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A smoke detector (sometimes called a "smoke
alarm") is a safety device that detects airborne smoke
and issues an audible alarm, thereby alerting nearby people
to the danger of fire. Most smoke detectors work either by
optical detection or by ionization, but some of them use both
detection methods to increase sensitivity to smoke.
History
In 1902 George Andrew Darby, an electrical engineer of 211
Bloomsbury Street, Birmingham, England, patented the electrical
Heat-Indicator and Fire Alarm. The device indicated any change
of temperature in the apartment where it was fixed. The device
operated by closing an electrical circuit to sound an alarm
if the temperature raised above the safe limit. The contact
was made by bridging a gap with a conductor, or allowing one
plate to fall on another: this movement was caused simply
by a block of butter which melted as the temperature raised.
This early device subsequently gave way to more modern fire
and eventually smoke alarms.
In 1969, the patent for this invention
was claimed by Kenneth House and Randolph Smith.
Optical Smoke
Detector
This type of detector includes a light source, a lens to collimate
the light into a beam, and a photodiode or other photoelectric
sensor at right-angles to the beam. In the absence of smoke,
the light passes in front of the detector but does not fall
on it. When visible smoke enters the beam, some light is scattered
by the smoke particles, and some of the scattered light is
detected by the sensor. An increased output from the sensor
sets off the alarm.
Ionization detector
This type of detector is cheaper than the optical detector,
and can detect particles of smoke that are too small to be
visible. It includes a tiny mass of radioactive americium-241,
which is a source of alpha radiation. The radiation passes
through an ionization chamber, which is an air-filled space
between two electrodes, and permits a small, constant current
to flow between the electrodes. Any smoke that enters the
chamber absorbs the alpha particles, which reduces the ionization
and interrupts this flow of current, setting off the alarm.
Hot air entering the chamber changes the rate of ionization
and therefore, the electric current level, which triggers
an alarm.
Batteries
Most residential smoke detectors run on alkaline batteries.
If these batteries run out, the smoke detector will become
inactive. Most smoke detectors are designed to signal a low
battery condition, but it is common for houses to have smoke
detectors with dead batteries. As a result, public information
campaigns have been created to remind people to change their
smoke detector batteries regularly. In regions using daylight
saving time, these campaigns usually suggest that people change
their batteries when they change their clocks. Some detectors
are also being sold with a lithium battery that can run for
about 7 to 10 years, though this might actually make it less
likely for people to change batteries since their replacement
is needed so infrequently. Most detectors today, particularly
those installed during new construction, are wired to the
main electricity flow of buildings. Many of these units also
include a battery backup to ensure operation during a power
outage.
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