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Spark gap
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<h2 id="breakdown-voltage">Breakdown voltage</h2> <p>For air, the breakdown strength is about 30 kV/cm at sea level.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p> <h2 id="spark-visibility">Spark visibility</h2> <p>The light emitted by a spark does not come from the current of <a href="/facts/Electron/L0KBo5cW">electrons</a> itself, but from the material medium <a href="/facts/Fluorescence/qsJVT2qP">fluorescing</a> in response to collisions from the electrons. When electrons collide with molecules of air in the gap, they excite their <a href="/facts/Atomic_orbital/tKhgl6mL">orbital</a> electrons to higher <a href="/facts/Energy_level/66x51Khc">energy levels</a>. When these excited electrons fall back to their original energy levels, they emit energy as light. It is impossible for a visible spark to form in a <a href="/facts/Vacuum/BTk3goyV">vacuum</a>. Without intervening matter capable of <a href="/facts/Electromagnetism/oAkmALHT">electromagnetic</a> transitions, the spark will be <a href="/facts/Invisible/yW8zGvKm">invisible</a> (see <a href="/facts/Vacuum_arc/3DM8nlIM">vacuum arc</a>). </p> <h2 id="applications">Applications</h2> <p>Spark gaps are essential to the functioning of a number of electronic devices. </p> <h3>Ignition devices</h3> <p>A <a href="/facts/Spark_plug/Y9lG10hg">spark plug</a> uses a spark gap to initiate <a href="/facts/Combustion/ohQoWj8z">combustion</a>. The heat of the ionization trail, but more importantly, UV radiation and hot free electrons (both cause the formation of reactive free radicals) ignite a fuel-air mixture inside an <a href="/facts/Internal_combustion_engine/Ju4BXzj0">internal combustion engine</a>, or a burner in a furnace, oven, or stove. The more UV radiation is produced and successfully spread into the combustion chamber, the further the combustion process proceeds. </p><p>The <a href="/facts/Space_Shuttle_Main_Engine/vGkj6Vxm">Space Shuttle Main Engine</a> hydrogen oxygen propellant mixture was ignited with a spark igniter.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> <h3>Protective devices</h3> <p class="note">See also: <a href="/facts/Surge_protector/o3LqlVNv">Surge protector</a></p> <p>Spark gaps are frequently used to prevent <a href="/facts/Voltage_spike/XYdAJkje">voltage surges</a> from damaging equipment. Spark gaps are used in high-voltage <a href="/facts/Switch/vqbNDgYm">switches</a>, large power <a href="/facts/Transformer/EYYfXWrF">transformers</a>, in <a href="/facts/Power_plant/vYeVY1fD">power plants</a> and <a href="/facts/Electrical_substation/Iy8NeYzy">electrical substations</a>. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two <a href="/facts/Leaf_spring/mE90B7wj">leaf springs</a> 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 and allows an arc to form, which sustains ionization and hence conduction. A <a href="/facts/Jacob%2527s_ladder_(electrical)/2eDHjA6Y">Jacob's ladder</a> on top of the switch will cause the arc to rise and eventually extinguish. One might also find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons. These are sometimes called horn gaps. If a spark should ever manage to jump over the insulator and give rise to an arc, it will be extinguished. </p><p>Smaller spark gaps are often used to protect sensitive electrical or electronic equipment from high-voltage <a href="/facts/Overvoltage/LsVwLvVQ">surges</a>. In sophisticated versions of these devices (called gas tube arresters),<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> 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 <a href="/facts/Telephone/P7uEgWQX">telephone</a> lines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of <a href="/facts/Lightning/dLTuxz8n">lightning</a> strikes. Less sophisticated (and much less expensive) spark gaps are made using modified <a href="/facts/Ceramic_capacitor/BRTB62yF">ceramic capacitors</a>; in these devices, the spark gap is simply an air gap <a href="/facts/Saw/mh8PG7xy">sawn</a> between the two lead wires that connect the capacitor to the circuit. A voltage surge causes a spark that 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 <a href="/facts/Cathode-ray_tube/8NVGx5C7">cathode-ray tube</a> (CRT). </p><p>Small spark gaps are very common in <a href="/facts/Telephone_switchboard/Xu1xeItl">telephone switchboards</a>, as the long phone cables are very susceptible to induced surges from <a href="/facts/Lightning/dLTuxz8n">lightning</a> strikes. Larger spark gaps are used to protect <a href="/facts/Electric_power_transmission/I9w3lRmM">power lines</a>. </p><p>Spark gaps are sometimes implemented on <a href="/facts/Printed_Circuit_Boards/muSR2p43">Printed Circuit Boards</a> in electronics products using two closely spaced exposed PCB traces. This is an effectively zero cost method of adding crude over-voltage protection to electronics products.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p><p><a href="/facts/Transil/3DG9FK2C">Transils</a> and <a href="/facts/Trisil/AdAkfWlP">trisils</a> are the solid-state alternatives to spark gaps for lower-power applications. <a href="/facts/Neon_lamp/dVz7h0B4">Neon bulbs</a> are also used for this purpose. </p> <h3>High speed photography</h3> <p>A triggered spark gap in an <a href="/facts/Air-gap_flash/ahKzQ8zj">air-gap flash</a> is used to produce photographic light flashes in the sub-microsecond domain. </p> <h3>Radio transmitters</h3> <p class="note">Main article: <a href="/facts/Spark-gap_transmitter/Ce9TvWcd">Spark-gap transmitter</a></p> <p>A spark radiates energy throughout the <a href="/facts/Electromagnetic_spectrum/24LARvFY">electromagnetic spectrum</a>. Nowadays, this is usually regarded as illegal <a href="/facts/Electromagnetic_interference/gv3DlKNr">radio frequency interference</a> and is suppressed, but in the early days of radio communications (1880–1920), this was the means by which radio signals were transmitted, in the <a href="/facts/Modulation/aAXPjINv">unmodulated</a> <a href="/facts/Spark-gap_transmitter/Ce9TvWcd">spark-gap transmitter</a>. Many radio spark gaps include cooling devices, such as the <a href="/facts/Rotary_gap/Ce9TvWcd">rotary gap</a> and <a href="/facts/Heat_sink/ccIEdHnq">heat sinks</a>, since the spark gap becomes quite hot under continuous use at high power. </p> <h3>Sphere gap for voltage measurement</h3> <p>A calibrated spherical spark gap will break down at a highly repeatable voltage, when corrected for air pressure, humidity and temperature. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. A spark gap can be used to measure high voltage AC, DC, or pulses, but for very short pulses, an <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> light source or <a href="/facts/Radioactivity/Ya6FVjt4">radioactive source</a> may be put on one of the terminals to provide a source of electrons.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h3>Power-switching devices</h3> <p>Spark gaps may be used as electrical switches because they have two states with significantly different electrical resistance. Resistance between the electrodes may be as high as 1012 <a href="/facts/Ohm/QNVrlGjw">ohms</a> when the electrodes are separated by gas or vacuum which means that little current flows even when a high voltage exists between the electrodes. Resistance drops as low as a 10-3 ohms low when the electrodes are connected by plasma which means that power dissipation is low even at high current. This combination of properties has led to the use of spark gaps as electrical switches in <a href="/facts/Pulsed_power/BTJN8xg5">pulsed power</a> applications where energy is stored at high voltage in a <a href="/facts/Capacitor/hyowqv7n">capacitor</a> and then discharged at high current. Examples include pulsed <a href="/facts/Laser/jk6PTvvT">lasers</a>, <a href="/facts/Railgun/fRjY3X5Y">railguns</a>, <a href="/facts/Marx_generator/6XhMqzIg">Marx generators</a>, <a href="/facts/Z_Pulsed_Power_Facility/zSCKHpXY">fusion</a>, ultrastrong pulsed <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> research, and <a href="/facts/Exploding-bridgewire_detonator/w0Glec5o">nuclear bomb triggering</a>. </p><p>When a spark gap consists of only two electrodes separated by gas, the transition between the non-conducting and conducting states is governed by <a href="/facts/Paschen%2527s_law/qSWw9dvP">Paschen's law</a>. At typical pressure and electrode distance combinations, Paschen's law says that <a href="/facts/Townsend_discharge/W69jPeBw">Townsend discharge</a> will fill the gap between the electrodes with conductive plasma whenever the ratio of the <a href="/facts/Electric_field/tt4MooBs">electric field</a> strength to the pressure exceeds a constant value determined by the composition of the gas. The speed with which pressure can be reduced is limited by <a href="/facts/Choked_flow/XySpdv3G">choked flow</a>, while increasing the electric field in a capacitor discharge circuit is limited by the capacitance in the circuit and the current available for <a href="/facts/Capacitor/hyowqv7n">charging the capacitance</a>. These limitations on the speed with which discharge may be initiated mean that spark gaps with two electrodes typically have high <a href="/facts/Jitter/UpLIJDVQ">jitter</a>. <a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>Triggered spark gaps are a class of devices with some additional means of triggering to achieve low jitter. Most commonly, this is a third electrode, as in a <a href="/facts/Trigatron/uD79YEDj">trigatron</a>. The voltage of the trigger electrode can be changed quickly because the capacitance between it and the other electrodes is small. In a triggered spark gap, gas pressure is optimized to minimize jitter while also avoiding unintentional triggering. Triggered spark gaps are made in permanently sealed versions with limited voltage range and in user-pressurized versions with voltage range proportional to the available pressure range. Triggered spark gaps share many similarities with other <a href="/facts/Gas-filled_tube/uPNhhy7I">gas-filled tubes</a> such as <a href="/facts/Thyratron/tkhB15CK">thyratrons</a>, <a href="/facts/Krytron/VjcEDoM2">krytrons</a>, <a href="/facts/Ignitron/M0tGxglG">ignitrons</a>, and <a href="/facts/Crossatron/TQvvsWIK">crossatrons</a>. </p><p>Triggered vacuum gaps, or <a href="/facts/Krytron/VjcEDoM2">sprytrons</a>, resemble triggered spark gaps both in appearance and construction but rely on a different operating principle. A triggered vacuum gap consists of three electrodes in an airtight glass or ceramic envelope that has been evacuated. This means that, unlike a triggered spark gap, a triggered vacuum gap operates in the parameter space to the left of the Paschen minimum where breakdown is promoted by increasing pressure. Current between the electrodes is limited to a small value by <a href="/facts/Field_electron_emission/ReNmucpE">field emission</a> in the non-conducting state. Breakdown is initiated by rapidly evaporating material from a trigger electrode or an adjacent resistive coating. Once the <a href="/facts/Vacuum_arc/3DM8nlIM">vacuum arc</a> is initiated, a triggered vacuum gap is filled with conductive plasma as in any other spark gap. A triggered vacuum gap has a larger operating voltage range than a sealed triggered spark gap because Paschen curves are much steeper to the left of the Paschen minimum than at higher pressures. Triggered vacuum gaps are also <a href="/facts/Radiation_hardening/nB4Wbctl">rad hard</a> because in the non-conducting state they do not contain any gas that could be <a href="/facts/Ionizing_radiation/vw0d9GaT">ionized by radiation</a>. <a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h3>Insect control</h3> <p class="note">Main article: <a href="/facts/Bug_zapper/NftgE1Gf">Bug zapper</a></p> <p>They are also used as insect zappers. The two electrodes are implemented as metal lattices placed a slightly too far apart for the voltage to jump. When an insect ventures between the electrodes the gap distance is reduced by the insects body, being conductive, and a spark discharge occurs to electrocute and burn the insect. </p><p>In this use the spark gap mechanism is often used in conjunction with a bait, such as a light, to attract the insect into the spark gap. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Arc_lamp/1y99ep6E">Arc lamp</a></li> <li><a href="/facts/Arcing_horns/QlTs6T6b">Arcing horns</a></li> <li><a href="/facts/Corona_discharge/caQ4gmoP">Corona discharge</a></li> <li><a href="/facts/Electric_arc/1hUPPAG0">Electric arc</a></li> <li><a href="/facts/Ignition_system/7Rqqltpr">Ignition system</a></li> <li><a href="/facts/Ignition_system/7Rqqltpr">Model T Spark Coil</a></li> <li><a href="/facts/List_of_electronics_topics/VTrwvXxp">List of electronics topics</a></li> <li><a href="/facts/Plasma_arc_loudspeaker/VhAu3VjM">Plasma arc loudspeakers</a></li> <li><a href="/facts/Radar/jk9BTbFA">Radar</a></li> <li><a href="/facts/Spark-gap_transmitter/Ce9TvWcd">Spark-gap transmitter</a></li> <li><a href="/facts/Spark_plug/Y9lG10hg">Spark plug</a></li> <li><a href="/facts/Spark_spread/u4LSIysd">Spark spread</a></li> <li><a href="/facts/Tesla_coil/NhGAeR7v">Tesla coil</a></li> <li><a href="/facts/Vacuum_arc/3DM8nlIM">Vacuum arc</a></li> <li><a href="/facts/Paschen%2527s_law/qSWw9dvP">Paschen's law</a></li></ul> <h2 id="external-links">External links</h2> <p>Jacob's Ladder videos: </p> <ul><li><a href="https://www.youtube.com/watch?v=PXiOQCRiSp0">Transformer in Nevada</a></li> <li><a href="https://www.youtube.com/watch?v=aGjWk9HjkGI">In someone's home</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Meek, J. (1940). "A Theory of Spark Discharge". Physical Review. 57 (8): 722–728. Bibcode:1940PhRv...57..722M. doi:10.1103/PhysRev.57.722. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Boeing: Space Shuttle Main Engine Orientation" (PDF). Boeing. June 1998. Retrieved 16 November 2019. <a href="http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf" target="_blank">http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>[1] Transient Protection Products, Gas Tubes, Hybrid Protection Systems | rated up to 20,000 amps ... sealed from dust and moisture ... some have small amount of amounts of Tritium Backfill Gas (10 microcuries) | Reynolds Industries Incorporated <a href="https://www.teledynedefenseelectronics.com/reynolds/resource-library/Documents/Spark%20Gaps.pdf" target="_blank">https://www.teledynedefenseelectronics.com/reynolds/resource-library/Documents/Spark%20Gaps.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"A guide to designing for ESD and EMC" (PDF). NXP Semiconductors. January 19, 2010. Archived (PDF) from the original on August 3, 2019. <a href="https://www.nxp.com/docs/en/application-note/AN10897.pdf" target="_blank">https://www.nxp.com/docs/en/application-note/AN10897.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Ryan, Hugh M. (ed) High Voltage Engineering and Testing (2nd Edition), Institution of Engineering and Technology 2001, ISBN 978-0-85296-775-1 pages <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Triggered Spark Gap Design". Retrieved 17 February 2019. <a href="https://hofstragroup.com/triggered-spark-gap-design.html" target="_blank">https://hofstragroup.com/triggered-spark-gap-design.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Gas Discharge Closing Switches. Springer Science+Business Media, LLC. 1990. ISBN 978-1-4899-2132-1. <a href="978-1-4899-2132-1" target="_blank">978-1-4899-2132-1</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> </ol>