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Electric spark
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures.
The rapid transition from a non-conducting to a conductive state produces a brief emission of light and a sharp crack or snapping sound. A spark is created when the applied electric field exceeds the dielectric breakdown strength of the intervening medium. For air, the breakdown strength is about 30 kV/cm at sea level. At the beginning stages, free electrons in the gap (from cosmic rays or background radiation) are accelerated by the electrical field. As they collide with air molecules, they create additional ions and newly-freed electrons which are also accelerated. The exponentially-increasing electrons and ions rapidly cause regions of the air in the gap to become electrically conductive in a process called dielectric breakdown. Once the gap breaks down, current flow is limited by the available charge (for an electrostatic discharge) or by the impedance of the external power supply. If the power supply continues to supply current, the spark will evolve into a continuous discharge called an electric arc. An electric spark can also occur within insulating liquids or solids, but the breakdown mechanisms are significantly different than for sparks in gases.
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.
==History==
Around 600 BC, Greek philosopher Thales of Miletus observed that amber could be electrified when rubbed with a cloth and attract other objects and produce sparks. In 1671, Leibniz discovered that sparks were associated with electrical phenomena. In 1708, Samuel Wall performed experiments with amber rubbed with cloth to produce sparks. In 1752, Thomas-François Dalibard and Benjamin Franklin independently demonstrated that lightning and electricity were equivalent. In Franklin's famous kite experiment, he successfully extracted sparks from a cloud during a thunderstorm.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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