An electrical insulator is a material whose internal electric charges do not flow freely. This contrasts with other materials and conductors, which conduct electric current more easily; the property that distinguishes an insulator is its resistivity. A perfect insulator does not exist, because insulators contain small numbers of mobile charges which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms; this is known as the breakdown voltage of an insulator. Some materials such as glass and Teflon, which have high resistivity, are good electrical insulators. A much larger class of materials though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at used voltages, thus are employed as insulation for electrical wiring and cables. Examples include rubber-like polymers and most plastics which can be thermoset or thermoplastic in nature.
Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation; the term insulator is used more to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground. Electrical insulation is the absence of electrical conduction. Electronic band theory says that a charge flows if states are available into which electrons can be excited; this allows electrons to gain energy and thereby move through a conductor such as a metal. If no such states are available, the material is an insulator. Most insulators have a large band gap; this occurs because the "valence" band containing the highest energy electrons is full, a large energy gap separates this band from the next band above it.
There is always some voltage. Once this voltage is exceeded the material ceases being an insulator, charge begins to pass through it. However, it is accompanied by physical or chemical changes that permanently degrade the material's insulating properties. Materials that lack electron conduction are insulators. For example, if a liquid or gas contains ions the ions can be made to flow as an electric current, the material is a conductor. Electrolytes and plasmas contain ions and act as conductors whether or not electron flow is involved; when subjected to a high enough voltage, insulators suffer from the phenomenon of electrical breakdown. When the electric field applied across an insulating substance exceeds in any location the threshold breakdown field for that substance, the insulator becomes a conductor, causing a large increase in current, an electric arc through the substance. Electrical breakdown occurs when the electric field in the material is strong enough to accelerate free charge carriers to a high enough velocity to knock electrons from atoms when they strike them, ionizing the atoms.
These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a chain reaction. The insulator becomes filled with mobile charge carriers, its resistance drops to a low level. In a solid, the breakdown voltage is proportional to the band gap energy; when corona discharge occurs, the air in a region around a high-voltage conductor can break down and ionise without a catastrophic increase in current. However, if the region of air breakdown extends to another conductor at a different voltage it creates a conductive path between them, a large current flows through the air, creating an electric arc. A vacuum can suffer a sort of breakdown, but in this case the breakdown or vacuum arc involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself. In addition, all insulators become conductors at high temperatures as the thermal energy of the valence electrons is sufficient to put them in the conduction band.
In certain capacitors, shorts between electrodes formed due to dielectric breakdown can disappear when the applied electric field is reduced. A flexible coating of an insulator is applied to electric wire and cable, this is called insulated wire. Wires sometimes don't use an insulating coating, since a solid coating may be impractical. However, wires that touch each other produce cross connections, short circuits, fire hazards. In coaxial cable the center conductor must be supported in the middle of the hollow shield to prevent EM wave reflections. Wires that expose voltages higher than 60 V can cause human shock and electrocution hazards. Insulating coatings help to prevent all of these problems; some wires have a mechanical covering with no voltage rating—e.g.: service-drop, doorbell, thermostat wire. An insulated wire or cable has a maximum conductor temperature rating, it may not have an ampacity rating. In electronic systems, printed circuit boards are made from epoxy plastic and fibreglass.
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. For diodes, the breakdown voltage is the minimum reverse voltage that makes the diode conduct appreciably in reverse; some devices have a forward breakdown voltage. Breakdown voltage is a characteristic of an insulator that defines the maximum voltage difference that can be applied across the material before the insulator conducts. In solid insulating materials, this creates a weakened path within the material by creating permanent molecular or physical changes by the sudden current. Within rarefied gases found in certain types of lamps, breakdown voltage is sometimes called the striking voltage; the breakdown voltage of a material is not a definite value because it is a form of failure and there is a statistical probability whether the material will fail at a given voltage. When a value is given it is the mean breakdown voltage of a large sample. Another term is withstand voltage, where the probability of failure at a given voltage is so low it is considered, when designing insulation, that the material will not fail at this voltage.
Two different breakdown voltage measurements of a material are the AC and impulse breakdown voltages. The AC voltage is the line frequency of the mains; the impulse breakdown voltage is simulating lightning strikes, uses a 1.2 microsecond rise for the wave to reach 90% amplitude drops back down to 50% amplitude after 50 microseconds. Two technical standards governing performing these tests are ASTM D1816 and ASTM D3300 published by ASTM. In standard conditions at atmospheric pressure, air serves as an excellent insulator, requiring the application of a significant voltage of 3.0 kV/mm before breaking down. In partial vacuum, this breakdown potential may decrease to an extent that two uninsulated surfaces with different potentials might induce the electrical breakdown of the surrounding gas; this may damage an apparatus. In a gas, the breakdown voltage can be determined by Paschen's law; the breakdown voltage in a partial vacuum is represented as V b = B p d ln − ln where V b is the breakdown potential in volts DC, A and B are constants that depend on the surrounding gas, p represents the pressure of the surrounding gas, d represents the distance in centimetres between the electrodes, γ s e represents the Secondary Electron Emission Coefficient.
A detailed derivation and some background information is given in the article about Paschen's law. Breakdown voltage is a parameter of a diode that defines the largest reverse voltage that can be applied without causing an exponential increase in the leakage current in the diode. Exceeding the breakdown voltage of a diode, per se, is not destructive. In fact, Zener diodes are just doped normal diodes that exploit the breakdown voltage of a diode to provide regulation of voltage levels. Rectifier diodes may have several voltage ratings, such as the peak inverse voltage across the diode, the maximum RMS input voltage to the rectifier circuit. Many small-signal transistors need to have any breakdown currents limited to much lower values to avoid excessive heating. To avoid damage to the device, to limit the effects excessive leakage current may have on the surrounding circuit, the following bipolar transistor maximum ratings are specified: VCEO (sometimes written BVCEO or VCEO The maximum voltage between collector and emitter that can be safely applied when no circuit at the base of the transistor is there to remove collector-base leakage.
Typical values: 20 volts to as high as 700 volts. VCBO The maximum collector-to-base voltage, with emitter open-circuit. Typical values 25 to 1200 volts. VCER The maximum voltage rating between collector and emitter with some specified resistance between base and emitter. A more realistic rating for real-world circuits than the open-base or open-emitter scenarios above. VEBO The maximum reverse voltage on the base with respect to the emitter. VCES Collector to emitter rating; some devices may have a maximum rate of change of voltage specified. Power transformers, circuit breakers and other electrical apparatus connected to overhead transmission lines are exposed to transi
In physics and electronic engineering, an electron hole is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or crystal lattice the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, the absence of an electron leaves a net positive charge at the hole's location. Holes are not particles, but rather quasiparticles. Holes in a metal or semiconductor crystal lattice can move through the lattice as electrons can, act to positively-charged particles, they play an important role in the operation of semiconductor devices such as transistors and integrated circuits. If an electron is excited into a higher state it leaves a hole in its old state; this meaning is used in Auger electron spectroscopy, in computational chemistry, to explain the low electron-electron scattering-rate in crystals. In crystals, electronic band structure calculations lead to an effective mass for the electrons, negative at the top of a band.
The negative mass is an unintuitive concept, in these situations a more familiar picture is found by considering a positive charge with a positive mass. In solid-state physics, an electron hole is the absence of an electron from a full valence band. A hole is a way to conceptualize the interactions of the electrons within a nearly full valence band of a crystal lattice, missing a small fraction of its electrons. In some ways, the behavior of a hole within a semiconductor crystal lattice is comparable to that of the bubble in a full bottle of water. Hole conduction in a valence band can be explained by the following analogy. Imagine a row of people seated in an auditorium, where there are no spare chairs. Someone in the middle of the row wants to leave, so he jumps over the back of the seat into another row, walks out; the empty row is analogous to the conduction band, the person walking out is analogous to a conduction electron. Now imagine someone else comes along and wants to sit down; the empty row has a poor view.
Instead, a person in the crowded row moves into the empty seat the first person left behind. The empty seat moves one spot closer to the person waiting to sit down; the next person follows, the next, et cetera. One could say. Once the empty seat reaches the edge, the new person can sit down. In the process everyone in the row has moved along. If those people were negatively charged, this movement would constitute conduction. If the seats themselves were positively charged only the vacant seat would be positive; this is a simple model of how hole conduction works. Instead of analyzing the movement of an empty state in the valence band as the movement of many separate electrons, a single equivalent imaginary particle called a "hole" is considered. In an applied electric field, the electrons move in one direction, corresponding to the hole moving in the other. If a hole associates itself with a neutral atom, that atom becomes positive. Therefore, the hole is taken to have positive charge of +e the opposite of the electron charge.
In reality, due to the uncertainty principle of quantum mechanics, combined with the energy levels available in the crystal, the hole is not localizable to a single position as described in the previous example. Rather, the positive charge which represents the hole spans an area in the crystal lattice covering many hundreds of unit cells; this is equivalent to being unable to tell. Conduction band electrons are delocalized; the analogy above is quite simplified, cannot explain why holes create an opposite effect to electrons in the Hall effect and Seebeck effect. A more precise and detailed explanation follows; the dispersion relation determines. A dispersion relation is the relationship between wavevector and energy in a band, part of the electronic band structure. In quantum mechanics, the electrons are waves, energy is the wave frequency. A localized electron is a wavepacket, the motion of an electron is given by the formula for the group velocity of a wave. An electric field affects an electron by shifting all the wavevectors in the wavepacket, the electron accelerates when its wave group velocity changes.
Therefore, the way an electron responds to forces is determined by its dispersion relation. An electron floating in space has the dispersion relation E=ℏ2k2/, where m is the electron mass and ℏ is reduced Planck constant. Near the bottom of the conduction band of a semiconductor, the dispersion relation is instead E=ℏ2k2/, so a conduction-band electron responds to forces as if it had the mass m*. Electrons near the top of the valence band behave; the dispersion relation near the top of the valence band is E=ℏ2k2/ with negative effective mass. So electrons near the top of the valence band behave; when a force pulls the electrons to the right, these electrons move left. This is due to the shape of the valence band, is unrelated to whether the band is full or empty. If you could somehow empty out the valence band and just put one electron near the valence band maximum, this electron would move the "wrong way" in response to forces. Po
The electron is a subatomic particle, symbol e− or β−, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, are thought to be elementary particles because they have no known components or substructure; the electron has a mass, 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant, ħ; as it is a fermion, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light; the wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy. Electrons play an essential role in numerous physical phenomena, such as electricity, magnetism and thermal conductivity, they participate in gravitational and weak interactions.
Since an electron has charge, it has a surrounding electric field, if that electron is moving relative to an observer, it will generate a magnetic field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law. Electrons absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields. Special telescopes can detect electron plasma in outer space. Electrons are involved in many applications such as electronics, cathode ray tubes, electron microscopes, radiation therapy, gaseous ionization detectors and particle accelerators. Interactions involving electrons with other subatomic particles are of interest in fields such as chemistry and nuclear physics; the Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms.
Ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the main cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of an indivisible quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge'electron' in 1891, J. J. Thomson and his team of British physicists identified it as a particle in 1897. Electrons can participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of radioactive isotopes and in high-energy collisions, for instance when cosmic rays enter the atmosphere; the antiparticle of the electron is called the positron. When an electron collides with a positron, both particles can be annihilated, producing gamma ray photons.
The ancient Greeks noticed. Along with lightning, this phenomenon is one of humanity's earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electrica, to refer to those substances with property similar to that of amber which attract small objects after being rubbed. Both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον. In the early 1700s, Francis Hauksbee and French chemist Charles François du Fay independently discovered what they believed were two kinds of frictional electricity—one generated from rubbing glass, the other from rubbing resin. From this, du Fay theorized that electricity consists of two electrical fluids and resinous, that are separated by friction, that neutralize each other when combined. American scientist Ebenezer Kinnersley also independently reached the same conclusion. A decade Benjamin Franklin proposed that electricity was not from different types of electrical fluid, but a single electrical fluid showing an excess or deficit.
He gave them the modern charge nomenclature of negative respectively. Franklin thought of the charge carrier as being positive, but he did not identify which situation was a surplus of the charge carrier, which situation was a deficit. Between 1838 and 1851, British natural philosopher Richard Laming developed the idea that an atom is composed of a core of matter surrounded by subatomic particles that had unit electric charges. Beginning in 1846, German physicist William Weber theorized that electricity was composed of positively and negatively charged fluids, their interaction was governed by the inverse square law. After studying the phenomenon of electrolysis in 1874, Irish physicist George Johnstone Stoney suggested that there existed a "single definite quantity of electricity", the charge of a monovalent ion, he was able to estimate the value of this elementary charge e by means of Faraday's laws of electrolysis. However, Stoney could not be removed. In 1881, German physicist Hermann von Helmholtz argued that both positive and negative charges were divided into elementary parts, each of which "behaves like atoms of electricity".
Stoney coined the term
A diode is a two-terminal electronic component that conducts current in one direction. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices; the discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used; the most common function of a diode is to allow an electric current to pass in one direction, while blocking it in the opposite direction. As such, the diode can be viewed as an electronic version of a check valve; this unidirectional behavior is called rectification, is used to convert alternating current to direct current.
Forms of rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers. However, diodes can have more complicated behavior than this simple on–off action, because of their nonlinear current-voltage characteristics. Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction; the voltage drop across a forward-biased diode varies only a little with the current, is a function of temperature. Diodes' high resistance to current flowing in the reverse direction drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture; these techniques are used to create special-purpose diodes. For example, diodes are used to regulate voltage, to protect circuits from high voltage surges, to electronically tune radio and TV receivers, to generate radio-frequency oscillations, to produce light.
Tunnel, Gunn and IMPATT diodes exhibit negative resistance, useful in microwave and switching circuits. Diodes, both vacuum and semiconductor, can be used as shot-noise generators. Thermionic diodes and solid-state diodes were developed separately, at the same time, in the early 1900s, as radio receiver detectors; until the 1950s, vacuum diodes were used more in radios because the early point-contact semiconductor diodes were less stable. In addition, most receiving sets had vacuum tubes for amplification that could have the thermionic diodes included in the tube, vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes that were available at that time. In 1873, Frederick Guthrie observed that a grounded, white hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope, but not a negatively charged electroscope. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb called Edison effect, was granted a patent on application of the phenomenon for use in a dc voltmeter.
About 20 years John Ambrose Fleming realized that the Edison effect could be used as a radio detector. Fleming patented the first true thermionic diode, the Fleming valve, in Britain on November 16, 1904. Throughout the vacuum tube era, valve diodes were used in all electronics such as radios, sound systems and instrumentation, they lost market share beginning in the late 1940s due to selenium rectifier technology and to semiconductor diodes during the 1960s. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, in musical instrument and audiophile applications. In 1874, German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral. Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894; the crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906.
Other experimenters tried a variety of other minerals as detectors. Semiconductor principles were unknown to the developers of these early rectifiers. During the 1930s understanding of physics advanced and in the mid 1930s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology. Researchers at Bell Labs, Western Electric, MIT, Purdue and in the UK intensively developed point-contact diodes during World War II for application in ra
A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, behaviour opposite to that of a metal, their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created; the behavior of charge carriers which include electrons and electron holes at these junctions is the basis of diodes and all modern electronics. Some examples of semiconductors are silicon and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor used in laser diodes, solar cells, microwave frequency integrated circuits, others. Silicon is a critical element for fabricating most electronic circuits. Semiconductor devices can display a range of useful properties such as passing current more in one direction than the other, showing variable resistance, sensitivity to light or heat.
Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification and energy conversion. The conductivity of silicon is increased by adding a small amount of trivalent atoms; this process is known as doping and resulting semiconductors are known as doped or extrinsic semiconductors. Apart from doping, the conductivity of a semiconductor can be improved by increasing its temperature; this is contrary to the behaviour of a metal in which conductivity decreases with increase in temperature. The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of charge carriers in a crystal lattice. Doping increases the number of charge carriers within the crystal; when a doped semiconductor contains free holes it is called "p-type", when it contains free electrons it is known as "n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants.
A single semiconductor crystal can have many p- and n-type regions. Although some pure elements and many compounds display semiconductor properties, silicon and compounds of gallium are the most used in electronic devices. Elements near the so-called "metalloid staircase", where the metalloids are located on the periodic table, are used as semiconductors; some of the properties of semiconductor materials were observed throughout the mid 19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of the cat's-whisker detector, a primitive semiconductor diode used in early radio receivers. Developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958. Variable electrical conductivity Semiconductors in their natural state are poor conductors because a current requires the flow of electrons, semiconductors have their valence bands filled, preventing the entry flow of new electrons.
There are several developed techniques that allow semiconducting materials to behave like conducting materials, such as doping or gating. These modifications have two outcomes: p-type; these refer to the shortage of electrons, respectively. An unbalanced number of electrons would cause a current to flow through the material. Heterojunctions Heterojunctions occur when two differently doped semiconducting materials are joined together. For example, a configuration could consist of n-doped germanium; this results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, the p-doped germanium would have an excess of holes; the transfer occurs until equilibrium is reached by a process called recombination, which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type. A product of this process is charged ions. Excited electrons A difference in electric potential on a semiconducting material would cause it to leave thermal equilibrium and create a non-equilibrium situation.
This introduces electrons and holes to the system, which interact via a process called ambipolar diffusion. Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes; such disruptions can occur as a result of a temperature difference or photons, which can enter the system and create electrons and holes. The process that creates and annihilates electrons and holes are called generation and recombination. Light emission In certain semiconductors, excited electrons can relax by emitting light instead of producing heat; these semiconductors are used in the construction of light-emitting diodes and fluorescent quantum dots. High thermal conductivitySemiconductors with high thermal conductivity can be used for heat dissipation and improving thermal management of electronics. Thermal energy conversion Semiconductors have large thermoelectric power factors making them useful in thermoelectric generators, as well as high thermoelectric figures of merit making them useful in thermoelectric coolers.
A large number of elements and compounds have semiconducting properties, including: Certain pure elements are found in Group 14 of the p
A spark gap consists of an arrangement of two conducting electrodes separated by a gap filled with a gas such as air, designed to allow an electric spark 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. An electric current flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current"; this happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and breaking the filament of ionized gas. The action of ionizing the gas is violent and disruptive leading to sound and heat. Spark gaps were used in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, X-ray machines, their most widespread use today is in spark plugs to ignite the fuel in internal combustion engines, but they are used in lightning arresters and other devices to protect electrical equipment from high-voltage transients.
The light emitted by a spark does not come from the current of electrons 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 levels; 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. Without intervening matter capable of electromagnetic transitions, the spark will be invisible. Spark gaps are essential to the functioning of a number of electronic devices. A spark plug uses a spark gap to initiate combustion; the heat of the ionization trail, but more UV radiation and hot free electrons ignite a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven, or stove. The more UV radiation is produced and spread into the combustion chamber, the further the combustion process proceeds. Spark gaps are used to prevent voltage surges from damaging equipment.
Spark gaps are used in high-voltage switches, large power transformers, 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 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 extinguish. One might 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 manage to jump over the insulator and give rise to an arc, it will be extinguished. Smaller spark gaps are 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 used for telephone lines as they enter a building. Less sophisticated spark gaps are made using modified ceramic capacitors. 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 used to prevent damaging arcs between the elements of the electron gun within a cathode ray tube. Small spark gaps are common in telephone switchboards, as the long phone cables are susceptible to induced surges from lightning strikes. Larger spark gaps are used to protect power lines. Spark gaps are implemented on Printed Circuit Boards in mains power electronics products using two spaced exposed PCB traces; this is an zero cost method of adding crude overload protection to electronics products. Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are used for this purpose. A triggered spark gap in an air-gap flash is used to produce photographic light flashes in the sub-microsecond domain.
A spark radiates energy throughout the electromagnetic spectrum. Nowadays, this is regarded as illegal 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 unmodulated spark-gap transmitter. Many radio spark gaps include cooling devices, such as the rotary gap and heat sinks, since the spark gap becomes quite hot under continuous use at high power. A calibrated spherical spark gap will break down at a repeatable voltage, when corrected for air pressure 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 short pulses, an ultraviolet light source or radioactive source may be put on one of the terminals to provide a source of electrons. Spark gaps may be used as electrical switches because they have two states with