question: Why are some electrical sparks blue while some are yellow?
comment: Same reason a hot object glows red, but an even hotter object glows orange. The shorter the wave is, the higher the energy is. If the spark transfers more energy, it will be blue, if it transfers less it will be yellow.
passages: Two kinds of sparks may occur during switch operation. On closure, a few sparks, like those from a flint-and-steel, may appear as a tiny bit of metal is heated by friction to incandescence, melted, and thrown off. On opening, a bluish arc may occur, with a detectable "electrical" (ozone) odor. Subsequently, the contacts may be seen to be darkened and pitted. Damaged contacts have higher resistance, rendering them more vulnerable to further damage and causing a cycle in which the contacts soon may fail completely.
   A list must be drawn up of every explosive material that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how explosive the material would be if it was ignited, with IIC being the most explosive Zone system gas group and IIA being the least. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for Zone system gas groups.
  
  Section::::Temperature classification.
   Section::::Observed effects.
  
  Section::::Observed effects.:Blue glow.
  
  It has been observed that many criticality accidents emit a blue flash of light.
  
  The blue glow of a criticality accident can result from the fluorescence of the excited ions, atoms and molecules of air (mostly oxygen and nitrogen) falling back to unexcited states, which produces an abundance of blue light. This is also the reason electrical sparks in air, including lightning, appear electric blue. The smell of ozone was said to be a sign of high ambient radioactivity by Chernobyl liquidators.
   Electric blue is a color whose definition varies but is often considered close to cyan, and which is a representation of the color of lightning, an electric spark, and the color of ionized argon gas; it was originally named after the ionized air glow produced during electrical discharges, though its meaning has broadened to include shades of blue that are metaphorically "electric" by virtue of being "intense" or particularly "vibrant". Electric arcs can cause a variety of color emissions depending on the gases involved, but blue and purple are typical colors produced in the troposphere where oxygen and nitrogen dominate.
   In pyrotechnics, charcoal, iron filings, aluminum, titanium and metal alloys such as magnalium may be used to create sparks. The quantity and style of sparks produced depends on the composition and pyrophoricity of the metal and can be used to identify the type of metal by spark testing. In the case of iron, the presence of carbon is required, as in carbon steel — about 0.7% is best for large sparks. The carbon burns explosively in the hot iron and this produces pretty, branching sparks. The color of sparks used in pyrotechnics is determined by the material that the sparks are made from, with the possibility of adding different chemical compounds to certain materials to further influence the color of the sparks. The basic color of sparks is limited to red/orange, gold (yellow) and silver (white). This is explained by light emission from a solid particle. Light emitted from solid particles is defined by black-body radiation. The temperature of the spark is controlled by the reactivity of the metal. Higher reactive metals lead to hotter sparks. The electronegativity has found to be an helpful indicator to estimate the temperature and consequently the color of a spark. To achieve colors differing from black body emitters, vapor phase combustion of the metal is necessary. A typical example is zinc, with a low boiling of 1180 K. Zinc sparks show an unusual bluish/greenish white appearance. Exotic sparks can be obtained from erbium powder. These sparks switch between surface and vapor phase combustion and accordingly between orange (black body) and green (element specific) emission. The duration of a spark’s existence is determined by the initial size of the particle, with a larger size leading to a longer-lasting spark.
   On a simple level, flame emission spectroscopy can be observed using just a flame and samples of metal salts. This method of qualitative analysis is called a flame test. For example, sodium salts placed in the flame will glow yellow from sodium ions, while strontium (used in road flares) ions color it red. Copper wire will create a blue colored flame, however in the presence of chloride gives green (molecular contribution by CuCl). 
  
  Section::::Emission coefficient.
   Section::::Urban Couture.
   Each chemical gas or vapour used in industry is classified into a gas group.
  
  Group IIC is the most severe Zone system gas group. Hazards in this group gas can be ignited very easily indeed. Equipment marked as suitable for Group IIC is also suitable for IIB and IIA. Equipment marked as suitable for IIB is also suitable for IIA but NOT for IIC. If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC
   Section::::Background.
   Section::::Composition.
   Metals with low thermal conductivity are especially good at producing sparks. Titanium and zirconium are especially good in this respect and so are now used in fireworks. Copper, on the other hand, has a high conductivity and so is poor at producing sparks. For this reason, alloys of copper such as beryllium bronze are used to make safety tools which will not spark so easily.
  
  Section::::Flint and steel.
   The color of sparks is limited to red/orange, yellow/gold and white/silver. This is explained by light emission from an incandescent solid particle in contrast to the element-specific emission from the vapor phase of a flame. Light emitted from a solid particle is defined by black-body radiation. Low boiling metals can form sparks with an intensively colored glowing shell surrounding the basic particle. This is caused by vapor phase combustion of the metal. 
   Section::::Reception and legacy.
   BULLET::::- Zinc – used in some smoke compositions, together with sulfur used in some early amateur rocket fuels, also in pyrotechnic stars; heavy, zinc-based compositions may require additional lift to fly high enough; moisture-sensitive; can spontaneously ignite; rarely used as primary fuel except in smoke compositions, can be encountered as a secondary enhancement fuel
  
  BULLET::::- Copper – used as a blue colorant with other fuels
  
  BULLET::::- Brass – a zinc-copper alloy used in some fireworks formulas, as a blue colorant for its copper content
   If a circuit includes a neutral or midpoint conductor, then it should be identified by a blue colour (preferably light blue). Light blue is the color used to identify intrinsically safe conductors, and must not be used for any other type of conductor.
  
  Section::::Use of colours.:AC phase conductors.
  
  The preferred colours for AC phase conductors are:
  
  BULLET::::- L1: brown
  
  BULLET::::- L2: black
  
  BULLET::::- L3: grey
  
  For a single AC phase: brown
  
  Section::::Use of colours.:Protective conductor.
   Section::::Decline.
  
  Blue light has been obsolete for signaling since early in the twentieth century, but pyrotechnic lighting is still popular for celebratory fireworks displays, and its synonyms, "Bengal light" or Bengal fire can still be found in modern pyrotechnic manuals. Such displays were also popular in nineteenth century civilian life: 200 blue lights were used in the first illumination of Niagara Falls during the 1860 North American visit of the Prince of Wales.
   Section::::Electrical ignition hazard.
  
  A household light switch may emit a small, harmless visible spark when switching. In an ordinary atmosphere this arc is of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery either is designed to contain any explosion within the device or is designed not to produce sparks with sufficient energy to trigger an explosion.
   BULLET::::- Ex nA II T3 X (Type n, non-sparking, Group 2 gases, Temperature class 3, special conditions apply).
   BULLET::::- High-speed steel: High-speed steel has a faint red spark that sparks at the tip.
  
  BULLET::::- 300-series stainless steel: These sparks are not so dense as the carbon steel sparks, do not fork, and are orange to straw in color.
  
  BULLET::::- 310-series stainless steel: These sparks are much shorter and thinner than the 300-series sparks. They are red to orange in color and do not fork.
  
  BULLET::::- 400-series stainless steel: 400-series sparks are similar to 300-series sparks, but are slightly longer and have forks at the ends of the sparks.
   The color of the spark depends upon the gases that make up the gaseous media. While the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. Lightning is an example of an immense spark that can be many miles long.
  
  Section::::Mechanism.:Persistent arcs.
   Sodium is a common component or contaminant in many compounds and its spectrum tends to dominate over others. The test flame is often viewed through cobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.
  
  Section::::Results.
   Other factors, particularly the fuel chemistry and its propensity for forming soot, have an influence on luminosity.
  
  The Victorian chemist, Edward Frankland, studied luminosity in flames and discovered that their luminosity also increased with pressure.
  
  Section::::Bunsen burner.
  
  One of the most familiar instances of a luminous flame is produced by a Bunsen burner. This burner has a controllable air supply and a constant gas jet: when the air supply is reduced, a highly luminous, and thus visible, orange 'safety flame' is produced. For heating work, the air inlet is opened and the burner produces a much hotter blue flame.
   These elements have low electrical resistivity so they are used for wiring. Copper is the cheapest and most widely used. Bond wires for integrated circuits are usually gold. Silver and silver-plated copper wiring are found in some special applications.
  
  Section::::Occurrence.
  
  Copper occurs in its native form in Chile, China, Mexico, Russia and the USA. Various natural ores of copper are: copper pyrites (CuFeS), cuprite or ruby copper (CuO), copper glance (CuS), malachite, (Cu(OH)CuCO), and azurite (Cu(OH)2CuCO).
  
  Copper pyrite is the principal ore, and yields nearly 76% of the world production of copper.
  
  Section::::Production.
   Section::::Blue light (nickname).
  
  "Blue light" was a derisive nickname given to military officers of the 18th and 19th centuries, whose evangelical Christian zeal burned as brightly as its namesake signal, to the chagrin of those less ardent who were subject to the perceived ostentatious piety. Confederate Gen. Stonewall Jackson carried the nickname "Old Blue Lights" during the American Civil War because of his overt religiosity.
  
  Section::::Blue lantern myth.
   Sometimes, sparks can be dangerous. They can cause fires and burn skin.
  
  Lightning is an example of an electric spark in nature, while electric sparks, large or small, occur in or near many man-made objects, both by design and sometimes by accident.
  
  Section::::History.
  