Electrostatic machines Rohit Singla#1 #

Electrical Engineering Dept., Indian Institute of Technology, Delhi Near Hauz Khas, New Delhi-110016 [email protected]

Abstract - Today, everybody uses electricity as a ready-for-use energy that is provided in a clean way. This is the result of long research and engineering work which can be traced back for centuries. The first generators of electricity were not electro dynamic as today's machines, but they were based on electrostatic principles. Due to their principle of operation, electrostatic generators produce high voltage, but low currents. The output is always a unipolar static voltage. Depending from the used materials, it may be positive or negative. Friction is the key of the operation. Although most mechanical energy needed to power an electrostatic generator is converted into heat, a fraction of the work (not the fraction of friction) is used to generate electric potential by splitting charges. In this term paper, we have discussed various electrostatic machines developed over time and analysed the evolution of electricity.

Keywords

– Electrostatic charge, Machines, electricity I. INTRODUCTION

An 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 behaviour 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. To understand electrostatic machines, we need to learn their basic principles. First is the charging by induction. If a polythene rod is rubbed with cloth, the rod can lift small pieces of paper. This can be explained by stating that the rod becomes charged. A charged object will attract a neutral object until it touches and then it is repelled. It is concluded by the experiment that there are two types of charge. As with magnetism, like charges repel and unlike charges attract. Polythene rod charged by rubbing a woollen cap, rod shows negative charge, the

woollen cap shows a positive charge. This implies that positive and negative charges are created in equal quantities by friction. If electrons move away from the wool to the polythene the rod will become negatively charged. The nuclei that remain in the wool have lost electrons and so they become positively charged. The process of rubbing effectively increases the area of contact between the wool and the polythene rod, since transfer of charge occurs when the two materials are in contact. A charged polythene rod can be used to transfer charge onto an insulated conductor without the two objects coming into contact. The negatively charged polythene rod is brought close to the conductor. The electrons within the conductor are repelled by the charged polythene rod and the positive ions are attracted to the polythene rod. The conductor has a separation of charge on it. If the negatively charged side is momentarily earthed, the negative charge is able to flow to earth but the positive charge remains attracted to the negatively charged polythene rod. When the rod is taken away from the conductor, the positive charge on the conductor distributes itself evenly around the surface of the conductor leaving it charged. This is called charging by induction.

Fig 1: Charging by induction

A device that uses this method of charging by induction to carry charge is the electrophorus as shown in Figure 2. The device was invented in 1800 by Volta and consists of an ebonite base which can be charged by rubbing it with cat skin which causes the base to become negatively charged. A brass plate with an insulating handle is placed onto the negatively charged base. This causes the electrons in the metal to be repelled and move as far away as possible from the negative charge leaving the positively charge layer attracted to the base. The negative charge is removed by touching the metal disc briefly, giving a small electric shock to the person as the negative charge flows to earth through the body. The result is a positive charge on the surface of the plate which can be used. The charge on the ebonite plate has not been reduced so the process can in theory be repeated indefinitely.

Fig 3: Distribution of charge on a pear shaped closed conductor

In ancient greece, the amber was known to attract little objects after being rubbed with cloth or fur. From the Greek expression elektron, the modern term ‘electrics’ is directly derived. In 1600, William Gilbert (1544-1603) coined the expression electrica in his famous book De Magnete. In ancient Greece, there was no effort to mechanize the rubbing of a piece of amber in order to get a continuous effect. Although light could be observed in the dark, nobody made a connection between this and the lightning which was regarded as Zeus' weapon. The knowledge about this type of electricity remained almost unchanged until the beginning of the seventeenth century. Several antique authors like Pliny the elder or Renaissance men like Giovanni Battista della Porta describe the effect but without drawing further conclusions. We can say that the history of electricity covers four large periods, If a 4th period could be mentioned, since the 1950s, with the fantastic possibilities of electronics; 1st period, mainly covers 17th and 18th century when Fig 2: Transferring charge using an electrophorus. electrostatic phenomena amazed people in the Curiosities Cabinets, 2nd period can be defined A gold-leaf electroscope is a device that can be from the early beginning of the 19th to the 1870s used to test for electrical potential and charge and and in this time, chemical generators gave enough the sign of the charge if it is first given a charge of energy for carrying out scientific experiments and known sign. A pear shaped closed surface finding the actual nature and the laws of electricity conductor as shown in Figure 4. is charged. If wire and finally the 3rd period can be defined since connected to a gold leaf electroscope is contacted 1870s and thanks to electromagnetic machine, it with the surface of the conductor, the electroscope became possible to turn mechanical energy to shows a constant deflection. This indicates that the electricity and the doors opened for universal uses. potential on the surface of the conductor is constant II. THE SULPHUR BALL all over the surface. These are the basic electrostatic principles which led to the development of Otto von Guericke (1602-1686) who became electrostatic machines. famous for his Magdeburg vacuum experiments invented a first simple electrostatic generator. It was made of a sulphur ball which rotated in a

wooden cradle. The ball itself was rubbed by hand. As the principles of electric conduction had not been discovered yet, von Guericke transported the charged sulphur ball to the place where the electric experiment should happen. Guericke made the ball by pouring molten sulphur into a hollow glass sphere. After the sulphur was cold, the glass hull was smashed and removed. Someday, a researcher found out that the empty glass sphere itself provided the same results.

William Hauksbee, both inventive and inquisitive, designed a rotor to rub a small disk of amber in a vacuum chamber. When the chamber contained some mercury vapour, it lit up! This was the first mercury gas discharge lamp! The engravings show surprising similarities to today's lightning spheres.

Fig 4: Von Guericke's first electrostatic generator III. A BAROQUE GAS DISCHARGING LAMP

In 1730 scientific research has discovered the Fig 6: Hauksbee's setup to demonstrate light principles of electric conduction. An inspiration for effects caused by static electricity electric research came from an area which at the IV. THE BEER GLASS GENERATOR first glance had absolutely nothing to contribute: the Glass proved to be an ideal material for an mercury barometric device invented by Evangelista Torricelli. If the mercury-filled tube was shaken electrostatic generator. It was cheaper than sulphur and the evacuated portion of the tube was observed and could easily be shaped to disks or cylinders. An ordinary beer glass turned out to be a good isolating in the dark, a light emission could be seen. rotor in Winkler's electrostatic machine.

Fig 5: Hauksbee's amber rotor

Fig 7: An electrostatic machine invented by Johann Heinrich Winkler (1703-1770)

Machines like these were not only made for scientific research, but a preferred toy for amusement. In the 18th century, everybody wanted to experience the electric shock. Experiments like the "electric kiss" were a salon pastime. Although the French Abbé Nollet demonstrated in 1745 that little animals like birds and fish were killed instantaneously by the discharge of a Leyden jar, nobody was aware of the latent dangers of this type of experiments.

V. THE LEYDEN JAR

In 1745, the so-called Leyden Jar (or Leyden Bottle) was invented by Ewald Jürgen von Kleist (1700-1748). Kleist searched for a way to store electric energy and had the idea to fill it into a bottle! The bottle contained water or mercury and was placed onto a metal surface with ground connection. No wonder: the device worked, but not because of the fact that electricity could be filled into bottles. One year after Kleist, the physicist Cunnaeus in Leyden/the Netherlands independently invented this bottle again. Thus the term Leyden Jar became more familiar, although in Germany, this device sometimes also was called Kleist's bottle. An intense research work began to find out which liquid is the most suitable. A few years later, researchers had learned that water is not necessary, but a metal hull inside and outside the jar was sufficient for storing electrostatic energy. Thus the first capacitors were born.

Fig 8: The electric kiss provided a very special thrill Soon after the effects of electrostatic discharge were found, researchers and charlatans started to cure diseases with electric shocks. Here we find parallels to the "Mesmerists" who claimed to use magnetic powers for therapy.

Fig 9: Toothache therapy around 1750 Being ill at that time was no fun!

Fig 10: Early Leyden jars and an advanced electrostatic battery in 1795 Frequently, several jars were connected in order to multiply the charge. Experimenting with this type of capacitors started to become pretty dangerous. In 1783, while trying to charge a battery during a thunderstorm, Prof. Richmann was killed by unintended getting too close to a conductor with his head. He is the first known victim of high voltage experiments in the history of physics. Benjamin Franklin had a good deal of luck not to win this honor when performing his kite experiments.

Fig 12: An early disk generator by Winter VII.

Fig 11: St. Petersburg, 6 August 1783. Prof. Richman and his assistant being struck by lightning while charging capacitors. The assistant escaped almost unharmed, whereas Richman was dead immediately. The pathologic analysis revealed that "he only had a small hole in his forehead, a burnt left shoe and a blue spot at his foot. [...] the brain being ok, the front part of the lung sane, but the rear being brown and black of blood." The conclusion was that the electric discharge had taken its way through Richmann's body. The scientific community was shocked.

THE ADVANCED ROTOR

Inventors found out that it is a good idea to laminate metal or cardboard sheets onto the isolating disks of electrostatic generators. Disks for advanced generators of this type were made of glass, shellac and ebonite (hard rubber). Especially hard rubber turned out to be a very suitable material as it did not get damaged so easily than glass or shellac.

VI. THE DISK ROTOR

Generators based on disks were invented around 1800 by Winter. Their characteristic construction element is a mercury-prepared leather cushion that covers approximately one fourth of the surface area. The leather cushion replaced the experimenter’s hand and gave a more continuous result. In 1799, first experiments of electrolysis by electrostatic energy were made. It turned out that the recently invented chemical elements caused same or better effect than many thousand electric discharges of a Leyden bottle battery. Experiments like these helped to shape the understanding of electric energy.

Fig 13: The so-called influence machine by Holtz, 1865

VIII.

THE WIMSHURST MACHINE

Wimshurst machines are the end point of the long development of electrostatic disk machines. They caused very good results and were frequently used to power X-ray tubes. The characteristic construction elements of these machines are disks which are laminated with radially arranged metal sheets. The advantage of disks is that they can be stacked onto one axle in order to multiply the effect.

IX. THE VAN-DEGRAAFF GENERATOR

The principle of this machine is to transport voltage by the aid of a tape made of isolating flexible material e.g. rubber. Early in the development of machinery, it was observed that mechanical transmission belts gave reason for unintended high voltage production, which harmed persons or buildings by igniting parts of a workshop. The same effect caused by transporting the highly inflammable celluloid films inside the projector was the reason for more than one cinema perishing in fire.

Fig 14: A Wimshurst machine around 1905 The invention of the electomagnetic inductor by Ruhmkorff in 1857 began to make the electrostatic disk machines obsolete. Today, both devices only serve as useful demonstration objects in physics lessons to show how electric charges accumulate. For technical applications, high voltages can be easier generated by electronic and electromagnetic methods.

Fig 16: A 5 Megavolt Van-deGraaff generator

Fig 15: A Ruhmkorff inductor to power an X-ray tube (1910)

The principle is based on an isolating endless tape which transports an electric charge to a conductor. Although the device can be operated without an additional electric power source, normally a DC high voltage is applied to the tape, thus considerably increasing the output voltage. Van DeGraaff generators are still in use in particle accelerator labs. The largest machines produce up to 10 million Volts.

X. THE STEAM ELECTROSTATIC GENERATOR

Wet steam which is pressed through a nozzle causes electric chargement. This was the origin of the idea to construct an electrostatic generator based on steam. Although these machines caused good results, they were difficult to maintain. As they also were expensive, comparatively few were built and have survived in museum collections.

REFERENCES

1. Oleg D. Jefimenko, "Electrostatic Motors: Their History, Types, and Principles of Operation" Electret Scientific, Star City, 1973. 2. C. L. Stong, "Electrostatic motors are powered by electric field of the Earth", IEEE proceedings, October, 1974. 3. Alfred W. Simon, "Quantitative Theory of the Influence Electrostatic Generator". Phys. Rev. 24, 690–696 (1924), Issue 6 – December 1924. 4. G. W. Francis (Author) and Oleg D. Jefimenko (Editor), "Electrostatic Experiments: An Encyclopedia of Early Electrostatic Experiments, Demonstrations, Devices, and Apparatus". Electret Scientific, Star City, 2005. 5. Schiffer, Michael Brian (2003). Bringing the Lightning Down: Benjamin Franklin and Electrical Technology in the Age of Enlightenment. Univ. of California Press. ISBN 0-520-24829-5.,p.18-19.

Fig 17: A steam electrostatic generator XI. CONCLUSION

Electrostatic generators have their place in the history of science. They accompanied the way to understand electricity. However, their efficiency is poor, compared to the mechanical effort which is needed to produce electrical energy. In this context, it involves large risk for the investors of electrostatic PMMs based on disk rotors or on the Van deGraaf principle. Machines of this type are no toy and even small devices can be dangerous if carelessly handled. As a rule of thumb, a charged Leyden jar of 1/2 liter (=1/8 gallon) volume can endanger your life!