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
breakdown voltage of the gas within the gap, a
spark forms,
ionizing the gas and drastically reducing its
electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the
voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to
sound (ranging from a
snap for a
spark plug to
thunder for a
lightning discharge),
light and
heat.