Charles-François de Cisternay Du Fay

1698-1739

French Physicist

Charles-François de Cisternay Du Fay is best known for his discovery of positive and negative electrical charges and some of their properties. He was one of the first to use an electroscope in his research, and his work inspired Benjamin Franklin's (1706-1790) famous kite experiment.

Du Fay was born in France in 1698, and virtually nothing is known about him until he began his experiments with electricity in the 1730s. Prior to that time, although electricity had been known for over 2,000 years (the Greeks first noticed it at about 400 b.c.), virtually nothing had been learned. The best theories of the day were that electricity, like "heaviness," was simply a property of all solid materials. However, Du Fay quickly discovered that there were at least two distinct types of electricity, obtained from rubbing amber and sulfur. The electricity from rubbing amber, when transferred to a cloth, would repel electricity from another piece of amber, but attracted that from sulfur. Du Fay named these "vitreous" and "resinous" electricity, from the types of materials they came from (sulfur being vitreous and amber resinous).

Working in collaboration with Jean Antoine Nollet (1700-1770), a priest, Du Fay tried to better characterize the "electrical fluids" he was investigating. And, at this point, he seriously thought he might be looking at two separate fluids since they behaved differently. To test his theory, he had to find some way to capture enough "fluid" to perform experiments. At the time, there was a storage device named the Leyden jar. This was a glass jar, partially filled with water, and covered inside and out with tin foil. A wire ran through a cork stopper and, as electricity was generated, it passed through the wire into the jar, where it was stored.

Since the Leyden jar could store a fairly large amount of electricity, Du Fay was able to perform some interesting (and potentially deadly) demonstrations. In one, he passed an electrical discharge through 180 soldiers who had joined hands in a circle (which also gave rise to the term "circuit") by having the first soldier hold the jab and the last soldier touch the center wire. He (and others) apparently enjoyed themselves greatly, charging Leyden jars and shocking their friends and relatives. Unfortunately, they did not know that such shocks could be deadly.

Some of Du Fay's experiments were performed with an electroscope, a device he did not invent, but was one of the first to use in research. An electroscope makes use of the fact that like-charges repel each other and that the strength of the repulsive force is proportional to the electrical charge. Therefore, two small metal leaves will be held at an angle against the pull of gravity if they both have the same electrical charge, and the angle separating them will be greater if the charge is greater. While simple in theory and in operation, electroscopes still find extensive use today in fields ranging from physics to radiation safety.

Du Fay's work was influential in Benjamin Franklin's later work with electricity, and he also influenced many other prominent scientists of the day. Today, we understand the importance of electricity and have found innumerable uses for it. This makes it difficult to realize that, until just a few hundred years ago, it was thought of as a curiosity and nothing more. It was not until Du Fay and a few others began investigating its properties in a systematic manner that its nature and potential began to be fully appreciated.

P. ANDREW KARAM