Electric spark
[1] The rapid transition from a non-conducting to a conductive state produces a brief emission of light and a sharp crack or snapping sound.
[2] Experimentally, this figure tends to differ depending upon humidity, atmospheric pressure, shape of electrodes (needle and ground-plane, hemispherical etc.)
At the beginning stages, free electrons in the gap (from cosmic rays or background radiation) are accelerated by the electrical field, resulting in a Townsend avalanche.
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.
Even inadvertent static-discharges, or small sparks that occur when switching on lights or other circuits, can be enough to ignite flammable vapors from sources like gasoline, acetone, propane, or dust concentrations in the air, such as those found in flour mills or more generally in factories handling powders.
The higher the voltage; the farther a spark can jump across a gap, and with enough energy supplied can lead to greater discharges such as a glow or an arc.
High-voltage sparks, even those with low energy such as from a stun gun, can overload the conductive pathways of the nervous system, causing involuntary muscle-contractions, or interfere with vital nervous-system functions such as heart rhythm.
Conductive liquids, gels or ointments are often used when applying electrodes to a person's body, preventing sparks from forming at the point of contact and damaging skin.
Similarly, sparks can cause damage to metals and other conductors, ablating or pitting the surface; a phenomenon which is exploited in electric etching.
Sparks also produce ozone which, in high enough concentrations, can cause respiratory discomfort or distress, itching, or tissue damage, and can be harmful to other materials such as certain plastics.
