Voltage/Potential Difference
Electricity and Magnetism

Van de Graaff generator

for 14-16

A Van de Graaff generator can produce extremely high potential differences (voltages). A small school version can pump a huge charge onto the top dome so that the potential difference between the dome and the earth can be 200 000 volts yet the total charge is so tiny that you only receive a small shock when you touch it. This makes possible dramatic demonstration experiments which convey important ideas about charges and currents.

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How the Van de Graaff generator works

Voltage/Potential Difference
Electricity and Magnetism

How the Van de Graaff generator works

Practical Activity for 14-16

Demonstration

The film explains the principle of the Van de Graaff generator, explains how to overcome problems when using it, and shows one simple but effective demonstration.

Apparatus and Materials

Health & Safety and Technical Notes

Note that any student with a pre-existing heart condition should remain at a safe distance from the generator – a metre should be adequate.

More detailed safety guidance can be found here:

Procedure

  1. For the simple demonstration, place a stack of metal cake tins, upside down, on top of the uncharged dome of the generator.
  2. Switch on the generator. As it charges up, the cake tins will one-by-one lift off the top of the stack.

Teaching Notes

  • The Van de Graaff generator can be used to demonstrate effects involving static electricity in a spectacular fashion. The film explains that it is essentially a device which transfers electrons from its base (i.e. from the earth) to the upper dome, where they accumulate. You can estimate the potential difference (voltage) between the dome and earth knowing that it takes about 30 kV per centimetre to produce a spark in air (this figure depends on humidity etc.). So a 4 cm spark requires 120 kV.
  • Although these voltages are high, the amount of charge stored is small so the current in any spark will be small, and lasts for a very short time. For this reason, it is unlikely that any fit person will be harmed by a spark but see the safety notes above.
  • The film explains that charging occurs by a process which is similar to the familiar charging which occurs when one material is rubbed against another. The difference is that, in the Van de Graaff generator, rubbing does not occur. Contact between two materials (the belt and the roller) is sufficient to start charges moving, and the machine is designed to make this a continuous process.
  • Note that some generators are arranged so that electrons are transferred downwards, leaving the dome positively charged. You can test whether the dome is positively or negatively charged using a coulombmeter; you can reverse the direction of charging by swapping over the two rollers.
  • You can use a hairdryer to remove water vapour on the generator. The acrylic uprights may also need cleaning with isopropyl alcohol.
  • See also our page on how the Van de Graaff works:
  • Van de Graaff generator: the basics


  • It is useful to know the triboelectric series; this will help you to decide which of two materials will become positively charged when they are rubbed together:
  • Triboelectric effect entry on Wikipedia


  • A series of experiments using the Van de Graaff generator, together with notes on safety:

    Collection of Van de Graaff generator practicals


  • The video below explains the construction and functioning of a Van de Graaff generator. It also shows how to clean it in order to avoid problems with dirt and humidity.

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Experiments with a Van de Graaff generator

Voltage/Potential Difference
Electricity and Magnetism

Experiments with a Van de Graaff generator

Practical Activity for 14-16

Demonstration

The Van de Graaff generator never fails to inspire.

Apparatus and Materials

Health & Safety and Technical Notes

It is unwise to operate a computer close to a running Van de Graaff generator, particularly a laptop which is not earthed.

Accessories for the Van de Graaff generator should include a ‘head of hair’ and an insulating handle with a conducting polystyrene sphere suspended from the top.

Procedure

  1. Show the Van de Graaff generator, and describe it as a machine transporting charges to its large sphere. Bring up the light, conducting polystyrene sphere, suspended on a long insulating nylon thread from an insulating rod. Let the small sphere touch the large sphere, sharing some of the charge and the repulsion between like charges will be apparent.
  2. Photo courtesy of Mike Vetterlein
  3. Fix the insulating rod into the top of the generator. Alternatively, the ‘head of hair’ can be put on the top, again showing repulsion.
  4. Photo courtesy of Mike Vetterlein
  5. Allow the sphere to spark to a neighbouring earthed sphere, and then direct to earth by a wire.

Teaching Notes

  • The potential difference between the dome and the earth can be 200,000 volts, enough to make a spark jump across a narrow air gap. However, the total charge is so tiny that you only receive a small shock if you touch the dome.
  • You might say "near to large concentrations of charge the electric field can be very strong. If a pointed object is attached to the dome then the electric field near to the point is very large indeed. It may be a strong enough field to tear electrons off nearby molecules. Each electron flies away, pulled by an electric field. Soon it smashes into an air molecule. If it has gained enough energy as it accelerated in the intense electric field it can knock an electron off that molecule. There are now two electrons that fly on to make more collisions; a chain reaction. That is a spark."
  • You could also demonstrate that the force of repulsion gets larger when the dome holds more charge. Place a fist full of paper punch-outs (from a hole punch) on the dome and turn the machine on. The small pieces of paper get the same charge as the dome and so fly away in all directions, creating a fountain effect. The faster the machine, the greater the force of repulsion and the further away the punch-outs fall. [This suggestion submitted by Dr B S Sidhu from Slough Grammar School.]
  • The lower end of the Van de Graaff generator and the base of the neighbouring sphere should be earthed effectively in these demonstrations. Since the discharge will give a sudden pulse, the earth connection (for example, to the earth terminal of a low voltage power supply connected to mains) should be free from sharp bends or kinks.
  • By electrostatic induction, you can obtain an opposite charge from that of the Van de Graaff store (the upper, charged, sphere). Bring an uncharged metal ball near to the store so that the store pulls an opposite charge, and pushes a like charge away on the ball. Touch the ball and let that like charge run away to earth; then bring the ball away with the remaining opposite charge on it. See how that attracts another ball with a sample charge direct from the store.

This experiment was safety-tested in July 2007

  • A video showing how to use a Van de Graaff generator:

Resources

Download photos of our website user Jorge Rebelledo's home-made Van de Graaff generator.

Up next

Moving charges are an electric current

Voltage/Potential Difference
Electricity and Magnetism

Moving charges are an electric current

Practical Activity for 14-16

Demonstration

Both the Van de Graaff and the EHT supply cause the ball to shuttle between the plates. They both affect an ammeter in the same way.

Apparatus and Materials

  • Metal plates with insulating handles, 2
  • Table-tennis (ping-pong) ball, coated with Aquadag
  • Nylon thread, e.g. fishing line, 1 reel
  • Power supply, EHT, 0–5 kV (with internal safety resistor)
  • Retort stands, bosses and clamps, 3
  • Van de Graaff generator


Health & Safety and Technical Notes

Read this comprehensive safety note:

As an alternative to the metal plates, a pair of copper plates bent as shown and placed upright on a dry bench can be used.

Procedure

  1. Fix the handles of the plates in clamps, so that the two plates are set up parallel to each other with their planes vertical. They should be approximately 10 cm apart.
  2. Connect one of the plates to the dome of the generator; earth the other, together with the base of the Van de Graaff machine. (The plates recommended have a small peg at the back to which a crocodile clip can be attached. This avoids having to connect the clip to the plate itself.)
  3. Hang the table-tennis ball, coated with Aquadag, from a suitable length of nylon thread between the two plates. When the generator is switched on, the ball transfers charges between the plates.
  4. If a sensitive galvanometer is available, insert it into this circuit between the second plate and the earth connection. Each transfer of charge from plate to plate will cause a deflection of the meter. For this to be significant, show that the galvanometer is capable of indicating the passage of a tiny momentary current derived from a dry cell. Connect a dry cell to the instrument through a student who completes the circuit momentarily by a brief touch on one terminal.
  5. Disconnect the Van de Graaff generator and temporarily earth all the equipment. Then connect the positive terminal of the EHT power pack (with the 50 megohm safety resistor in circuit} to the first plate and repeat the experiment, again showing charge transferred.
  6. If one is available, connect the sensitive galvanometer between the second plate and the earth connection. Observe the current.

Teaching Notes

  • Try out this demonstration in advance to determine the optimum separation of the plates and the length of the thread. You may have to give the ball a gentle nudge to start it moving.
  • The ball gains charge of one sign when it touches one plate. It is attracted to the other plate, where it gives up its charge and gains charge of the opposite sign. It is then attracted in the opposite direction, and so on. We can picture positive charge being carried in one direction, and negative charge in the opposite direction. Both contribute to the current (which flows in one direction).

This experiment was safety-tested in February 2005

  • A video showing how to use a Van de Graaff generator:

Up next

Model of ions in motion

Voltage/Potential Difference
Electricity and Magnetism

Model of ions in motion

Practical Activity for 14-16

Demonstration

A visual model showing that particles with one charge move in one direction, while those with opposite charge move in the reverse direction.

Apparatus and Materials

Health & Safety and Technical Notes

Read this comprehensive safety note:

The cylinder is made of clear insulating plastic (e.g. Perspex), with metal top and ends. One end is equipped to plug into the dome of the Van de Graaff generator. Inside are three or four metallized polystyrene spheres.

Procedure

  1. Plug the cylinder into the top of the Van de Graaff generator. Turn on the generator. The spheres, initially lying on the metal base, will become charged and rise up the cylinder until they are suspended between the two parallel plates formed by the two caps.
  2. Put your finger on the top metal cap and keep it there. The spheres will be set in motion as they carry charge back and forth. (Resting the other hand on a wooden bench to ensure you do not become charged up.)

Teaching Notes

  • The balls collect charge from the top of the dome and are repelled by it. Their weight is then balanced by the upward repulsion.
  • When the upper plate is earthed, the balls are attracted to it; they give up their charge, so that a tiny current flows through the teacher. The balls then fall back down, to become charged again.

This experiment was safety-tested in April 2006

  • A video showing how to use a Van de Graaff generator:

Up next

Clearing smoke

Voltage/Potential Difference
Electricity and Magnetism

Clearing smoke

Practical Activity for 14-16

Demonstration

A demonstration of the principle of electrostatic precipitation.

Apparatus and Materials

  • Transparent plastic cup (e.g. Perspex picnic cup)
  • Drawing pin
  • Aluminium foil
  • Smoke source (e.g. paper drinking straw)
  • Van de Graaff generator


Health & Safety and Technical Notes

Read this comprehensive safety note:

Drill a hole in the bottom of the cup. Run a bare wire through the hole to make contact with the Van de Graaff’s dome. Place a small piece of foil at the bottom of the cup, to make contact with the wire. Place a metal lid of foil on top of the cup.

Procedure

  1. Place the cup on top of the machine's sphere. Connect its lid to earth.
  2. Place a drawing pin, point upwards, in the cup. This will ensure a strong local electric field, and ‘electric wind’ to stir the smoke.
  3. Fill the cup with smoke and run the machine.

Teaching Notes

  • The electric field between the top and the bottom of the container clears the container of smoke as the particles are attracted to the top.
  • This principle is used in clearing smoke in a factory chimney. There is a wire down the centre of the chimney and a potential difference between the wire and the chimney walls. The smoke particles collect on the walls and so do not escape into the atmosphere.

This experiment was safety-tested in April 2006

  • A video showing how to use a Van de Graaff generator:

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Electrostatics

Electrostatic Force
Electricity and Magnetism

Electrostatics

Practical Activity for 14-16

You need to avoid planning an electrostatics lesson and then finding that the atmosphere and the equipment are so damp that they get no effect at all, or else the results you do get appear to give the ‘wrong’ result so are confusing. There are precautions that can be taken which will usually ensure success.

All dusters used to charge objects should be freshly laundered and fluffy, and kept in a clean bag. When laundering, do not put fabric conditioner in the water as it's an anti-static agent. Students should have clean hands. Polystyrene balls, balloons, acetate and polythene rods should all be cleaned regularly. It is helpful to store all electrostatic equipment, including the Van de Graaff generator, in a cupboard which is kept warm and dry with a low wattage lamp burning. However even on a wet day, putting all the equipment near to an electric heater for some time before the lesson ensures that it is dry enough.

Today’s synthetic materials are well-known for becoming charged very easily, so that cars and carpets can give quite a nasty shock. Try separating bed-clothes in the dark of night and you will really see sheet lightning!

In modern laboratories with water fed through plastic pipes, it may be very difficult to find any point electrically bonded to earth. In such cases, an earth for electrostatics experiments can be provided by burying a substantial metal rod in the ground with a wire running through the wall to a terminal in the laboratory.

It helps to be familiar with the electrostatic series:

  • Perspex (acrylic) ELECTROPOSITIVE
  • Glass
  • Silk
  • Wood
  • Sulphur
  • Cotton
  • Ebonite
  • Indian rubber
  • Polythene ELECTRONEGATIVE

If two materials are rubbed together, the material higher in the list will gain a positive charge, while the lower material will gain a negative charge.

Good luck: these lessons can be fun!

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Electric charge and current - a short history

Charge
Electricity and Magnetism

Electric charge and current - a short history

Teaching Guidance for 14-16

Electrical phenomena result from a fundamental property of matter: electric charge. The atoms that constitute most matter we encounter contain charged particles. Protons and electrons each have one unit charge, but of opposite sign. Atoms are generally neutral because the number of electrons and protons are the same.

Electric charges at rest have been known much longer than electric currents.

The amber effect

The property now called static electricity was known to the philosophers of ancient Greece. In fact the word electricity comes from ‘elektron’, the Greek name for amber. Amber is a resinous mineral used to make jewellery. It is probable that small fibres of clothing clung to amber jewels and were quite difficult to remove. Trying to rub the fibres off made the situation worse, causing early philosophers to wonder why.

William Gilbert mentioned the amber effect in his ground-breaking book On Magnetism, published in 1600. He noticed that the attraction between electrics was much weaker than magnetism and wrongly said that electrics never repelled.

Benjamin Franklin

A giant leap of understanding was required to explain observations like these in terms of positive and negative electrical charge. In the 18th century, Benjamin Franklin in America tried experiments with charges. It was Franklin who named the two kinds of electricity ‘positive’ and ‘negative’. He even collected electric charges from thunderstorm clouds through wet string from a kite.

Franklin was an advocate of a ‘single fluid’ model of electric charge. An object with an excess of fluid would have one charge; an object with a deficit of fluid would have the opposite charge. Other scientists had advocated a ‘two fluid’ theory, with separate positive and negative fluids moving around. It took over a century for the debate to come down on Franklin’s side.

It is interesting to note that Franklin coined several electrical terms which we still use today: battery, charge, conductor, plus, minus, positively, negatively, condenser (= capacitor), among others.

Electric currents

Electric currents were not fully investigated until batteries were invented in about 1800. Passing currents through salt solutions provides evidence that there are two kinds of charge carriers, positive and negative. The charge carriers that boil out of white hot metals are negative electrons, and movements of electrons produce current in a cool, metal wire.

For a time electric currents seemed so different from electric charges at rest that the two were studied separately. It seemed as if there were four kinds of electricity: positive and negative electrostatic charges, and positive and negative moving charges in currents. Now scientists know better. There are just two kinds, positive and negative, exerting the same kind of forces whether they were ‘electrostatic charges from friction’ or ‘moving charges from power supplies’.

A modern view

Electric forces are what hold together atoms and molecules, solids and liquids. In collisions between objects, electric forces push things apart.

Today we understand that electrons may be transferred when two different materials contact each other and then separate. You can list materials in order, from those “most likely to lose electrons” (gaining positive charge) to "those most likely to gain electrons” (gaining negative charge). This is called the triboelectric series.

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Van de Graaff generator - the basics

Voltage
Electricity and Magnetism

Van de Graaff generator - the basics

Teaching Guidance for 11-14 14-16

Principle of operation

Some insulating materials when separated from the surface of others, leave those surfaces electrically charged, each with the opposite sign of charge and with a high potential difference (p.d.).

A machine to make charges was invented in 1929 by a young American called Van de Graaff. Huge machines, some over 30 m high, based on his ideas have been built to produce extremely high potential differences.

Belts and rollers

A flexible belt made from an insulating material and running continuously over two rollers can, by the same process, produce a supply of charge where the surfaces separate. The two rollers have to have different surfaces (often acrylic and metal) and together with the belt-rubber, are chosen by experiment.

Combs

Charges are “sprayed” on to and removed from the moving belt by “combs” situated adjacent to the rollers. Actual contact between the combs and the belt is not essential because of the high potential differences. Combs can be simply a stretched wire, or a sharp or serrated edge: action depends on very high potential gradients due to their small radii (similar action to lightning conductors).

The lower comb is maintained at or close to earth potential and is a drain for negative charge, leaving the belt with positive charges that are carried up to the top comb.

Collecting sphere

The top comb is connected to a collecting sphere which, having inherent electrical capacity (proportional to its radius) will collect and store the charge on its outer surface until discharged either by breakdown of the surrounding air as a spark, or by conduction to an adjacent earthed object.

Charging current

So long as the belt continues to move, the process continues, the drive (motor or manual) supplying the power to overcome the electrical repulsion between the charges collected on the sphere and those arriving on the belt.

The charging current is usually a few mA and potential difference achieved by “junior generators” will be 100-150 kV and by “senior” generators up to about 300 kV.

The whole apparatus

The mechanical arrangement of the belt/roller system is very simple. The lower roller is driven either manually or by motor. The former usually involves a hand wheel and pulley with belt-drive; this pulley can be mounted directly on the motor spindle. In “junior” models, fixed speed, shaded-pole induction motors are usual; “senior” models often incorporate small H.P. variable-speed (sewing-machine) motors, with carbon brushes, control being by either a simple rotary rheostat or a solid state circuit. The motors, control switches and mains input socket are housed in a metal or plastic enclosure, although some junior models have used a transparent plastic cake-cover.

The support column for the collecting sphere can be a simple PVC plastic rod or acrylic tube or a pair of acrylic strips with separators. In some models the belt is enclosed within a plastic pipe with “windows” along its length. Not all generators have means of adjusting the separation of the upper and lower rollers i.e. the belts have to be tailored for a particular machine.

Since the diameter of the collecting sphere determines the maximum p.d. (voltage) achievable, large spheres are mounted on taller columns to be more remote from the earth motor and control box.

Machines are usually supplied with a “discharger", often another, smaller, sphere mounted on a metal rod that has to be earthed to draw sparks from the collecting sphere.

Demonstrations and accessories

Certainly the Van de Graaff generator can produce striking demonstrations. The usual experiments are:

Faraday’s cylinder to show electric charge resides on the outer surface of a charged hollow conductor.

Bouncing ball. Suspend a conducting ball a non-conducting thread. When the ball touches the charging sphere, it will become charged and be repelled away from the sphere. If the ball is then allowed to discharge (touching an earthed surface, or leaking charge to the air) it will be attracted once more to the sphere, to be recharged ... and so the process continues.

The head of hair is another demonstration of repulsion. Real hair or shredded paper strips bunched at one end are used and provide a sensitive means of detecting charge.

The electric wind is produced by release of ions at the end of a pointed conductor and is enough to deflect a candle’s flame.

Hamilton’s mill utilizes the electric wind at the pointed ends of four arms to cause rotation about a pivot. This is similar to the action of a lightning conductor, which allows charge transfer at sharp points.

Kinetic theory model You can show random motion of metallic balls continuously affected by repulsion and loss of charge within a transparent vessel.

Neon indicator shows luminous discharge from the gaseous excitation by the high electric fields near the generator.

An apparatus note on the Van de Graaff generator gives information about good housekeeping and repairs:

Van de Graaff generator


Up next

Van de Graaff generator safety

Voltage
Electricity and Magnetism

Van de Graaff generator safety

Teaching Guidance for 14-16

Van de Graaff generator demonstrations can provide useful insights into electrical phenomena, which are at the same time memorable.

  • It is essential the Van de Graaff generators for school science are obtained through reputable school science equipment suppliers. The sphere has a capacitance, and will store energy electrostatically as a result of the separation of charge. The energy stored electrostatically by the sphere should not exceed 0.5 J.
  • Do not add devices to the sphere that increase the capacitance.
  • Van de Graaff generators with mains powered pulleys must be electrically inspected and tested in the same way as other mains powered equipment.
  • When carrying out the hair-standing-on-end demonstration, do it with one person at a time. After the demonstration, to avoid a sudden discharge, the person should take their hand off the sphere and touch the surface of a wood bench top (avoiding metal fittings such as gas taps). Alternatively, hand the charged person a wooden metre rule. After a few moments, they will be discharged.
  • It is not advisable for people to participate in practical work with Van de Graaff generators if they have heart conditions, or a pacemaker, or other electronic medical equipment fitted.
  • The electrical discharge from a Van de Graaff generator can wreck electronic circuits, so equipment such as computers and instrumentation with electronic circuits should be kept well away.

The Van de Graaff generators designed for schools are usually the triboelectric type; these are the most suitable. The transfer of charge is achieved by a rubber belt driven by a plastic pulley, with an arrangement of metal combs at either end of the belt. Charge is transferred to a metal sphere (a capacitor) and very high voltages are achieved between the sphere and ground, typically in the range 200 kV to 300 kV.

Using a Van de Graaff generator, one is quite likely to receive a short shock by accidental or intentional contact with the charged dome. An enquiry to CLEAPSS has revealed no recorded incident of direct injury caused by shocks from the correct use of school Van de Graaff generators. However, some people are more sensitive than others and can find the shocks very unpleasant and painful. For this reason, only volunteers should take direct part in the practical work.

The shock is a single unidirectional pulse of short duration; a capacitor discharge through the resistance of the body and contacts. Using rough values of sphere capacitance, 10 pF, and body resistance, 1000 ohms, the discharge time through a person will be of the order of nanoseconds and certainly less than a millisecond. The capacitance of the sphere needs to be low enough that at maximum voltage, the energy transferred by the discharge current through the body causes at most no more than an unpleasant or painful sensation. The current flowing and energy transferred to the body should be well below that which could cause any risk of ventricular fibrillation.

For an impulse current I Amps of short duration t seconds ( t < 10 ms) through the body, the principal factor for the initiation of ventricular fibrillation is the value of I  ×  t or I 2  ×  t (IEC 2007). At high applied voltages, the resistance of the adult body (left hand to right hand) is at least 575 ohms for 95% of the population (IEC 2005). The total body resistance for children is expected to be higher but of the same order of magnitude. The IEC gives a threshold value of 'Specific Fibrillation Energy', for a 1 ms current impulse, of ValueExponent{2}{-3}A 2s. Below this threshold there is no evidence of fibrillation. The Specific Fibrillation 'Energy' can be regarded as the energy dissipated per unit resistance of the body through which the current flows. Note that ‘specific’ here means ‘per unit resistance’ rather than ‘per unit mass’.

At 575 ohms and discharge time not exceeding 1 ms, the energy stored electrostatically by the capacitor would need to be at least 1.1 J to reach the Specific Fibrillation Energy. Although this is a very conservative estimate because the discharge time is likely to be much less than 1 ms, Van de Graaff generators that can store more than 1 J of energy electrostatically by the sphere should be avoided. A discharge of 1 J affects everybody severely (BSI 1991).

An estimate of the energy stored electrostatically by the sphere can be made by calculating the sphere capacitance, C = 4 π ε0 r (where C is in farads, r is the sphere radius in metres and ε0 is the permittivity of free space), estimating the voltage, V , using the length of spark gap, and calculating the energy E , in joules, from E = 0.5 CV 2.

Generally speaking, sphere diameters of Van de Graaff generators should be about 20 cm or less. Using data from one manufacturer’s specification, the sphere diameter is given as 20 cm and the maximum voltage 250 kV, the energy stored electrostatically would be 0.35 J, well below 1 J. If this is compared to a sphere diameter of, say, 25 cm and a maximum voltage of 350 kV, then energy stored electrostatically would be 0.85 J. This would still be below 1 J, but the shocks would be correspondingly more unpleasant and painful, and this may put off some people from using the generator.

If you wish to estimate the voltage across the sphere and ground, you can do this by finding the maximum spark length. Wait until the sphere is fully charged, then bring up a grounded sphere slowly until you obtain a spark discharge. This technique has limitations and you should do it carefully several times to find the maximum spark length. Do the test on a dry day with low relative humidity so the Van de Graaff generator is working at its best.

Note that the rule-of-thumb 3 kV/mm is only a reasonable rule for voltages below 100 kV.

References

IEC 2007. IEC/TS 60479-2:2007. Effects of human beings and livestock – Part 2: Special aspects.

IEC 2005. IEC/TS 60479-1:2005. Effects of human beings and livestock – Part 1: General aspects.

BSI 2002. BS EN 60052:2002. Voltage measurement by means of standard air gaps.

BSI 1991. BS5958-1:1991. Code of practice for control of undesirable static electricity Part 1: General considerations. [Replaced by PD CLC/TR 50404:2003 but remains current.]

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