Editing Spark plug (section)

====Central electrode====
[[File:Bougie3.jpg|thumb|right|Central and lateral electrodes]]
The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of [[Radio frequency|RF]]  noise from the sparking.  Non-resistor spark plugs, commonly sold without an "R" in the plug type part number, lack this element to reduce electro-magnetic interference with radios and other sensitive equipment. The tip can be made of a combination of [[copper]], [[nickel]]-[[iron]], [[chromium]], or [[noble metal]]s.  

In the late 1970s, the development of engines reached a stage where the heat range of conventional spark plugs with solid nickel alloy centre electrodes was unable to cope with their demands.  A plug that was cold enough to cope with the demands of high speed driving would not be able to burn off the carbon deposits caused by stop–start urban conditions, and would foul in these conditions, making the engine misfire.  Similarly, a plug that was hot enough to run smoothly in town could melt when called upon to cope with extended high speed running on motorways.  The answer to this problem, devised by the spark plug manufacturers, was to use a different material and design for the centre electrode that would be able to carry the heat of combustion away from the tip more effectively than a solid nickel alloy could.  Copper was the material chosen for the task and a method for manufacturing the copper-cored centre electrode was created by [[Floform]].

The central electrode is usually the one designed to eject the electrons (the [[cathode]], i.e. negative polarity<ref name="Hillier, Spark plug" >{{Cite book
  |title=Fundamentals of Motor Vehicle Technology
  |last=V.A.W.  |first=Hillier
  |edition=4th
  |year=1991
  |publisher=Stanley Thornes
  |isbn=0-7487-05317
  |ref={{harvid|Hillier|1991}}
  |chapter=74: The ignition system
  |page=450
}}</ref> relative to the engine block) because it is normally the hottest part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces (see [[thermionic emission]]).<ref name=ih>International Harvester, [http://www.liberatedmanuals.com/TM-5-4210-230-14-and-P-1.pdf Truck Service Manual TM 5-4210-230-14&P-1] - Electrical - Ignition Coils and Condensers, CTS-2013-E p. 5 (PDF page 545)</ref>  In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, from a sharp point or edge rather than a flat surface (see [[corona discharge]]).<ref name=ih />  Using the colder, blunter side electrode as negative requires up to 45 percent higher voltage,<ref name=ih /> so few ignition systems aside from [[wasted spark]] are designed this way.<ref name=wastedspark>NGK, [https://www.ngk.com/glossary/8/spark-plug/W Wasted Spark Ignition]</ref>  Waste spark systems place a greater strain upon spark plugs since they alternately fire electrons in both directions (from the ground electrode to the central electrode, not just from the central electrode to the ground electrode).  As a result, vehicles with such a system should have precious metals on both electrodes, not just on the central electrode, in order to increase service replacement intervals since they wear down the metal more quickly in both directions, not just one.<ref>See p. 824 of the 2015 Champion Master Catalog.  http://www.fme-cat.com/catalogs.aspx {{Webarchive|url=https://web.archive.org/web/20180601144828/http://fme-cat.com/catalogs.aspx |date=2018-06-01 }}</ref>

It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds.  Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.

At one time it was common to remove the spark plugs, clean deposits off the ends either manually or with specialized [[sandblasting]] equipment and file the end of the electrode to restore the sharp edges, but this practice has become less frequent for three reasons:
# cleaning with tools such as a wire brush leaves traces of metal on the insulator which can provide a weak conduction path and thus weaken the spark (increasing emissions).
# plugs are so cheap relative to labor cost, economics dictate replacement, particularly with modern long-life plugs.
# iridium and platinum plugs that have longer lifetimes than copper have become more common.

The development of noble metal high temperature electrodes (using metals such as [[yttrium]], [[iridium]], [[tungsten]], [[palladium]], or [[ruthenium]], as well as the relatively high value [[platinum]],  [[silver]] or [[gold]]) allows the use of a smaller center wire, which has sharper edges but will not melt or corrode away.  These materials are used because of their high melting points and durability, not because of their electrical conductivity (which is irrelevant in series with the plug resistor or wires).  The smaller electrode also absorbs less heat from the spark and initial flame energy.

[[Polonium]] spark plugs were marketed by [[Firestone Tire and Rubber Company|Firestone]] from 1940 to 1953. While the amount of radiation from the plugs was minuscule and not a threat to the consumer, the benefits of such plugs quickly diminished after approximately a month because of polonium's short half-life, and because buildup on the conductors would block the radiation that improved engine performance. The premise behind the polonium spark plug, as well as [[Alfred Matthew Hubbard]]'s prototype [[radium]] plug that preceded it, was that the radiation would improve ionization of the fuel in the cylinder and thus allow the plug to fire more quickly and efficiently.<ref>{{cite web|url=https://www.orau.org/health-physics-museum/collection/consumer/miscellaneous/spark-plugs.html|title=Radioactive spark plugs|publisher=Oak Ridge Associated Universities|date=January 20, 1999|access-date=October 7, 2021}}</ref><ref>{{cite web|url=http://www.utoledo.edu/nsm/ic/elements/polonium.html|first=Cassandra|last=Pittman|title=Polonium|work=The Instrumentation Center|publisher=University of Toledo|date=February 3, 2017|access-date=August 23, 2018}}</ref>