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With lower cost and easier installation, sales of home security cameras increased in the early 21st century. Following the September 11 attacks, the use of video surveillance in public places became more common to deter future terrorist attacks. Under the Homeland Security Grant Program, government grants are available for cities to install surveillance camera networks. In 2009, there
were an
estimated 15,000 CCTV systems in Chicago, many linked to an integrated camera network. New York City's Domain Awareness System has 6,000 video surveillance cameras linked together, there are over 4,000 cameras on the subway system although nearly half of them do not work, and two thirds of large apartment and commercial buildings use video surveillance cameras. In the Washington D. C. area, there are more than 30,000 surveillance cameras in schools, and the Metro has nearly 6,000 cameras in use across the system. The Cheshire figure is regarded as more dependable than a previous study by Michael McCahill and Clive Norris of UrbanEye published in 2002. Based on a small sample in Putney High Street, McCahill and Norris extrapolated the number of surveillance cameras in Greater London to be around 500,000 and the total number of cameras in the UK to be around 4,200,000. According to their estimate the UK has one camera for every 14 people.
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The type of sensor installed in a smoke detector may also dictate the price range, since there are sensors that have more expensive components than those of the
other types. Among the types of sensors presently used by several manufacturers of smoke detectors are the ionization type, the optic sensor – also called photo electric sensor, the air sampling sensor, and combination type sensors. A smoke detector with an ionization type sensor uses what is called
radio isotopes that produce ionization in the air. The process is quite technical, but what this type of detector contains is ionized air molecules in a chamber inside the device. The ionized air carries electrical current and maintains that current, until something disrupts the air molecules. When the air molecules are disrupted, usually by smoke, the current drops, and an electrical circuit sound the alarm.