Gray Discovers Principles of Electric Conductivity

Date 1729

Stephen Gray discovered that electricity could flow from one object to another and that while some materials were conductors of electricity, other materials were insulators of electricity. His meticulous and imaginative experiments transformed the study of static electricity from a parlor amusement to a science.

Locale London and Cambridge, England

Key Figures

• Stephen Gray (1666-1736), amateur experimental scientist
• John Flamsteed (1646-1719), England’s astronomer royal and Gray’s role model, who probably tutored Gray
• Francis Hauksbee (1660-1713), a demonstrator of experiments for the Royal Society, who awakened Gray’s interest in static electricity
• Jean Théophile Desaguliers (1683-1744), a Royal Society lecturer, whom Gray assisted
• Granville Wheler (1701-1770 ), a friend of Gray who provided him working space and help with his experiments
• Charles-François de Cisternay Du Fay (1698-1739), French scientist who repeated many of Gray’s experiments and continued his work

Summary of Event

Stephen Gray’s father made a decent income dyeing cloth, and Gray carried on the family business while dabbling in science. At some time he seems to have studied with John Flamsteed, England’s astronomer royal. Gray ground his own telescope lenses and measured eclipses, sunspots, and the revolutions of Jupiter’s satellites. He made a microscope using as a lens a water droplet in a tiny hole made in a brass plate. It worked well enough for him to see microscopic organisms swimming in the water, and he became widely known as a careful and talented observer. Although he was not a member, the Royal Society of London still published Gray’s microscope observations in 1696.

The fellows of the Royal Society met weekly to discuss items of scientific interest, including the amber effect, in which rubbing amber enables it to attract bits of fluff, an effect that had been known since ancient times. More recently, William Gilbert showed that glass, sulfur, precious stones, resin, and sealing wax could also be “electrified,” a term derived from the Greek word for amber, elektron. In 1706, Francis Hauksbee used a flint glass tube to demonstrate some effects of static electricity to the society, and hearing of this, Gray performed his own experiments and sent an account of them to the society in 1708. He later became friends with several society members when he became an assistant to Jean Théophile Desaguliers, who performed demonstrations for the organization.

When Gray hurt his back and could no longer work at the family trade, he applied to become a resident of Sutton’s Hospital, also known as the London Charterhouse. This request was finally granted in 1719. The Charterhouse was a day school for poor boys and provided room and board for eighty male pensioners who were required to be educated enough to teach the boys. Gray continued to dabble with electrostatics but made relatively little progress. Finally, in 1729, at the age of sixty-three, he obtained a flint glass tube similar to the one used by Hauksbee. Just over 3 feet long and 1 inch in diameter, the tube acquired a static charge when rubbed with a cloth or by a dry hand. To keep dust out when the tube was not in use, Gray placed corks in the ends. He could tell when the tube had acquired “electric virtue” because it would then attract bits of feather, thread, or leaf-brass. When the tube was tested in the dark, a crackling sound and a flash of light (a spark) would sometimes occur, and Gray wondered if that light falling on a metal would convey “electric virtue” to the metal. He had previously attempted, but without success, to electrify metals by rubbing them.

Preparing for this experiment, Gray rubbed the tube, then noticing, however, that small feathers were attracted to the corks as well as to the tube. This simple observation was a breakthrough: Electric virtue could be conducted from one body to another. Flush with excitement, Gray tried every object he had at hand: An ivory ball with a hole in it was stuck on a short stick and the stick inserted in the cork; the ball suspended by wire or 3 feet of twine, silver coins, a lead ball, a fire shovel, an iron poker, metal tongs, a copper tea kettle (empty or full of hot or cold water), brick, tile, chalk, and a head of cabbage. All could be electrified when connected to the tube. Gray later found that materials such as wood or twine conduct only when the humidity is high enough.

Next, Gray wondered how far the electric virtue could be made to travel. Limited by the size of his room, he assembled 18 feet of sticks and canes and connected one end to the tube and the other to the ivory ball, which became electrified. Needing more space, during the next few months he visited his friends John Godfrey and Granville Wheler, both of whom had large homes and lands. They eventually succeeded in transmitting electric virtue down 886 feet of twine, but two more important discoveries were made along the way. Godfrey and Gray prepared a horizontal line of twine suspended by twine from nails in a ceiling beam and had absolutely no success. Gray reasoned correctly that the electric virtue flowed up the twine and was dissipated in the ceiling beam.

When Gray visited Wheler and explained the problem of suspending a horizontal line, Wheler suggested that the twine line could be suspended with silk threads; this strategy was successful. Gray supposed that the silk thread blocked the flow of electric virtue because it had a much smaller diameter than the twine. Later, however, when the silk support lines broke under the weight of long lengths of twine and they used brass wire no thicker than the silk lines to replace them, there was once again no electric flow. Gray now correctly reasoned that it was not the diameter of the support lines that was important, but that some materials allowed the flow of electric virtue into the ceiling while others blocked this flow.

As Gray continued to experiment, he wondered if people could be electrified. This led to a dramatic demonstration, called “the charity boy,” which is still associated with Gray’s name. The spectacle made great after-dinner entertainment for guests, and it was educational. Gray suspended two large loops of horsehair clothesline from hooks in the ceiling. He had a charity boy (a student in a private charitable school for the poor) lie horizontally, suspended in the air by one loop about his chest and the other about his legs. Then Gray used his excited glass tube to electrify the boy. Any guest who tried to touch the boy was zapped by a sharp spark, and when the boy extended his hands over insulated stands holding bits of feathers and leaf-brass, the bits jumped upward to his hands. If a guest brought a finger near the boy’s nose, the finger would draw sparks from the boy’s nose. The spectacle left no doubt that people could be electrified. Gray wrote a careful description of his experiments and findings, which was published by the Royal Society in 1732.

The principles of conductivity can be demonstrated in modern terms. For example, when different materials, such as a wool cloth and a glass rod, are placed into close contact and then separated (helped by rubbing), some electrons will be transferred from the glass to the wool. This leaves the glass charged positively and the wool negatively. If a small feather is brought close to the positively charged glass, it will act on the atoms of the feather to pull some electrons in each atom to the side of the atom that is closest to the glass rod. Since the average negative charge in the feather is now slightly closer than the average positive charge, the feather will be attracted to the glass. If the feather now touches the glass rod, some electrons from the feather may transfer to the rod, leaving the feather positive and the rod less positive. Both rod and feather are positively charged and the feather will now be repelled from the rod. Gray, and others, observed both the attractive and repulsive effects. In fact, one of Gray’s favorite demonstrations was to repel a feather and make it float in the air by holding the tube horizontally and keeping it beneath the feather.

Significance

Stephen Gray continued to experiment and publish until he became too ill. He died in 1736. He had received the Royal Society’s greatest honor, the Copley Medal, in 1731, the first year it was awarded. He had received it again in 1732 and was also elected a fellow of the Royal Society. Gray’s friend Desaguliers gave the modern names “conductors” and “insulators” to the two classes of materials Gray had discovered.

The French scientist Charles-François de Cisternay Du Fay visited Wheler and Gray in 1732. Inspired by their work, Du Fay came up with the two-fluid theory of electricity, which remained popular until it was superseded by Benjamin Franklin’s one-fluid theory. Since Gray was the first to send electrical signals hundreds of feet down a line, he is an ancestor of the telegraph, the telephone, and long-distance communication. Perhaps most important, just as William Gilbert made the study of magnets scientific, Gray, more than any of his predecessors, made the study of electricity scientific.

Bibliography

Bodanis, David. Electric Universe: The Shocking True Story of Electricity. New York: Crown, 2005. A fascinating exploration of the history of electricity that looks at inventions from the lightbulb to the computer. Includes a helpful annotated bibliography.

Fara, Patricia. An Entertainment for Angels: Electricity in the Enlightenment. New York: Columbia University Press, 2002. Fara describes early electrical experiments and examines Gray and his work. Includes bibliographical references.

Heilbron, J. L. Electricity in the Seventeenth and Eighteenth Centuries: A Study in Early Modern Physics. Mineola, N.Y.: Dover, 1999. An excellent science history book that places Gray’s work in context. Although there are no equations, better-prepared students will benefit most from this work. Contains an extensive bibliography.