Sparks in a Vacuum
Date: Around 1993
Can a spark travel across a vacuum?
What happens to the air between two charged objects to allow a
spark to jump between them?
In the air between two highly charged objects there is a large
electric field. If there happens to be a free electron in that space it will
be accelerated to high speeds by the electric field. While it is being
accelerated it will collide with the gas atoms and be slowed down. When the
field gets so strong that it gains enough energy between collisions so that it
can ionize (excite another electron from the colliding atom) the atom it hits,
the number of electrons in the air can increase very quickly (called an
electron avalanche) This large number of electrons increases quickly as each
electron frees a new electron and the whole group makes up the spark. The
spark can move in a vacuum, but the creation of the spark requires the gas
atoms to be present as source of new electrons and the avalanche. The
breakdown field depends on the density and how tightly held are the atomic
The previous answer describes a spark that occurs in air or another medium,
and describes it well.
The question is actually fairly tricky because the word "spark" is not well
defined. Usually by "spark" we mean a momentary flow of electricity through
a medium that does not usually conduct electricity. We have sparks through
air, but not through metal. When we think of the word "spark" we think of
a bright flash. The word "arc" usually connotes a continuous flow of
electricity. Like an arc welder.
A good vacuum is a very good insulator. Much better than air because there
are no molecules to ionize and participate in an avalanche. However,
researchers who work with high-vacuum, high-voltage equipment know that
little "sparks" occur in vacuum with a few thousand volts or more. We do not
know why it happens, but we think it has something to do with dirt or dust
on charged surfaces. The strange thing is that these sparks are little
points of light that do not apparently jump between anything, like a normal
It is possible to have a stream of electrons travel in a vacuum. To do this
the emitter must be sharply pointed and the applied voltage must be large.
Electrons come off the sharply pointed emitter. This is the basis of the
field-emission electron microscope. But these electrons do not make light,
so they are not a "spark" or "arc" as commonly used. So, no, it does not
seem possible to have a 'spark' in the conventional sense in a good vacuum.
There is something called a "triggered vacuum spark-gap switch." It at
first seems like a switch based on a spark through a vacuum, but the current
is really conducted by a tiny burst of evaporated metal atoms, which carry
the current like air ions do.
Click here to return to the Physics Archives
Update: June 2012