- Spark gap
A spark gap consists of an arrangement of two conducting
electrodes separated by a gap usually filled with a gassuch as air. When a suitable voltageis supplied, a sparkforms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the ' holding current'. This usually happens when the voltagedrops, but in some cases occurs when the heated gas rises, stretching out and then breaking the of ionized gas. Usually the action of ionizing the gas is violent and disruptive, often leading to sound(ranging from a "snap" for a spark plugto thunderfor a lightningdischarge), lightand heat.
The emitted light does not come from the electron current, but the material medium
fluorescingin response to collisions from the electrons exciting its electronorbitals to high, excited states and dropping them repeatedly. It is impossible for a visible spark to form in a vacuum. Without intervening matter capable of electromagnetictransitions, the spark will be invisible(see vacuum arc).
Spark gaps are essential to the functioning of a number of electronic devices.
A spark plug uses a spark gap to initiate
combustion. The heat of the ionization trail ignites a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven, or stove.
A spark radiates energy throughout the
electromagnetic spectrum. Nowadays, this is usually regarded as radio frequency interferenceand is suppressed, but in the early days of radio communications, this was the means by which radio signals were transmitted, in the spark-gap transmitter. Many radio spark gaps include cooling devices such as the rotary gapand heat sinks, since the spark gap becomes quite hot under continuous use at high power.
park gaps as protective devices
Spark gaps are frequently used to prevent voltage surges from damaging equipment. Spark gaps are used in high-voltage
switches, for example, in power plants and electrical substations. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two leaf springs holding the other end as second contact. If the blade is opened, a spark may keep the connection between blade and spring conducting. (The spark ionizes the air, which becomes conductive, allowing an arc to form, which sustains ionization and hence conduction.) Here, a Jacob's ladder (see below) on top of the switch will pull the arc apart and so extinguish it. You might also find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons. If a spark should ever manage to jump over the insulator and give rise to an arc, it will be extinguished.
Smaller spark gaps are often used to protect sensitive electrical or electronic equipment from high voltage surges. In sophisticated versions of these devices (called gas tube arresters), a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment. These devices are commonly used for
telephonelines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of lightningstrikes. Less sophisticated (and much less expensive) spark gaps are made using modified ceramic capacitors; in these devices, the spark gap is simply an air gap sawn between the two lead wires that connect the capacitor to the circuit. A voltage surge causes a spark which jumps from lead wire to lead wire across the gap left by the sawing process. These low-cost devices are often used to prevent damaging arcs between the elements of the electron gun(s) within a cathode ray tube(CRT).
Small spark gaps are very common in
telephone switchboards, as the long phone cables are very susceptible to induced surges from lightningstrikes. Larger spark gaps are used to protect power lines. Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are also used for this purpose.
Special purpose, high-energy triggerable spark gaps are also used to rapidly switch high voltages and very high currents for certain pulsed power applications, such as pulsed
lasers, railguns, fusion and ultrastrong pulsed magnetic fieldresearch, and in the triggering of nuclear bombs. These often have higher power capabilities than any thyristor, thyratron, krytron, or sprytron. One such type of switch is known as a trigatron.
A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top. It was named for the "ladder to heaven" described in the Bible.
When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an
electric arc. Air breaks down at about 30kV/cm,cite journal |author=J. J. Lowke |title=Theory of electrical breakdown in air |journal=Journal of Physics D: Applied Physics |volume=25 |issue= |pages=202–210 |year=1992 |url=http://www.iop.org/EJ/article/0022-3727/25/2/012/jd920212.pdf?request-id=XIfZw4zI3BGDoxCz2wi7Kg|doi=] depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supplycan deliver, and the heavy load dramatically reduces the voltage across the gap.
The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.
This cycle leads to an exotic-looking display of electric
white, yellow, blueor purplearcs which is often seen in movies about mad scientists. The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil, or any other source of high voltage in the 10,000 volt - 30,000 volt range, like a neon sign transformer or circuit (10-30 kV) or a television picture tube circuit ( flyback transformer) (10-28 kV), and two coat hangers or rods built into a "V" shape. For larger ladders, microwave oven transformers connected in series or utility pole transformers (pole pigs) run in reverse (step-up) are used.The sparks can burn through thin paper and plastic and start fires; contact with the exposed high voltage can be lethal.
Exposure to an arc-producing device can pose health hazards. In a closed space such as a classroom or home, the continuous arc formation of an open-air Jacob's Ladder will ionize oxygen and nitrogen, which then reforms into reactive molecules such as
ozoneand nitric oxide. These free radicalscan be damaging to the mucous membranesof people near the spark gap.
These hazards are not present when the arc is formed outdoors since the heated ionized gases will rise up into the air and dissipate into the atmosphere. Spark gaps which only intermittently produce short spark bursts are also minimally hazardous because the volume of ions generated is very small.
Arcs can also produce a broad spectrum of wavelengths spanning the visible light and the invisible ultraviolet and infrared spectrum. Very intense arcs generated by means such as
arc weldingcan produce significant amounts of ultraviolet which is damaging to the retina of the observer. These arcs should only be observed through special dark filters which reduce the arc intensity and shield the observer's eyes from the ultraviolet rays.
* Model T Spark Coil
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Plasma arc loudspeakers
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Look at other dictionaries:
Spark gap — (Elec.) The space filled with air or other dielectric between high potential terminals (as of an electrostatic machine, induction coil, or condenser), through which the discharge passes; the air gap of a jump spark. [Webster 1913 Suppl.] … The Collaborative International Dictionary of English
spark gap — n. a space between two electrodes through which a spark discharge may take place … English World dictionary
spark gap — spark′ gap n. 1) elm a space between two electrodes, across which a discharge of electricity may take place 2) elm the electrodes and the space between, considered as a unit: used in ignition systems … From formal English to slang
spark gap — noun 1. a component of an ignition system; consists of two shaped electrodes and the space between them • Hypernyms: ↑component, ↑constituent, ↑element • Part Holonyms: ↑ignition, ↑ignition system 2. the gap between two high potential terminals … Useful english dictionary
spark gap — noun Date: 1889 a space between two high potential terminals (as of an induction coil or spark plug) through which pass discharges of electricity; also a device having a spark gap … New Collegiate Dictionary
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spark gap — noun a) A gap, between two electrical terminals, across which sparks are generated. b) Such a gap as a component of an engines ignition system, especially that between the electrodes of a spark plug … Wiktionary
spark gap — The space between the center and side electrode tips on a spark plug. Also see gap … Dictionary of automotive terms