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. When a suitable
voltage is supplied, 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 filament of ionized gas.
Encyclopedia
A
spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air. When a suitable
voltage is supplied, 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 filament of ionized gas. Usually the action of ionizing the gas is violent and disruptive, often leading to
sound ,
light and
heat.
Spark visibility
The emitted light does not come from the electron current, but the material medium
fluorescing in response to collisions from the electrons exciting its
electron orbitals to high, excited states and dropping them repeatedly. It is possible for a spark to form in a
vacuum and many comprise the
solar wind. Without intervening matter capable of electromagnetic transitions, the spark will be
invisible .
Applications
Spark gaps are essential to the functioning of a number of electronic devices.
Ignition devices
A spark plug uses a spark gap to initiate combustion. The heat of the ionization trail ignites a fuel-air mixture inside an
internal combustion engine, or a burner in a furnace, oven, or stove.
Radio transmitters
A spark radiates energy throughout the
electromagnetic spectrum. Nowadays, this is usually regarded as radio frequency interference and is suppressed, but in the early days of radio communications, this was the means by which radio signals were transmitted, in the
spark-gap transmitter. Many radio spark gaps include machined fins for cooling as the spark gap becomes quite hot with continued use.
Spark gaps as protective devices
Spark gaps are frequently used to prevent voltage surges from damaging equipment.Spark gaps are used in high-voltage
switches, for example, in
power plants and
electrical substations. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two
leaf springs holding the other end as second contact. If the blade is opened, a spark may keep the connection between blade and spring conducting. Here, a
Jacob's ladder on top of the switch will pull the arc apart and so extinguish it. You might also find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons. If a spark should ever manage to jump over the insulator and give rise to an arc, it will be extinguished.
Smaller spark gaps are often used to protect sensitive electrical or electronic equipment from high voltage surges. In sophisticated versions of these devices , a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment. These devices are commonly used for
telephone lines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of
lightning strikes. Less sophisticated spark gaps are made using modified
ceramic capacitors; in these devices, the spark gap is simply an air gap
sawn between the two lead wires that connect the capacitor to the circuit. A voltage surge causes a spark which jumps from lead wire to lead wire across the gap left by the sawing process. These low-cost devices are often used to prevent damaging arcs between the elements of the electron gun within a
cathode ray tube .
Small spark gaps are very common in
telephone switchboards, as the long phone cables are very susceptible to induced surges from
lightning strikes. Larger spark gaps are used to protect power lines.
Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications.
Neon bulbs are also used for this purpose.
Power-switching devices
Special purpose, high-energy triggerable spark gaps are also used to rapidly switch high voltages and very high currents for certain pulsed power applications, such as pulsed
lasers,
railguns,
fusion and ultrastrong pulsed
magnetic field research. These often have higher power capabilities than any
thyristor,
thyratron,
krytron, or
sprytron. One such type of switch is known as a
trigatron.
Visual entertainment
A
Jacob's ladder is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top.
When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an
electric arc. Air breaks down at about 24kV/in, depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a
short circuit, drawing as much current as the electrical
power supply can deliver, and the heavy
load dramatically reduces the voltage across the gap.
The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.
This cycle leads to an exotic-looking display of electric
white,
yellow,
blue or purple arcs which is often seen in movies about
mad scientists. The device was a staple in schools and science fairs of the
1950s and
1960s, typically constructed out of a
Model T spark coil, or any other source of high voltage in the 10,000 volt - 30,000 volt range, like a
neon sign transformer or circuit or a television picture tube circuit , and two coat hangers or rods built into a "V" shape.
The sparks can burn through thin paper and plastic and start fires; contact with the exposed high voltage can be lethal.
Spark gaps in nature
Lightning is a large spark leaping between a
cloud and the
earth or between clouds.
See also
External links
