THE ELECTROSTATIC GENERATOR

An American physicist, R. J. Van de Graaff, developed one of the first particle accelerators in the early 1930’s, which is known as the Van de Graaff generator, electrostatic generator. This device accelerates electrons to produce high energy radiation. Initially, the generator was capable of producing X-radiation in the 1 to 2 MeV range. Continued design changes produced even higher energies.

The generator operates by projecting electrons onto a moving belt. The electrons ride on the belt and are collected at the opposite end on a high voltage terminal. Here, a heated filament supplies electrons for acceleration. A glass/metal tube with a vacuum provides a path for particle acceleration away from the high voltage terminal. At the end of the tube is a target, whereby the accelerated particles can interact producing high energy radiation. Not long after the development of the Van de Graaff generator, it was determined that charged particles could be accelerated to very high speeds by driving them in a circular path. This was accomplished by the application of strong electromagnets. A variety of these have been developed to produce even higher energy radiation than that of the Van de Graaff generator.

History of Electrostatic Generators

Today, everybody is familiar with electricity. Let's say, almost 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 electrodynamic as today's machines, but they were based on electrostatic principles. Long before electrodynamic generators were invented, electrostatic machines and devices had their place in science. 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 positve 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 a fraction of friction - got the point?) is used to generate electric potential by splitting charges.

The Beginnings

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 continous 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.

The Sulphur Ball

Otto von Guericke (1602-1686) who became 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.

von Guericke's first electrostatic generator around 1660

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. Some day, a researcher found out that the empty glass sphere itself provided the same results.

A Baroque Gas Discharging Lamp

In 1730 scientific research has discovered the principles of electric conduction. An inspiriation for electric research came from an area which at the first glance had absoluteley nothing to contribute: the mercury barometric device invented by Evangelista Torricelli. If the mercury-filled tube was shaken and the evacuated portion of the tube was observed in the dark, a light emission could be seen. 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.

Hauksbee's amber rotor

Hauksbee's setup to demonstrate
light effects caused by static electricity.

 The Beer Glass Generator

Glass proved to be an ideal material for an electrostatic generator. It was cheaper than sulphur and could easily be shaped to disks or cylinders. An ordinary beer glass turned out to be a good isolating rotor in Winkler's electrostatic machine.

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 instantaneuosly by the discharge of a Leyden jar, nobody was aware of the latent dangers of this type of experiments.

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.

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

 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.

Early Leyden jars	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 unintendedly 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 honour when performing his kite experiments.

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 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 forth of the surface area. The leather cushion replaced the experimentor's hand and gave a more continous 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.

An early disk generator by Winter

 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.

The so-called influence machine by Holtz, 1865

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.

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 element 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.

A Wimshurst machine around 1905.
The end point of a long development.

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.

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

 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.

A 5 Megavolt Van-deGraaff generator

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 accellerator labs. The largest machines produce up to 10 million Volts.

 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.

A steam electrostatic generator

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, I'd like to seriously warn all would-be inventors 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!