Electrostatic Devices

Electrostatics, in physics, is the study of the behavior of electric charges that are at rest (static). 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.

Ancient Greek philosopher Thales (624–546 BC) discovered that when a piece of amber was rubbed, it could pick up light objects, a process known as tribo-electrification. The Greek name for amber, elektron, gave rise to many of the words used 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, English physician William Gilbert (1544–1603) began experimenting with static electricity, pointing out the difference between static electric attraction and magnetic attraction. Later, in the mid-1600s, German scientist and inventor Otto von Guericke (1602–1686) 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 English scientist Francis Hawkesbee (1666–1713) 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 (England), 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 American author, scientist, publisher, and inventor Benjamin Franklin (1706–1790), 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.

French chemist Charles Francois de Cisternay du Fay (1698–1739) discovered that suspended bits of cork, electrified with a statically charged glass rod, repelled each other. Du Fay concluded that any two objects that had the same charge repelled each other, while unlike charges attracted. The science of electrostatics, so named by French scientist Andre´ Marie Ampeère (1775–1836), is based on this fact.

French physicist Charles Coulomb (1736–1806) became interested in the work of English chemist Joseph Priestley (1733–1804), who had built an electro-static 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 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.

Italian physicist Alessandro Volta (1745–1827) 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, England, 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 that 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 American physicist Robert J. Van de Graaff (1901– 1967), 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, Massachusetts, 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.