Coulomb

By the early 1700s, Sir Isaac Newton's law of gravitational force had been widely accepted by the scientific community, which realized the vast array of problems to which it could be applied. During the period 1760-1780, scientists began to search for a comparable law that would describe the force between two electrically charged bodies. Many assumed that such a law would follow the general lines of the gravitational law, namely that the force would vary directly with the magnitude of the charges and inversely as the distance between them.

The first experiments in this field were conducted by the Swiss mathematician Daniel Bernoulli around 1760. Bernoulli's experiments were apparently among the earliest quantitative studies in the field of electricity, and they aroused little interest among other scientists. A decade later, however, two early English chemists, Joseph Priestley and Henry Cavendish, carried out experiments similar to those of Bernoulli and obtained qualitative support for a gravitation-like relationship for electrical charges.

Conclusive work on this subject was completed by Coulomb in 1785. The French physicist designed an ingenious apparatus for measuring the relatively modest force that exists between two charged bodies. The apparatus is known as a torsion balance. The torsion balance consists of a non-conducting horizontal bar suspended by a thin fiber of metal or silk. Two small spheres are attached to opposite ends of the bar and given an electrical charge. A third ball is then placed adjacent to the ball at one end of the horizontal rod and given a charge identical to those on the rod.

In this arrangement, a force of repulsion develops between the two adjacent balls. As they push away from each other, they cause the metal or silk fiber to twist. The amount of twist that develops in the fiber can be measured and can be used to calculate the force that produced the distortion.