Leader (spark)
A leader is a hot, highly conductive channel of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 that plays a critical part during dielectric breakdown within in a long electric spark
Electric spark
An electric spark is a type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air...



When a gas is subjected to high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

 stress, the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 is often quite non-uniform near one, or both, of the high voltage electrodes making up a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

. Breakdown initially begins with the formation of corona discharge
Corona discharge
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

s near the electrode with the highest electrical stress. If the electrical field is further increased, longer length cold discharges (called streamers or burst corona) sporadically form near the stressed electrode. Streamers are actually electron avalanche
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a medium are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision , thereby forming "new" electrons to undergo the same process in successive cycles...

s, each one typically lasting only a few nanoseconds. Streamers redistribute charge within the surrounding gas, temporarily forming regions of excess charge (space charge
Space charge
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges...

s) in the regions surrounding the discharges.

If the electrical field is sufficiently high, the individual currents from multitudes of streamers combine to create a hot, highly conductive path that projects from the electrode, going some distance into the gap. The projecting channel of hot plasma is called a leader, and it can have an electrical conductivity approaching that of an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

. The leader effectively projects the electrical field from the nearby electrode further into the gap, similar to introducing a short length of wire into the gap. The tip of the conductive leader now forms a new region from which streamers can entend even further into the gap. As new streamer discharges feed the tip of the leader, the streamer currents help to keep the leader hot and conductive. Under sufficiently high voltages, the leader will continue to extend itself further into the gap, doing so in a series of jumps until the entire gap has been bridged. Although leaders are most often associated with the initial formative stages of a lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 stroke, they are actually characteristic of the development of all long sparks. In the case of a lightning leader, each extension (called a step leader) is typically 10 - 50 meters in length. The branching, visible blue-white discharges from a Tesla Coil
Tesla coil
A Tesla coil is a type of resonant transformer circuit invented by Nikola Tesla around 1891. It is used to produce high voltage, low current, high frequency alternating current electricity. Tesla coils produce higher current than the other source of high voltage discharges, electrostatic machines...

are also examples of leaders.

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