An electric ark might be something that holds electrons
safely through a great flood. An electric arc, however,
is an event where the insulating effect preventing current
from flowing between two conductors and different potentials
is broken down and suddenly allows current to flow. Under
this condition, an insulator, which previously prevented the
flow of electricity, becomes a conductor.
Arcing almost always results in a discharge of light and heat.
The degree to which this happens generally depends upon the
material that is breaking down. Neon lamps generate light
by having a voltage applied and controlling an arc. Very little
noise is generated but considerable light is output.
Lightning is an arc generated when the voltage potential between
clouds or between a cloud and the Earth reach such high values
that the surrounding air that is insulating the two breaks down.
The light, heat, and noise generated is very large.
Paul Leonard, B.A., Sr. Software Engineer
I believe the phenomenon referred to is the electric 'arc.' This occurs when a
large enough voltage is built up to cause dielectric breakdown of a media,
generally air. Basically, normally current won't flow through air, but if you have
a big enough negative charge piled up in one place, and a big enough positive
charge close by, the electrons (charge carriers) will be pulled so hard that they
are ripped through the normally non-conductive medium between them. These
electrons rip through the air so hard that they create a plasma (a state of matter
where electrons and nuclei are not bound together) along the path they travel.
This path is the glowing area associated with an arc/spark, and is the same
phenomenon as lightning, although on a much smaller scale. Electrons then flow
along this path until the voltage is mostly dissipated.
Gregory Ogin, Physics Undergraduate Student, UST, St. Paul, MN
'The difference between what the most and the least learned people know is inexpressibly trivial in relation to that which is unknown.'