Electrostatic generator

An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th Century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity. Electrostatic generators operate by using manual (or other) power to transform mechanical work into electric energy. They develop electrostatic charges of opposite signs rendered to two conductors, using only electric forces. They work by using moving plates, drums, or belts to carry electric charge to a high potential electrode. The charge is generated by one of two methods: either the triboelectric effect (friction) orelectrostatic induction.

Electrostatic machines are typically used in science classrooms to safely demonstrate electrical forces and high voltage phenomena. The elevated potential differences achieved have been also used for a variety of practical applications, such as operating X-ray tubes, medical applications, sterilization of food, and nuclear physics experiments. Electrostatic generators such as the Van de Graaff generator, and variations as the Pelletron, also find use in physics research.
Description

Electrostatic generators can be divided into two categories depending on how the charge is generated:

• Friction machines use the triboelectric effect (electricity generated by contact or friction)
• Influence machines use electrostatic induction

Friction machines

History

Typical friction machine using a glass globe, common in the 18th century

Martinus van Marum's Electrostatic generator at Teylers Museum

The first electrostatic generators are called friction machines because of the friction in the generation process. A primitive form of frictional electrical machine was constructed around 1663 by Otto von Guericke, using a sulphur globe that could be rotated and rubbed by hand. It may not actually have been rotated during use.^[1] , but inspired many later machines that used rotating globes. Isaac Newton suggested the use of a glass globe instead of a sulphur one (Optics, 8th Query). Francis Hauksbee improved the basic design ^[2].

Generators were further advanced when G. M. Bose of Wittenberg added a collecting conductor (an insulated tube or cylinder supported on silk strings). In 1746, Watson's machine had a large wheel turning several glass globes with a sword and a gun barrel suspended from silk cords for its prime conductors. J. H. Winkler, professor of physics at Leipzig, substituted a leather cushion for the hand. Andreas Gordon of Erfurt, a Scottish Benedictine monk, used a glass cylinder in place of a sphere. Jesse Ramsden, in 1768, constructed a widely used version of a plate electrical generator. By 1784, the van Marum machine could produce voltage with either polarity. Martin van Marum constructed a large electrostatic machine of high quality for his experiments (currently on display at the Teylers Museum in the Netherlands).

In 1785, N. Rouland constructed a silk belted machine which rubbed two grounded hare fur covered tubes. Edward Nairne developed an electrostatic generator for medical purposes in 1787 which had the ability to generate either positive or negative electricity, the first named being collected from the prime conductor carrying the collecting points and the second from another prime conductor carrying the friction pad. The Winter machine possessed higher efficiency than earlier friction machines. In the 1830s, Georg Ohm possessed a machine similar to the van Marum machine for his research (which is now at the Deutsches Museum, Munich, Germany). In 1840, the Woodward machine was developed from improving the Ramsden machine (placing the prime conductor above the disk(s)). Also in 1840, the Armstrong hydroelectric machine was developed and used steam as a charge carrier.

Friction operation

The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which leads to static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena ofcontact electrification and the triboelectric effect. Rubbing two non-conductive objects generates a great amount of static electricity. This is not just the result of friction; two non-conductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some examples of these substances are rubber, plastic, glass, and pith. Conductive objects in contact generate charge imbalance too, but retain the charges only if insulated. The charge that is transferred during contact electrification is stored on the surface of each object. Note that the presence of electric current does not detract from the electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can exist simultaneously in the same system.

Influence machines

History

Frictional machines were, in time, gradually superseded by the second class of instrument mentioned above, namely, influence machines. These operate by electrostatic induction and convert mechanical work into electrostatic energy by the aid of a small initial charge which is continually being replenished and reinforced. The first suggestion of an influence machine appears to have grown out of the invention of Volta's electrophorus. The electrophorus is a single-plate capacitor used to produce imbalances of electric charge via the process of electrostatic induction. The next step was when Abraham Bennet, the inventor of the gold leaf electroscope, described a "doubler of electricity" (Phil. Trans., 1787), as a device similar to the electrophorus, but that could amplify a small charge by means of repeated manual operations with three insulated plates, in order to make it observable in an electroscope. Erasmus Darwin, W. Wilson, G. C. Bohnenberger, and (later, 1841) J. C. E. Péclet developed various modifications of Bennet's device. In 1788, William Nicholson proposed his rotating doubler, which can be considered as the first rotating influence machine. His instrument was described as "an instrument which by turning a winch produces the two states of electricity without friction or communication with the earth". (Phil. Trans., 1788, p. 403) Nicholson later described a "spinning condenser" apparatus, as a better instrument for measurements.

Others, including T. Cavallo (who developed the "Cavallo multiplier", a charge multiplier using simple addition, in 1795), John Read, Charles Bernard Desormes, and Jean Nicolas Pierre Hachette, developed further various forms of rotating doublers. In 1798, The German scientist and preacher Gottlieb Christoph Bohnenberger, described the Bohnenberger machine, along with several other doublers of Bennet and Nicholson types in a book. The most interesting of these were described in the "Annalen der Physik" (1801). Giuseppe Belli, in 1831, developed a simple symmetrical doubler which consisted of two curved metal plates between which revolved a pair of plates carried on an insulating stem. It was the first symmetrical influence machine, with identical structures for both terminals. This apparatus was reinvented several times, by C. F. Varley, that patented a high power version in 1860, by Lord Kelvin (the "replenisher") 1868, and by A. D. Moore (the "dirod"), more recently. Lord Kelvin also devised a combined influence machine and electromagnetic machine, commonly called a mouse mill, for electrifying the ink in connection with his siphon recorder, and a water-drop electrostatic generator (1867), which he called the "water-dropping condenser".

Holtz's influence machine.

Between 1864 and 1880, W. T. B. Holtz constructed and described a large number of influence machines which were considered the most advanced developments of the time. In one form, the Holtz machine consisted of a glass disk mounted on a horizontal axis which could be made to rotate at a considerable speed by a multiplying gear, interacting with induction plates mounted in a fixed disk close to it. In 1865, August J. I. Toepler developed an influence machine that consisted of two disks fixed on the same shaft and rotating in the same direction. In 1868, the Schwedoff machine had a curious structure to increase the output current. Also in 1868, several mixed friction-influence machine were developed, including the Kundt machine and the Carré machine. In 1866, the Piche machine (or Bertsch machine) was developed. In 1869, H. Julius Smith received the American patent for a portable and airtight device that was designed to ignite powder. Also in 1869, sectorless machines in Germany were investigated by Poggendorff.

The action and efficiency of influence machines were further investigated by F. Rossetti, A. Righi, and F. W. G. Kohlrausch. E. E. N. Mascart, A. Roiti, andE. Bouchotte also examined the efficiency and current producing power of influence machines. In 1871, sectorless machines were investigated by Musaeus. In 1872, Righi's electrometer was developed and was one of the first antecedents of the Van de Graaff generator. In 1873, Leyser developed the Leyser machine, a variation of the Holtz machine. In 1880, Robert Voss (a Berlin instrument maker) devised a form of machine in which he claimed that the principles of Toepler and Holtz were combined. The same structure become also known as the Toepler-Holtz machine. In 1878, the British inventor James Wimshurst started his studies about electrostatic generators, improving the Holtz machine, in a powerful version with multiple disks. The classical Wimshurst machine, that become the most popular form of influence machine, was reported to the scientific community by 1883, although previous machines with very similar structures were previously described by Holtz and Musaeus. In 1885, one of the largest-ever Wimshurst machines was built in England (it is now at the Chicago Museum of Science and Industry). In 1887, Weinhold modified the Leyser machine with a system of vertical metal bar inductors with wooden cylinders close to the disk for avoiding polarity reversals. M. L. Lebiez described the Lebiez machine, that was essentially a simplified Voss machine (L'Électricien, April 1895, pp. 225-227). In 1894, Bonetti^[3]designed a machine with the structure of the Wimshurst machine, but without metal sectors in the disks. This machine is significantly more powerful than the sectored version, but it must usually be started with an externally-applied charge.

In 1898, the Pidgeon machine was developed with a unique setup by W. R. Pidgeon. In October 28 of that year, Pidgeon presented this machine to the Physical Society after several years of investigation into influence machines (beginning at the start of the decade). The device was later reported in the Philosophical Magazine (Dec. 1898, pg. 564) and the Electrical Review(Vol. XLV, pg. 748). A Pidgeon machine possesses fixed inductors arranged in a manner that increases the electrical induction effect (and its electrical output is at least double that of typical machines of this type [except when it is overtaxed]). The essential features of the Pidgeon machine are, one, the combination of the rotating support and the fixed support for inducing charge, and, two, the improved insulation of all parts of the machine (but more especially of the generator's carriers). Pidgeon machines are a combination of a Wimshurst Machine and Voss Machine, with special features adapted to reduce the amount of charge leakage. Pidgeon machines excite themselves more readily than the best of these types of machines. In addition, Pidgeon investigated higher current "triplex" section machines (or "double machines with a single central disk") with enclosed sectors (and went on to receive British Patent 22517 (1899) for this type of machine).

Multiple disk machines and "triplex" electrostatic machines (generators with three disks) were also developed extensively around the turn of the century. In 1900, F. Tudsbury discovered that enclosing a generator in a metallic chamber containing compressed air, or better, carbon dioxide, the insulating properties of compressed gases enabled a greatly improved effect to be obtained owing to the increase in the breakdown voltage of the compressed gas, and reduction of the leakage across the plates and insulating supports. In 1903, Alfred Wehrsen patented an ebonite rotating disk possessing embedded sectors with button contacts at the disk surface. In 1907, Heinrich Wommelsdorf reported a variation of the Holtz machine using this disk and inductors embedded in celluloid plates (DE154175; "Wehrsen machine"). Wommelsdorf also developed several high-performance electrostatic generators, of which the best known were his "Condenser machines" (1920). These were single disk machines, using disks with embedded sectors that were accessed at the edges.

Modern electrostatic generators

An example of a common modern device using high voltage (a "plasma globe", that does not use static electricity)

Electrostatic generators had a fundamental role in the investigations about the structure of matter, starting at the end of the 19th century. By the 1920s, it was evident that machines able to produce greater voltage were needed. The Van de Graaff generator was developed, starting in 1929, at MIT. The first model was demonstrated in October 1929. The basic idea was to use an insulating belt to transport electric charge to the interior of an insulated hollow terminal, where it could be discharged regardless of the potential already present on the terminal, that does not produce any electric field in its interior. The idea was not new, but the implementation using an electronic power supply to charge the belt was a fundamental innovation that made the old machines obsolete. The first machine used a silk ribbon bought at a five and dime store as the charge transport belt. In 1931 a version able to produce 1,000,000 volts was described in a patent disclosure. Nikola Tesla wrote a Scientific American article, "Possibilities of Electro-Static Generators" in 1934 concerning the Van de Graaff generator (pp. 132-134 and 163-165). Tesla stated, "I believe that when new types [of Van de Graaff generators] are developed and sufficiently improved a great future will be assured to them". High-power machines were soon developed, working on pressurized containers to allow greater charge concentration on the surfaces without ionization. Variations of the Van de Graaff generator were also developed for Physics research, as the Pelletron, that uses a chain with alternating insulating and conducting links for charge transport. Simplified Van de Graaff generators are commonly seen in demonstrations about static electricity, due to its high-voltage capability, producing the curious effect of making the hair of people touching the terminal, standing over an insulating support, stand up.

Between 1945 and 1960, the French researcher Noël Felici developed a series of high-power electrostatic generators, based on electronic excitation and using cylinders rotating at high speed and hydrogen in pressurized containers.

Fringe science and devices

These generators have been used, sometimes inappropriately and with some controversy, to support various fringe science investigations. In 1911, George Samuel Piggott received a patent for a compact double machine enclosed within a pressurized box for his experiments concerning radiotelegraphy and "antigravity". Much later (in the 1960s), a machine known as "Testatika" was built by German engineer, Paul Suisse Bauman, and promoted by a Swiss community, the Methernithans. Testatika is an electromagnetic generator based on the 1898 Pidgeon electrostatic machine, said to produce "free energy" available directly from the environment

Wimshurst machine

Wimshurst machine with two Leyden jars. Suppose that the conditions are as in the figure that is the segment A1 is positive and the segment B1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A1 comes opposite the segment B2 of the B plate, which is now in contact with the brush Y, it will be at a high positive potential, and will therefore cause a displacement of electricity along the conductor between Y and Y1 bringing a large negative charge on B1 and sending a positive charge to the segment touching. As A1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half.

The Wimshurst machine is an electrostatic device for generating high voltages developed between 1880 and 1883 by British inventorJames Wimshurst (1832 – 1903).

It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane, two cross bars with metallic brushes, and a spark gap formed by two metal spheres.

Description

These machines belong to a class of generators called influence machines, which separate electric charges through electrostatic induction, or influence. Earlier machines in this class were developed by Wilhelm Holtz (1865 and 1867), August Toepler (1865), and J. Robert Voss (1880). The older machines were less efficient and exhibited an unpredictable tendency to switch their polarity. The Wimshurst did not have this defect.

In a Wimshurst machine, the two insulated disks and their metal sectors rotate in opposite directions passing the crossed metal neutralizer bars and their brushes. An imbalance of charges is induced, amplified, and collected by two pairs of metal combs with points placed near the surfaces of each disk. These collectors are mounted on insulating supports and connected to the output terminals. The positive feedback increases the accumulating charges exponentially until the dielectric breakdown voltage of the air is reached and a spark jumps across the gap.

The machine is theoretically not self-starting, meaning that if none of the secors on the disks has any electrical charge there is nothing to induce charges on other sectors. In practice, even a small residual charge on any sector is enough to start the process going once the discs start to rotate. The machine will only work satisfactorily in a dry atmosphere. It does require mechanical power to turn the disks against the electric field, and it is this energy that the machine converts into electric power. The output of the Wimshurst machine is essentially a constant current that is proportional to the area covered by the metal sectors and to the rotation speed. The insulation and the size of the machine determine the maximum output voltage that can be reached. The accumulated spark energy can be increased by adding a pair of Leyden jars, an early type of capacitor suitable for high voltages, with the jars’ inner plates independently connected to each of the output terminals and the jars’ outer plates interconnected. A typical Wimshurst machine can produce sparks that are about a third of the disk's diameter in length and several tens of microamperes.

Modus operandi

The two contra rotating insulating discs (usually made of glass) have a number of metal sectors stuck onto them. The machine is provided with 4 small earthed brushes (2 on each side of the machine on shafts at 90 degrees to each other, plus a pair of charge collection combs.

Assume that a single sector on one glass plate has a positive electrostatic charge. As the sector passes a sector on the other plate, the positively charged sector attracts the electrons in the neighbouring sector to the surface next to the positive sector. This process repeats as the discs revolve until one of sector on the other disc passes one of the earthed brushes. This brush discharges the positive charge on the opposite side of the sector to where the electrons have migrated. As the sectors part, this second sector now retains a net negative charge. This negative charge will now repel electrons on sectors on the other disc as they pass. Once again earthed brushes dissipate the negative charge on the outside of those sectors leaving a net positive charge. The repeated induction of charges is cumulative and if it were not for the current drawn would continue until the insulation broke down. The positive and negative charges are collected by combs to charge the Leyden jars. It is the mechanical energy required to separate the opposing charges on the adjacent sectors that provides the energy source for the electrical output.

Contact electrification

Contact electrification is an obsolete scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at the time.^[1] In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge imbalance. The phenomenon of electrification by contact, or contact tension, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Then it was discovered that 'piles' of dissimilar metal disks separated by acid-soaked cloth, Voltaic piles, could also produce charge differences. Although it was later found that these effects were caused by different physical processes - triboelectricity, the Volta effect, differing work functions of metals, and others - at the time they were all thought to be caused by a common 'contact electrification' process.^{[citation needed]}

Contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generators such as Ramsden's or Winter's machines, but it also led directly^{[citation needed]}to the development of much modern electrical technology such as batteries, fuel cells, electroplating, thermocouples, and semiconductor junction devices including radio detector diodes,photocells, LEDs, and thermoelectric cells

History

The theory held that static electricity was generated by means of contact between dissimilar materials, and was in close agreement with the principles of static electricity as then understood. It was eventually replaced by the current theory of electrochemistry, namely, that electricity is generated by the action of chemistry and the exchange of electrons betweenatoms making up the battery. An important fact leading to the rejection of the theory of contact tension was the observation that corrosion, that is, the chemical degradation of the battery, seemed unavoidable with its use, and that the more electricity was drawn from the battery, the faster the corrosion proceeded.

In fact, the Volta effect does correspond to a weak electric potential difference developed by the contact of different metals. Nowadays, this is often known as a contact potential difference. This effect was first discovered by Alessandro Volta, and can be measured using a capacitance electroscope comprising different metals. However, this effect does not, by itself, account for the action of electric batteries.

A number of high voltage dry piles were invented between the early 1800s and the 1830s in an attempt to determine the answer to this question, and specifically to support Volta’s hypothesis of contact tension. The Oxford Electric Bell is one example.

Triboelectric contact

If two different insulators are touched together, such as when a piece of rubber is touched against a piece of glass, then the surface of the rubber will acquire an excess negative charge, and the glass will acquire an equal positive charge. If the surfaces are then pulled apart, a very high voltage is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via quantum tunneling, or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution.

Electrolytic-metallic contact

If a piece of metal is touched against an electrolytic material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on each metal piece will be forced to a different level, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which lead from one metal part to the other, and leading out through the chemical reactions on the metal surface, through the electrolyte, then back into the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell or battery.See also: Dry pile

Metallic contact

If two metals having differing work functions are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta effect. The process is halted when the difference in electric potential (electrostatic potential) between the two metals reaches a particular value (the difference in work function values - usually less than one volt.) At this point, the Fermi levels for the two metals are equal, and there is no voltage difference between them. [If there were a voltage difference between them, then a current would flow between them: so "zero current" means "zero voltage difference"

Semiconductor contact

If metal touches a semiconductive material, or if two different semiconductors are placed into contact, one becomes charged slightly positive and the other slightly negative. It is found that if this junction between semiconductors is connected to a power supply, and if the power supply is set to a voltage slightly higher than the natural voltage appearing because of contact electrification, then for one polarity of voltage there will be a current between the two semiconductor parts, but if the polarity is reversed, the current stops. Thus contact electrification leads to the invention of the diode or rectifier and triggers the revolution in Semiconductor electronics and physics.

In materials with a direct band gap, if bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When light meets contact electrification, the light energy is changed directly into electrical energy, allowing creation of solar cells. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the LED.

Triboelectric effect

The triboelectric effect (also known as 'triboelectric charging') is a type of contact electrification in which certain materials become electrically chargedafter they come into contact with another different material and are then separated (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.

Thus, it is not very predictable, and only broad generalizations can be made. Amber, for example, can acquire an electric charge by contact and separation (or friction) with a material like wool. This property, first recorded by Thales of Miletus, suggested the word "electricity", from the Greek word for amber, ēlektron. Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur.

Triboelectric series

Materials are often listed in order of the polarity of charge separation when they are touched with another object. A material towards the bottom of the series, when touched to a material near the top of the series, will attain a more negative charge, and vice versa. The further away two materials are from each other on the series, the greater the charge transferred. Materials near to each other on the series may not exchange any charge, or may exchange the opposite of what is implied by the list. This depends more on the presence of rubbing, the presence of contaminants or oxides, or upon properties other than on the type of material. Lists vary somewhat as to the exact order of some materials, since the charge also varies for nearby materials.

Cause

Although the word comes from the Greek for "rubbing", tribos, the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away, though the imbalance will be partially destroyed by tunneling or electrical breakdown (usually corona discharge). In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules.

The triboelectric effect is related to friction only because they both involve adhesion. However, the effect is greatly enhanced by rubbing the materials together, as they touch and separate many times. For surfaces with differing geometry, rubbing may also lead to heating of protrusions, causingpyroelectric charge separation which may add to the existing contact electrification, or which may oppose the existing polarity. Surface nano-effects are not well understood, and the atomic force microscope has made sudden progress possible in this field of physics.

Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause an electrical discharge of the built-up static electricity; a spark. A person simply walking across a carpet may build up a charge of many thousands of volts, enough to cause a spark one centimeter long or more. Low relative humidity in the ambient air increases the voltage at which electrical discharge occurs by increasing the ability of the insulating material to hold charge and by decreasing the conductivity of the air, making it difficult for the charge build-up to dissipate gradually. Simply removing a nylon shirt or corset can also create sparks, and car travel can lead to a build-up of charge on the metal car body (which acts as a Faraday cage). When the driver alights, sparks jump from frame to driver as he makes contact with the ground.

This type of discharge is often harmless because the energy ((V² * C)/2) of the spark is very small, being typically several tens of micro joules in cold dry weather, and much less than that in humid conditions. However, such sparks can ignite methane-air mixtures, and is a danger when leaks of natural gas occur in domestic buildings, for example. Gas leaks are a serious hazard and can cause physical damage and death.

In aircraft and spacecraft

Aircraft flying in weather will develop a static charge from air friction on the airframe. The static can be discharged with static dischargers or static wicks.

NASA follows what they call the Triboelectrification Rule whereby they will cancel a launch if the launch vehicle is predicted to pass through certain types of clouds. Flying through high-level clouds can generate “P-static” (P for precipitation), which can create static around the launch vehicle that will interfere with radio signals sent by or to the vehicle. This may prevent transmitting of telemetry to the ground or if the need arises to send a signal to the vehicle, particularly any critical signal for the flight termination system. When a hold is put in place due to the triboelectrification rule, it remains untilSpace Wing and observer personnel such as those in reconnaissance aircraft indicate that the skies are clear

Risks and counter-measures

The effect is of considerable industrial importance in terms of both safety and potential damage to manufactured goods. The spark produced is fully able to ignite flammable vapours, for example, petrol, ether fumes as well as methane gas.

Means have to be found to discharge carts which may carry such liquids in hospitals. Even where only a small charge is produced, this can result in dust particles being attracted to the rubbed surface. In the case of textile manufacture this can lead to a permanent grimy mark where the cloth has been charged. Some electronic devices, most notably CMOS integrated circuits and MOSFET transistors, can be accidentally destroyed by high-voltage static discharge. Such components are usually stored in a conductive foam for protection. Grounding oneself by touching the workbench, others, or using a special bracelet or anklet is standard practice while handling unconnected integrated circuits. Another way of dissipating charge is by using conducting materials such as carbon black loaded rubber mats in operating theatres, for example.

GATE EE – 2009 Electrical Engineering Question Paper