Electrostatic Motors

Gallery opened: 21 Aug 2012

Updated 18 July 2022

Franklin's Electric Wheel added


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The first electric motor was an electrostatic motor; an early electric whirl was demonstrated by that remarkable man Benjamin Franklin in 1748. He is often said to be the inventor, but it had already been described by Andrew Gordon in 1745, in Versuch einer Erklarung der Electricitat (Erfurt 1745). Michael Faraday's electromagnetic motor did not appear until 1821.

An electrostatic motor or capacitor motor is powered by the attraction and repulsion of electric charges. Electrostatic motors are the electrical dual of conventional electromagnetic motors that are powered by magnetic forces. Electrostatic motors typically require a very high voltage power supply, to generate enough attraction and repulsion to even overcome their own friction, and such supplies are difficult and expensive to make. The power output (if indeed any is available for external use) is usually very low. Unless a freely obtainable supply of high-voltage electricity exists the motors are impractical; this has not of course deterred inventors. The prospect of distributing 20 kV or so to every house has some daunting safety issues; what we do in the UK is to distribute at 11 kV to local substations that that change it into a more manageable 230 V. Having said that, the early electricity supply to Cambridge was at 2 kV, with each consumer having their own transformer; this system was introduced in 1891, but it is not currently clear how long it endured.

At a much smaller scale electrostatic motor are, however, an essential part of the molecular machinery of living cells. The bacterial flagellum is driven by a rotary engine built of protein, located at the flagellum's anchor point on the inner cell membrane. This engine is powered by proton motive force, (the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by cell metabolism. The process is complicated, and whether it is strictly speaking an electrostatic motor is unclear. To me, at least. A flagellum incudes a mechanical clutch that disengages when motion is not required; I think that is rather awe-inspiring.

There is now much interest in electrostatic motors working at a nano-scale.

Electrostatic motors have a Wikipedia page.


Left: An eight-point electric whirl: date unknown

An electric whirl consists of two or more sharp-pointed arms, rotating on a vertical point bearing to minimise friction. The operation of the electric whirl is normally described thus:

"The sharp points cause a concentration of electric field and electrons escaping from the points ionise the air locally. The resulting space charge is at the polarity as the points and so there is a repulsivew force that rotates the whirl."

I seem to recall being told many years ago that corona discharge from the points caused the local air to move and that gave a repelling force. This is called an electric wind and probably comes to the same thing.

As noted above, the electric whirl was first described by the Benedictine priest Andrew Gordon in 1745, in Versuch einer Erklarung der Electricitat (Erfurt 1745)

There is no prospect of getting any useful work out of the electric whirl.

Left: A three-point electric whirl: now

This is a modern whirl, with a 4mm socket for connection to a Van der Graaf generator or similiar.

I would have thought the points ought to be at right-angles to the spokes to make best use of the available force, as in the picture above. The points here seem to be set at 45 degrees.


Left: Benjamin Franklin's Electric Wheel: 1748

The wheel consisted of thirty glass spokes, each tipped with a brass thimble. As each thimble approached the knob of a Leyden jar it would be carrying an opposite charge and was therefore attracted; as the thimble passed the knob it was close enough for a spark to jump, giving the thimble the same polarity of charge as the jar. It was then repelled, and was carried around to the opposite jar where the same process reversed the thimble polarity again. This continued as long as there was enough charge on the Leyden jars to make a spark pass.

Franklin claimed in a letter that the wheel turned at 12 to 15 rpm, while carrying the weight of one hundred Spanish dollars; this unfortunately gives very little idea of the available power as the effect of the added weight depended on the quality of the lower point bearing. He also wrote that: "If a large fowl were spotted on the upright shaft, it would be carried round before a fire with a motion fit for roasting." I am afraid Ben was being a bit optimistic about the torque available.

Left: Franklin's Electric Wheel: modern replica

Built by Oleg D. Jefimenko and his graduate students in the Physics Department of West Virginia University.


Left: An electrostatic motor built at the University College of South Wales: 1968

This remarkable machine was a research project at the School of Engineering Science, University College of South Wales. It was shown at the 1968 Physics exhibition at Alexandra Palace, London.

The text gives the details, but the basic idea is that charges on the rotor move at a defined speed, controlled by carefully adjusting the conductivity of the fluid This allows a pattern of charges to build up that moves synchronously with the rotor, giving a sustained torque. The power supply was 10 kV or 20 kV.

Although safety is not referred to in the article text, this seems to me something of an Infernal Engine. The moving parts are immersed in a mixture of hexane and ethyl alcohol. Both are highly inflammable, and you then apply 10,000 Volts. Surely the smallest spark would have been catastrophic; I note that the fluid does not fill the lower box so there must have been a space at the top saturated with inflammable vapour. The little electrical generator it drives appears to have been put in its own air-tight box on top; sparking from its commutator must have been a worry.

Actually, there was even more to worry about. See below for information on hexane toxicity.

Onmature reflection I wouldn't have wanted to go anywhere near one of these things.

From Wireless World, May 1968, p102

Left: An electrostatic motor built at the University College of South Wales: 1968

The moving parts of the motor are immersed in a mixture of hexane and ethyl alcohol. Hexane exists in five isomeric forms, They are all colorless liquids at room temperature, with boiling points between 50 and 70 °C, and having a petrol-like odour. It is impossible to guess which isomer was used here, but the plain description "hexane" strongly suggests it was n-hexane. This substance shows long-term toxicity, leading to degeneratin of the peripheral nervous system, causing tingling and cramps in the arms and legs, followed by general muscular weakness. In severe cases, atrophy of the skeletal muscles occurs, with loss of coordination and vision problems.

In February 2010, it reported that an employee of Wintek Corporation, an Apple contractor in China, died in August 2009 due to hexane poisoning. Hexane was used to replace alcohol for cleaning touch-screens. 137 other employess suffered but survived.

One hopes that the staff at University College knew about hexane toxicity and took suitable precautions.

From Wireless World, May 1968, p103

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