Electrostatic Generators

THE FRICTIONAL MACHINES

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.

THE ARMSTRONG ELECTROSTATIC BOILER

Left: Armstrong's Electrostatic Boiler: 1841 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.

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: Modern version of the Belli Doubler Belli's doubler was the first symmetrical electrostatic generator using the influence technique rather than friction, and using rotary motion. It was originated by Giuseppe Belli in Italy and first described in 1838. This modern version was built by Dr. Antônio Carlos M. de Queiroz in October 2012. This is the man with a remarkably comprehensive website on electrostatic generators; there is more information on the Belli doubler on his website here website. The thing with the scale on the right is a modern version of a Henley electrometer, which measures the output voltage by electrostatic repulsion of same-polarity charges. It was introduced in 177o by William Henry. When a charge is applied the lower part of the light arm with two black balls is repelled by the stem of the electrometer as its charge is of the same polarity, and the upper arm indicates voltage on the quadrant scale.The most familiar form of electrometer is the gold-leaf electroscope. The Belli Doubler does not appear to have a Wikipedia page, but Wikipedia does yield several pictures of this modern machine. Electroscopes/electrometers do have a Wikipedia page. The Belli Doubler should be avoided by those concerned about obesity.

THE TOEPLER MACHINE

THE HOLTZ MACHINE

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: 1893 The Bonetti machine had two contra-rotating discs. There is more info on the de Queiroz website. The Bonetti machine does not have a Wikipedia page to itself, but there is some info on the general electrostatic generator .

THE VARLEY MACHINE

Left: The Varley electrostatic generator: 1900 The Varley machine had sheets of tin-foil attached to a glass plate . In front of them a rotating disk of ebonite or glass, having carriers of metal fixed to its edge. There is more info on the de Queiroz website. The Varley machine does not have a Wikipedia page to itself, but there is some info on the general electrostatic generator page of Wikipedia.

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 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). There is more info on James on Graces Guide 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, sources of high DC voltage were required to run the tubes. High voltage transformrs and mechanical rectifiers wre one method, but the distribution of mains electricity had barely begun. Induction coils could be used but were noisy, and expensive, the secondary required many turns of wire. Wimshurst machines gave a viable alternative, and unlike the other two methods could be hand-cranked, though given the long exposures requied for early X-ray work, it was probably rather arduous. Edward George Spencer-Churchill (1876–1964) was a first cousin of Winston Churchill and an undergraduate at Magdalen College, Oxford where he experimented with X-rays in 1898. He purchased a Jackson X-ray tube, a Wimshurst machine, and a fluorescent screen. Spencer-Churchill was taking images of the limbs of his friends for amusement, there then being no realisation that X-rays caused cumulative tissue damage. On the outbreak of the Boer War in 1899, he became a Lieutenant in the Grenadier Guards and his apparatus was taken to South Africa, where it was used to assist the Royal Army Medical Corps to locate bullets, shrapnel, or fractures in wounded soldiers. It used there between 1900 and 1902, and survived to be 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 Wimshurst machines were made with up to 12 plates (and probably more- research by the Museum staff is ongoing) the aim being more current rather than more voltage, which is limited by the physical dimensions and the breakdown voltage of air.

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, used for applying an initial charge to the Wimshurst machine, making starting-up more certain. Clearly the friction generator 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. Low vacuum makee electric breakdown occur sooner. 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: 1907

Left: The Wommelsdorf electrostatic generator: 1900 In 1903, Alfred Wehrsen patented an ebonite rotating disk with sectors embedded in it, and having button contacts on the disk surface. In 1907, Heinrich Wommelsdorf reported a variation of the Holtz machine using this form of disk and inductors embedded in celluloid plates. There is a page on the Wommelsdorf machine on the de Queiroz website. The Wommelsdorf electrostatic generator dos not have a Wikipedia apge of its own.

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.