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Electrostatic Devices

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Electrostatics is the study of the behavior of electric charges that are at rest. The phenomenon of static electricity has been known for well over 2,000 years, and a variety of electrostatic devices have been created over the centuries.

The ancient Greek philosopher Thales (624-546 B.C.) discovered that when a piece of amber was rubbed, it could pick up light objects, a process known as triboelectrification. The Greek name for amber, elektron, gave rise to many of the words we use in connection with electricity. It was also noted that lodestone had the natural ability to pick up iron objects, although the early Greeks did not know that electricity and magnetism were linked.

In the late sixteenth century, William Gilbert (1544-1603) began experimenting with static electricity, pointing out the difference between static electric attraction The Vivitron electrostatic particle accelerator under construction at the Centre des Recherches Nucleaires, Strasbourg, France. Vivitron, the largest Van de Graaff generator in the world, can generate a potential of up to 35 million volts. The accelerator will be used to fire ions of elements such as carbon at other nuclei. Under the right conditions this creates superdeformation, a relatively stable state in which the rotating nuclei have an elliptical form. Gamma rays given off by these nuclei reveal much about the internal structure of the nucleus. Photograph by Philippe Plailly/Eurelios. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. and magnetic attraction. Later, in the mid-1600s, Otto von Guericke built the first electrostatic machine. His device consisted of a sulfur globe that was rotated by a crank and stroked by hand. It released a considerable static electric charge with a large spark.

A similar device was invented by Francis Hawkesbee in 1706. In his design, an iron chain contacted a spinning globe and conducted the electric charge to a suspended gun barrel; at the other end of the barrel another chain conducted the charge.

In 1745, the first electrostatic storage device was invented nearly simultaneously by two scientists working independently. Peter von Muschenbrock, a professor at the University of Leyden, and Ewald von Kleist of the Cathedral of Camin, Germany, devised a water-filled glass jar with two electrodes. A Leyden student who had been using a Hawkesbee machine to electrify the water touched the chain to remove it and nearly died from the electric shock. This device, known as the Leyden jar, could accumulate a considerable electric charge, and audiences willingly received electric shocks in public displays. One of these displays aroused the curiosity of Benjamin Franklin, who obtained a Leyden jar for study. He determined that it was not the water that held the electric charge but the glass insulator. This is the principle behind the electrical condenser (capacitor), one of the most important electrical components in use today.

Charles F. DuFay (1698-1739) discovered that suspended bits of cork, electrified with a statically charged glass rod, repelled each other. DuFay concluded that any two objects which had the same charge repelled each other, while unlike charges attracted. The science of electrostatics, so named by André Ampère (1775-1836), is based on this fact.

French physicist Charles Coulomb (1736-1806) became interested in the work of Joseph Priestley (1733-1804), who had built an electrostatic generator in 1769, and studied electrical repulsion. Coulomb used his torsion balance to make precise measurements of the force of attraction between two electrically charged spheres A Van de Graaff generator is a device that is capable of building up a very high electrostatic potential. In this photo, the charge that has accumulated in the dome is leaking into the hair of a wig that has been placed on top of the generator. Because the charge is of one polarity, the hairs repel each other. © Adam Hart-Davis/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission. and found they obeyed an inverse square law. The mathematical relationship between the forces is known as Coulomb's law, and the unit of electric charge is named the coulomb in his honor.

Alessandro Volta invented a device in 1775 that could create and store an electrostatic charge. Called an electrophorus, it used two plates to accumulate a strong positive charge. The device replaced the Leyden jar, and the two-plate principle is behind the electrical condensers in use today.

Several other electrostatic machines have been devised. In 1765 John Reid, an instrument maker in London, built a portable static electric generating machine to treat medical problems. In 1783, John Cuthbertson built a huge device that could produce electrical discharges 2 ft (61 cm) in length. The gold leaf electroscope, invented in 1787, consists of two leaves which repel each other when they receive an electric charge. In 1881, British engineer James Wimshurst invented his Wimshurst machine, two glass discs with metal segments spinning opposite each other. Brushes touching the metal segments removed the charge created and conducted it to a pair of Leyden jars where it was stored for later use.

The most famous of all the electrostatic devices is the Van de Graaff generator. Invented in 1929 by Robert J. Van de Graaff, it uses a conveyor belt to carry an electric charge from a high-voltage supply to a hollow ball. It had various applications. For his experiments on properties of atoms, Van de Graaff needed to accelerate subatomic particles to very high velocity, and he knew that storing an electrostatic charge could result in a high potential. Another generator was modified to produce x rays for use in the treatment of internal tumors. It was installed in a hospital in Boston in 1937. Van de Graaff's first generator operated at 80,000 volts, but was eventually improved to five million volts. It remains one of the most widely used experimental exhibits in schools and museums today.

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