Electrostatic Generators

Gallery opened: 21 July 2022

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The first electric generators were electrostatic generators; electromagnetic generators did not appear until much later. The first electrostatic generators were of the frictional type. Leaving aside just rubbing bits of amber, probably the first frictional generator was made by Otto von Guericke, around 1660. He is better known for his experiments with vacuum. Later, influence machines, working by repeated electric induction, were more effective and gave higher voltages and currents.

There is a vast amount of information here, on a website by Antonio Carlos M. de Queiroz.

Electrostatic generators have a Wikipedia page.


THE FRICTIONAL MACHINES

In the early days of electricity, it was generated by rubbing a sphere made of sulphur, with the sphere was rotated by hand. Glass discs were also used. The frictional generation of electricity depends on the triboelectric effect, which describes the transfer of electric charge between two objects when they contact or slide against each other.

Left: The Large Electrostatic Generator in the Teylers Stichting Institute

One of the best-known frictional machines is the Large Electrostatic Generator of the Teylers Stichting Institute. It is in the Teylers Museum in Haarlem, Holland.

The two glass disks of the generator, which can just be seen in the centre of the picture, are 1.65 meters in diameter; the machine is capable of generating 330,000 Volts. A two-man crank handle can be seen to the left; there does not appear to be any step-up ratio from crank to disks.

Note that the generator stands on a table with insulating glass legs.

Note also the formidable array of Leyden jars in the distance; this is actually only a quarter of the jars used with the generator. I would imagine they could easily hold enough charge to be lethal.

Left: The Large Electrostatic Generator in the Teylers Stichting Institute

Left: Demonstration of frictional electricity

This classic illustration shows a woman standing on an insulating stand (possibly on a layer of solid sulphur) while the balding chap on the right rotates the sulphur or glass globe (with a serious step-up ratio from the flywheel) and rests his hand on it. Electric charge is them picked up from the globe and transmitted to the young lady via a horizontal iron rod suspended on silk strings. She seems to be about to touch something with her right hand, which may not be a good idea.

L'Electrisee translates as 'The Electrified Female'. The doggerel at the bottom tell us nothing about static electricity.

The young lady looks less than confident and I don't blame her.


THE RAMSDEN FRICTION MACHINE

Left: Ramsden frictional electrostatic generator

There were many frictional machines and this is just one version. The modern model here was built by de Queiroz in 1975. It has an 18 cm disc made of perspex, with two pairs of pads rubbing against it. The pad material is unknown but is probably wool.

In operation the charge builds up on both sides of the disc, and is collected by the U-shaped metal parts with spikes on, (just visible here) which are connected to the output balls.

The Museum Staff have not so far neem able to find any figures for the efficiency of these machines, but it must have been very low. It seems likely that most of the work input was converted to heat.

There is more info on the Ramsden machine on the de Queiroz website.

The Ramsden machine does not have a Wikipedia page, but there is more info in the 1911 edition of Encyclopædia Britannica.

Left: frictional electrostatic generator

High voltages can also be generated by the friction of wet steam with a solid body; this sounds unlikely, as humidity is the undoing of most electrostatic machines, but see the Gallery displaying Armstrong's Electrostatic Boiler, which could easily generate lethal amounts of electricity.


THE INFLUENCE MACHINES

Generating electricity at high voltages by electrostatic induction rather than friction.

There is a general overview of electrostatic machines here.

Left: The principle of electrostatic induction

Here we have two traditional Leyden jars, their outside conductors connected to ground, and two balls hanging from insulating strings. It goes like this:

1) Assume there is a small positive charge on the jar A, and a small negative charge on the jar B. (This assumption immediately tells us that these machines could be difficult to start; an initial charge was sometimes given from a friction rod)

2) The induced charges on the balls A & B therefore arrange themselves as shown.

3) The two balls are briefy connected together with the neutralising wire N, and all the positive charges flow to ball B and all the negative charges flow to ball A.

4) The position of the balls is swopped; this requires an energy input as the electrical attractions oppose this.

5) Ball B is now connected to jar A, and most of its charge flows into jar A because the jar has a much greater capacitance than the ball. The same occurs with ball A and jar B, but the charge is of the opposite polarity.

If this process is repeated, the charges on the jars continue to grow, until a spark jumps across a gap. If the repeating is done by mechanical means, so that the repeated process can be carried out rapidly, you have an electrostatic generator.

I hope that makes sense; if not, please tell me.


BOHNENBERGER'S MACHINE

Left: The Bohnenberger electrostatic generator: 1798

Bohnenberger's machine was an early method of performing the induction process mechanically.

There is more info on the de Queiroz website.


THE BELLI DOUBLER

Left: r

There is more info on the Belli doubler on the de Queiroz website.

The Belli Doubler Wikipedia page

The Belli Doubler should be avoided by those concerned about obesity.


THE TOEPLER MACHINE

Left: Modern Toepler electrostatic generator

August Toepler (1836 – 1912) was an academic physicist. He is also famous for inventing the Toepler pump for creating a vacuum.

There is more info on the Toepler machine on the de Queiroz website.

The Toepler machine Wikipedia page


THE HOLTZ MACHINE

Left: The Holtz electrostatic generator: 1865

The Holtz machine had one fixed and one rotating disc. Wilhelm_Holtz">Wilhelm Holtz

There is more info on the de Queiroz website.

The Holtz machine Wikipedia page


THE VOSS MACHINE

Left: The Voss generator: 1870s

This Voss influence machine was made by the firm of E S Ritchie & Sons, based on the design of German instrument maker Robert Voss. It has two glass plates; one is fixed and has two paper sectors with foil contacts and locations for edge-mounted brushes. The hand-cranked rotating plate has six foil points for contacts and there is black paint leading from each contact point to the axle, which may or may not be relevant to its operation.

In 1880 Robert Voss of Berlin combined the ideas of August Toepler and Wilhelm Holtz and produced an influence machine with two plates.

Voss received US Patent 410,053 In 1889 for a similar influence machine with two rotating plates.

There is more info National Museum of American History.

There is more info on the de Queiroz website.

Wikipedia page.


THE BONETTI MACHINE

Left: The Bonetti electrostatic generator: 1900

The Bonetti machine

There is more info on the de Queiroz website.

The Bonetti machine does not have a Wikipedia page.


THE VARLEY MACHINE

Left: The Varley electrostatic generator: 1900

The Varley machine

There is more info on the de Queiroz website.

The Varley machine Wikipedia page.


THE BONETTI MACHINE

Left: The Bonetti electrostatic generator: 1900

The Bonetti machine

There is more info on the de Queiroz website.

The Bonetti machine does not have a Wikipedia page.


LEYSER MACHINE

Left: The Leyser electrostatic generator: 1873

Leyser machine

There is a page on the Leyser machine on the de Queiroz website.


THE WIMSHURST MACHINE

Left: Wimshurst electrostatic generator: 1898

The Wimshurst machine is certainly the best-known of the rotary generators. It was developed between 1880 and 1883 by British inventor James Wimshurst (1832–1903).

This machine was built by Harvey & Peak of London.

When Wilhelm Röentgen (1845–1923) announced his discovery of X-rays in 1895, and their utility became obvious,

There are three items here, which were purchased and used together by an Oxford student in 1898. Edward George Spencer-Churchill (1876–1964) was a first cousin of Winston Churchill and an undergraduate at Magdalen College when he decided to try out the still-novel phenomenon of X-rays. He acquired a Jackson X-ray tube, a Wimshurst machine, and a fluorescent screen.

What earnest scientific subject was Spencer-Churchill pursuing? He was primarily taking images of the limbs of his friends – for fun! But amusement soon gave way to more serious concerns. On the outbreak of the Boer War in 1899, he became a Lieutenant in the Grenadier Guards and the apparatus was taken to South Africa, where it was used to assist the Royal Army Medical Corps to locate bullets, shrapnel, splinters or fractures in wounded servicemen. It was in use there between 1900 and 1902, and was still unscathed when it was presented to the Museum in 1939 by Spencer-Churchill himself.

There is a page on the Wimshurst machine on the de Queiroz website.

The Wimshurst Machine has a Wikipedia page.

Left: Wimshurst 12-plate electrostatic generator: 1898

The

Left: Wimshurst 12-plate electrostatic generator: 1898

Note the small disc machine in the right side of the case, spun by a separate crank; this is pretty clearly a Ramdsen friction generator. Clearly this was regarded as more dependable than an induction machine.


THE TUDSBURY ENCLOSED ELECTROSTATIC GENERATOR: 1900

Left: The Tudsbury enclosed electrostatic generator: 1900

The voltage that an electrostatic generator can produce is limited by the electrical breakdown of the air. There are three ways to tackle this:

  • 1) Run the machine in a high vacuum, but it needs to be a really high vacuum; see the Paschen graph at the end of this page.
  • 2) Run the machine in a high-pressure gas (up to hundreds of PSI)
  • 3) Run the machine in a dielectric liquid, like the University College of South Wales electrostatic motor.

In 1900 F. Tudsbury showed that if an influence machine is enclosed in a metallic chamber containing compressed air, or better, carbon dioxide, the insulating properties of compressed gases enable a greatly improved effect to be obtained. Sparks could be obtained more than twice as long as at atmospheric pressure. In one case a machine with plates 8 inches diameter which gave sparks of 2.5 in. at atmospheric pressure gave sparks of 5, 7, and 8 in. as the pressure was raised to 15, 30 and 45 psi above the normal atmosphere.


THE WOMMELSDORF GENERATOR: 1

Left: The Wommelsdorf electrostatic generator: 1900

There is a page on the Wommelsdorf machine on the de Queiroz website.

Wikipedia


THE PIDGEON GENERATOR: 18

Left: The Pidgeon electrostatic generator: 1900

There is a page on the Pidgeon machine on the de Queiroz website.

Wikipedia


THE VAN DE GRAAF GENERATOR: 1929

In 1931 a version able to produce 1,000,000 volts was described in a patent disclosure.

The Van De Graaf Generator has a Wikipedia entry.


THE PHYSICS OF HIGH VOLTAGE IN AIR
Left: Paschen's breakdown results:

This shows how if you want to work at voltages above the breakdown of atmospheric air, you need to either increase the pressure (ten time the pressure allowing roughly ten times the voltage) or achieve a pretty good vacuum.

There is a Wikipedia page on the electrical breakdown of gases.

The equation at the top of the graph is Paschen's Law, which has its own Wikipedia page. It is a correlation rather than an exact equation, the 'constants' A and B being roughly constant over a restricted range for a given gas.

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