The first single-station smoke detector was invented in 1970 and was brought out the next year. It was an ionization detector powered by a single 9-volt battery. It cost about US$125 (equivalent to $1,036 in 2025) and sold at a rate of a few hundred thousand units per year. Several developments in smoke detector technology occurred between 1971 and 1976, including the replacement of cold-cathode tubes with solid-state electronics. This greatly reduced the detectors' cost and size and made it possible to monitor battery life. The previous alarm horns which required special batteries were replaced with horns that were more energy-efficient and allowed the use of widely available batteries. These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned to make the operation more effective. The rechargeable batteries were often replaced by a pair of AA batteries along with a plastic shell encasing the detector. The photoelectric (optical) smoke detector was invented by Donald Steele and Robert Emmark from Electro Signal Lab and patented in 1972. The 10-year-lithium-battery-powered smoke alarm was introduced years later, in 1995.
Design Smoke can be detected using a photoelectric sensor or an ionization process. Fire without smoke can be detected by sensing carbon dioxide. Incomplete burning can be detected by sensing carbon monoxide.
Photoelectric
… excerpt ends here. Continue reading the full article.
