Sparks in air and uses

The dielectric breakdown strength of dry air between spherical electrodes is approximately 33 kV/cm at standard temperature and pressure (STP).This is only a rough guide, as the actual breakdown voltage depends largely on the shape and size of the electrodes. Strong electric fields (generated by high voltages applied to small or pointed conductors) often produce a purple corona discharge in the air,along with visible sparks.Voltages below about 500-700 volts cannot produce a noticeably visible spark or glow in air at atmospheric pressure,so these voltages are "low" by this rule.However, under conditions of low air pressure,such as in high-altitude aircraft,or in an environment of inert gases such as argon or neon, sparks can occur at much lower voltages.500 to 700 volts is not a fixed minimum for spark breakdown,but it is a rule of thumb.For air at STP, the minimum sparkover voltage is about 327 volts,as described by Friedrich Paschen.While a lower voltage will not normally jump across a gap that existed before the voltage was applied, interrupting an existing current with a gap will often create a low voltage spark or arc.As the contacts separate, some small contacts become the last points to separate.Electric current is confined to these small hot spots,causing them to become incandescent,which emits electrons (via thermionic emission).In a dark room, even a small 9 V battery can produce a noticeable spark through this mechanism.Ionized air and metal vapor (from the contacts) form a plasma that temporarily bridges the expanding gap.If the source and load allow sufficient current to flow,a self-sustaining arc may form.Once formed,the arc may extend to a considerable length before breaking the circuit.Attempts to open an inductive circuit often result in arcing because the inductor provides a high voltage pulse whenever the current is interrupted.AC systems make sustained arcing less likely because the current returns to zero twice per cycle.The arc is extinguished each time the current crosses zero and must be re-ignited for the next half-cycle to maintain the arc.

Unlike an ohmic conductor,the resistance of an arc decreases as the current increases.This makes accidental arcing in electrical equipment dangerous because even a small arc can become large enough to damage equipment and start a fire if enough current is available.Intentionally created arcs,such as for lighting or welding,require certain components in the circuit to stabilize the current/voltage characteristics of the arc. 

Uses

Electricity transmission and distribution lines typically use voltages between tens and hundreds of kilovolts.These lines may be overhead or underground.High voltage is used for power distribution to reduce ohmic losses when transmitting power over long distances.

Industry

It is used in semiconductor production to sputter thin metal films on wafer surfaces.It is also used for electrostatic flocking,covering objects with small fibers that stand up at the edges.

Science

Spark gaps have historically been used as an early form of radio transmission.Likewise,lightning discharges in Jupiter's atmosphere are thought to be the source of the planet's powerful radio frequency radiation.High voltages have been used in landmark experiments and discoveries in chemistry and particle physics.The electric arc is used to separate and find the element argon from the atmosphere.Induction coils powered early X-ray tubes. Moseley used an X-ray tube to determine the atomic number of selected metal elements from the spectrum it emitted when used as an anode.High voltage is used to generate the electron beam used in the microscope.Cockcroft and Walton invent the voltage multiplier to convert lithium atoms in lithium oxide into helium by accelerating hydrogen atoms.

Safety

If voltages in excess of 50 V are applied to dry,unbroken human skin, an electrical current is induced in the body tissue just across the chest area, resulting in heart fibrillation.The voltage at which an electric shock hazard exists depends on the conductivity of dry human skin.The insulating properties of dry skin can protect living human tissue from damage of up to 50 volts or so.Even a voltage source below 40 V can be fatal if that same skin gets wet,if there is a wound, or if voltage is applied to an electrode that penetrates the skin.Accidental contact with any high voltage providing sufficient energy could result in serious injury or death. This occurs when a person's body provides a path for an electrical current, leading to tissue damage and heart failure.Other injuries may include arcing burns from accidental contact.These burns can be especially dangerous if the victim's airway is affected.People can also be injured from physical exertion when they fall from a height or are thrown a considerable distance.Low energy exposures to high voltages are probably harmless,such as a spark in a dry climate when you touch a doorknob after walking across a carpeted floor.The voltage can be in the kilovolt range,but the average current is low.Standard precautions to avoid injury include working in conditions that avoid the flow of electrical energy through the body,especially through areas of the heart,such as between the arms or between the arms and legs.Electricity can flow between two conductors of a high-voltage device,and the body completes the circuit.To avoid this, workers should wear insulated clothing such as rubber gloves,use insulated tools,and avoid touching equipment with more than one hand at a time.Electricity can also flow between the device and the earth.To prevent this from happening,workers should stand on an insulating surface,such as a rubber mat.Security equipment is regularly tested to ensure it still protects users.Testing regulations vary by country. The testing company can test at voltages up to 300,000 volts and offer services from glove testing to elevated work platform (or EWP) testing.