Electrostatic Motors

Gallery opened: 21 Aug 2012
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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. Power output is usually very low and the motors are impractical in normal use.

At a much smaller scale they 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.

In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon.

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.

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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