VII.

Electricity and Magnetism

The final stage in the development of classical physics involved the study of electricity. In the 19th century, electricity was considered almost a science unto itself.

Electricity was a mysterious force. At first appearance, it did not seem to occur naturally, except in the frightening form of lightning. Researchers had to do an unnatural thing to study electricity; they had to manufacture the phenomenon before they could analyze it. We have come to realize that electricity is everywhere and that all matter is electrical in nature. In the 19th century, however, electricity was considered quite exotic.

Many heroes in the study of electricity and magnetism emerged between the late 1700s and the early 1800s, many of whom left their names on various electrical units. These scientists include Charles Augustin de Coulomb (the unit of charge), André Ampère (current), Georg Ohm (resistance), James Watt (electrical power), and James Joule (energy). Luigi Galvani gave us the galvanometer, a device for measuring currents, and Alessandro Volta gave us the volt, a unit of potential, or electromotive force. Similarly C. F. Gauss, Hans Christian Oersted, and W. E. Weber all made their mark and left their names on electrical engineering. Only Benjamin Franklin failed to get his name on any electrical unit, despite his significant contributions. All of these scientists contributed to the study of electricity. But the real giants in the field were two 19th-century Englishmen, Michael Faraday and James Clerk Maxwell.

The story of electricity begins in the late 1700s with Galvani’s invention of the battery, which was later improved by Volta, another Italian. Galvani studied frog reflexes by hanging frog muscles on a metal latticework outside his window and watching them twitch during thunderstorms. Volta called this a demonstration of “animal electricity.” Volta discovered that the frog electricity was caused by the action of an electrical current through two dissimilar metals separated by animal tissues, for Galvani’s frogs had hung on brass hooks attached to an iron latticework. Volta was able to produce an electrical current without the frog parts by experimenting with different pairs of metals separated by pieces of leather soaked in brine. He then created a “pile” of zinc and copper plates, realizing that the larger the pile, the more current he could drive through an external circuit. Crucial to this work was Volta’s invention of an electrometer for measuring the current. This research yielded two important results: a laboratory tool for producing currents and a realization that electricity could be produced by chemical reactions.

Another important development was Coulomb’s measurement of the strength and behavior of the electrical force between two charge balls. In 1777 Coulomb invented the torsion balance, a measuring device that was exquisitely sensitive to tiny forces. The force he was after, of course, was electricity. In the mid-1700s Benjamin Franklin had explained that there are two kinds of electricity, which he named plus (+) and minus (-). Franklin called the amount of electricity on an object electric charge. Coulomb discovered that objects with opposite electric charge (+ and -) attract each other and objects with the same electric charge (+ and +) repel one another. Using his torsion balance, he determined that the force between electrical charges varied inversely as the square of the distance between them. This determination, subsequently known as Coulomb’s law, would play a crucial role in our understanding of the atom.

In a brief period from 1820 to 1870, scientists conducted a series of experiments on what they first believed to be the separate phenomena of electricity and magnetism. These experiments led to an understanding that electricity and magnetism were the same phenomenon, which they called electromagnetism, and that light was a form of electromagnetic energy.