searchspell:fencecorrected for electric fence
An electric fence is a barrier that uses a painful or even lethal high-voltage electric shocks to deter animals or people from crossing a boundary.
HistoryAn early application of the electric fence was developed in 1936–1937 by New Zealand inventor William "Bill" Gallagher Snr. Built from a car's ignition coil and a meccano set, Gallagher used the device to keep his horse from scratching itself against his car. Gallagher later started a company to improve and market his invention. Today the Gallagher Group of companies is still heavily involved in electric fencing for livestock control. The concept of the electric fence was first described in Mark Twain's A Connecticut Yankee in King Arthur's Court, in 1889, as a defensive weapon. Electric fences were used to control stock in the United States in the early 1930's, and developed further in both the United States and New Zealand. Electric fences have improved significantly since the early days. Improvements include:
Design and functionElectric fences are designed to create an electrical network when touched by a person or animal. A component called a power energizer converts power into a brief high-voltage pulse. One terminal of the power energizer releases the electrical pulse along a connected bare wire about once per second. Another terminal is connected to a metal rod implanted in the earth, called a ground rod. An animal (which are mostly water and thus quite conductive) touching the wire and the earth simultaneously will complete an electrical circuit and will conduct the pulse, causing a painful electric shock.The effects of the electrical shock depend upon the voltage and electrical current used, and can range from painful to lethal. Early mains-driven fence chargers used a transformer and a mechanically-driven switch to generate the electrical pulses. The pulses were wide and the voltage unpredictable, with no-load peaks in excess of 10,000 volts and a rapid drop in voltage as the fence leakage increased. The switch mechanism was prone to failure. Later systems replaced the switch with a solid-state circuit, with an improvement in longevity but no change in pulse width or voltage control. "Weed burner" fence chargers were popular for a time and featured a longer-duration output pulse that would destroy weeds touching the fence. These were responsible for many grass fires when used during dry weather. Though still available, they have declined in popularity. Later "low impedance" fence chargers use a different design. A capacitor is charged by a solid-state circuit, then released using a thyristor or similar solid-state component. Voltage is consisent due to electronic output controls, within the limits of output power. Pulse width is much narrower, often about 10 microseconds. This design works for either battery or mains power sources. Permanent electric fencing may be constructed using conventional HT fencing techniques, with plain steel wire serving as the conducting wire. The wire must be kept insulated from the earth. Typical methods for doing so involve the mounting of the fence wire on plastic or porcelain insulators; other techniques include using fence posts that are themselves insulators. In the U.S., permanent electric fence is most often run using soft steel wire, above or in front of a woven wire or barbed wire fence that provides a physical barrier. Permanent electric fencing is popular in many agricultural areas, as construction of electric fences (using plain wire and lighter construction, as the fence does not need to physically restrain animals) is much cheaper and faster than conventional fences. Its disadvantages include the potential for the entire fence to be disabled due to a break in the conducting wire, power failure, or forced disconnection due to the risk of fires starting by dry grass touching the electrified wire. In practice, once animals have learned of the unpleasant consequences of touching the fence they tend to avoid it for considerable periods even when inactive. Substandard conventional fencing can also be repaired quickly and cheaply by the addition of a single electric wire mounted as a "stand-off" using spring-loaded insulated wire mounts from the original fence. Electric fencing is probably more popular, however, for the construction of temporary fencing, particularly to support the practice of strip grazing. Typically, a single strand of wire, or flexible plastic tape embedded with conducting wire, is mounted on espeically-designed posts designed to be pressed in by the fencer's feet. Within a few minutes a large area can be fenced off. Portable, battery-powered fence "energiser" units are made for this purpose. RanchingElectric fencing is commonplace in ranching and livestock-management settings. It is used both to keep livestock within designated boundaries and also to keep predators and other unwanted animals out. It is an attractive choice for ranchers because few materials are required compared to traditional physical barrier fences, resulting in lower cost. Additionally, the risk of damage to livestock is lower compared to barbed wire physical barriers. Wild-animal managementElectric fences are useful for controlling the movements of wild animals. Examples include deterring deer from entering private property, keeping animals off airport runways, and preventing geese from soiling areas used by people. SecurityNon-lethal electric fences are used to prevent trespass by both private and government-sector bodies. These include housing communities, commercial factories or warehouses, prisons, military bases, and government buildings. Lethal electric fences are sometimes used by prisons as a barrier to prevent inmates from escaping. Electric fences were infamously used to guard the concentration camps of Nazi Germany during World War II, where potentially lethal voltages and currents were employed, continuously rather than in pulses. They continue to be used in like fashion at high-security prisons and certain other installations to this day. Typically a nonelectric fence is constructed on either side of such an installation, or the deadly current is carried out of casual reach atop a wall. Livestock-type electric fences are occasionally employed to discourage suicide attempts on tall structures, graffiti, and other petty crime. Recent innovations have included the use of electricity to monitor fencing for intruder detection as opposed to providing an electric shock to discourage entry. Electrifed fences were also used in the book and film Jurassic Park,to contain the dinosaurs. External links
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