A spark gap consists of an arrangement of two

conducting Conducting is the art of directing a musical performance, such as an orchestral or Choir, choral concert. It has been defined as "the art of directing the simultaneous performance of several players or singers by the use of gesture." The primary d ...

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

s separated by a gap usually filled with a gas such as air, designed to allow an

electric spark An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an Ionization, ionized, Electric current, electrically conductive channel through a normally-insulating medium, often air or other ga ...

to pass between the conductors. When the potential 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 The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...

. 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 Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

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 In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

(ranging from a ''snap'' for a spark plug to

thunder Thunder is the sound caused by lightning. Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning pr ...

for a

lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...

discharge),

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

, and

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

. Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, and

X-ray machine An X-ray machine is a device that uses X-rays for a variety of applications including medicine, X-ray fluorescence, electronic assembly inspection, and measurement of material thickness in manufacturing operations. In medical applications, X-ra ...

s. Their most widespread use today is in spark plugs to ignite the fuel in

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

s, but they are also used in lightning arresters and other devices to protect electrical equipment from high-voltage transients.

Breakdown voltage

For air, the breakdown strength is about 30 kV/cm at sea level.

Spark visibility

The light emitted by a spark does not come from the current of

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s itself, but from the material medium fluorescing in response to collisions from the electrons. When electrons collide with molecules of air in the gap, they excite their orbital electrons to higher

energy level A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...

s. When these excited electrons fall back to their original energy levels, they emit energy as light. It is impossible for a visible spark to form in a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

. Without intervening matter capable of

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

transitions, the spark will be invisible (see vacuum arc).

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 Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

. The heat of the ionization trail, but more importantly, UV radiation and hot free electrons (both cause the formation of reactive free radicals) ignite a fuel-air mixture inside an

, or a burner in a furnace, oven, or stove. The more UV radiation is produced and successfully spread into the combustion chamber, the further the combustion process proceeds. The Space Shuttle Main Engine hydrogen oxygen propellant mixture was ignited with a spark igniter.

Protective devices

Spark gaps are frequently used to prevent voltage surges from damaging equipment. Spark gaps are used in high-voltage

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

es, large power transformers, in

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an electr ...

s 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. The spark ionizes the air, which becomes conductive and allows an arc to form, which sustains ionization and hence conduction. A Jacob's ladder on top of the switch will cause the arc to rise and eventually extinguish. One might also find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons. These are sometimes called horn gaps. 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 (called gas tube arresters), 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 A telephone, colloquially referred to as a phone, is a telecommunications device that enables two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most ...

lines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of

strikes. Less sophisticated (and much less expensive) 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 that 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(s) within a

cathode-ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...

(CRT). Small spark gaps are very common in telephone switchboards, as the long phone cables are very susceptible to induced surges from

strikes. Larger spark gaps are used to protect

power line An overhead power line is a structure used in electric power transmission and Electric power distribution, distribution to transmit electrical energy along large distances. It consists of one or more electrical conductor, conductors (commonly mu ...

s. Spark gaps are sometimes implemented on Printed Circuit Boards in electronics products using two closely spaced exposed PCB traces. This is an effectively zero cost method of adding crude over-voltage protection to electronics products. Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are also used for this purpose.

High speed photography

A triggered spark gap in an air-gap flash is used to produce photographic light flashes in the sub-microsecond domain.

Radio transmitters

A spark radiates energy throughout the

electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...

. Nowadays, this is usually regarded as illegal radio frequency interference and is suppressed, but in the early days of radio communications (1880–1920), this was the means by which radio signals were transmitted, in the unmodulated

spark-gap transmitter A spark-gap transmitter is an obsolete type of transmitter, radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the m ...

. Many radio spark gaps include cooling devices, such as the rotary gap and

heat sink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is thermal management (electronics), ...

s, since the spark gap becomes quite hot under continuous use at high power.

Sphere gap for voltage measurement

A calibrated spherical spark gap will break down at a highly repeatable voltage, when corrected for air pressure, humidity and temperature. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. A spark gap can be used to measure high voltage AC, DC, or pulses, but for very short pulses, an

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

light source or radioactive source may be put on one of the terminals to provide a source of electrons.

Power-switching devices

Spark gaps may be used as electrical switches because they have two states with significantly different electrical resistance. Resistance between the electrodes may be as high as 10 ohms when the electrodes are separated by gas or vacuum which means that little current flows even when a high voltage exists between the electrodes. Resistance drops as low as a 10 ohms low when the electrodes are connected by plasma which means that power dissipation is low even at high current. This combination of properties has led to the use of spark gaps as electrical switches in

pulsed power Pulsed power is the science and technology of accumulating energy over a relatively long period of time and releasing it instantly, thus increasing the instantaneous power. They can be used in some applications such as food processing, water treatme ...

applications where energy is stored at high voltage in a

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

and then discharged at high current. Examples include pulsed

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

railgun A railgun or rail gun, sometimes referred to as a rail cannon, is a linear motor device, typically designed as a ranged weapon, that uses Electromagnet, electromagnetic force to launch high-velocity Projectile, projectiles. The projectile norma ...

s, Marx generators, fusion, ultrastrong pulsed

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

research, and nuclear bomb triggering. When a spark gap consists of only two electrodes separated by gas, the transition between the non-conducting and conducting states is governed by Paschen's law. At typical pressure and electrode distance combinations, Paschen's law says that Townsend discharge will fill the gap between the electrodes with conductive plasma whenever the ratio of the

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

strength to the pressure exceeds a constant value determined by the composition of the gas. The speed with which pressure can be reduced is limited by

choked flow Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the fluid velocity. Choked flow is a Fluid dynamics, fluid dynamic condition associated with the Venturi effect. When a flowing fluid at a given pressu ...

, while increasing the electric field in a capacitor discharge circuit is limited by the capacitance in the circuit and the current available for charging the capacitance. These limitations on the speed with which discharge may be initiated mean that spark gaps with two electrodes typically have high

jitter In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a signifi ...

. Triggered spark gaps are a class of devices with some additional means of triggering to achieve low jitter. Most commonly, this is a third electrode, as in a trigatron. The voltage of the trigger electrode can be changed quickly because the capacitance between it and the other electrodes is small. In a triggered spark gap, gas pressure is optimized to minimize jitter while also avoiding unintentional triggering. Triggered spark gaps are made in permanently sealed versions with limited voltage range and in user-pressurized versions with voltage range proportional to the available pressure range. Triggered spark gaps share many similarities with other

gas-filled tube A gas-filled tube, also commonly known as a discharge tube or formerly as a Julius Plücker, Plücker tube, is an arrangement of electrodes in a gas within an dielectric, insulating, temperature-resistant envelope. Gas-filled tubes exploit phen ...

s such as thyratrons, krytrons, ignitrons, and crossatrons. Triggered vacuum gaps, or sprytrons, resemble triggered spark gaps both in appearance and construction but rely on a different operating principle. A triggered vacuum gap consists of three electrodes in an airtight glass or ceramic envelope that has been evacuated. This means that, unlike a triggered spark gap, a triggered vacuum gap operates in the parameter space to the left of the Paschen minimum where breakdown is promoted by increasing pressure. Current between the electrodes is limited to a small value by field emission in the non-conducting state. Breakdown is initiated by rapidly evaporating material from a trigger electrode or an adjacent resistive coating. Once the vacuum arc is initiated, a triggered vacuum gap is filled with conductive plasma as in any other spark gap. A triggered vacuum gap has a larger operating voltage range than a sealed triggered spark gap because Paschen curves are much steeper to the left of the Paschen minimum than at higher pressures. Triggered vacuum gaps are also rad hard because in the non-conducting state they do not contain any gas that could be ionized by radiation.

Insect control

They are also used as insect zappers. The two electrodes are implemented as metal lattices placed a slightly too far apart for the voltage to jump. When an insect ventures between the electrodes the gap distance is reduced by the insects body, being conductive, and a spark discharge occurs to electrocute and burn the insect. In this use the spark gap mechanism is often used in conjunction with a bait, such as a light, to attract the insect into the spark gap.

References

External links

Jacob's Ladder videos:
Transformer in Nevada

In someone's home
{{Authority control Electric arcs Electrical breakdown Gas-filled tubes Switching tubes Articles containing video clips