Electricity in 18th Century Experimental Physics
Mechanical philosophers began experimental investigations of static electricity during the 1600s, noting the attractions and repulsions (similar to magnetism) that they could produce by rubbing objects such as glass and amber. Otto von Guericke, for example, in 1672 made a sphere that could be charged by friction, and then floated a feather above it. Theories proliferated about what kind of "virtue" could account for the property of attraction, as mechanists sought to explain it without appeal to occult (hidden) qualities. It did not help that a wide range of substances, from amber to various gemstones and minerals, to resins and wax, micah and glass, could produce the electric effect of attraction. Mechanistic hypotheses generally invoked some sort of vaporous matter that was given off and circulated around. They wanted a direct material explanation. Some, like Guerecke, to explain the oddity of attraction without apparent physical connection, simply appealed to a property of action-at-a-distance, without a material explanation. But such an occult force was just the kind of explanation distrusted within the new mechanical philosophy.
The efforts of natural philosophers by the early 1700s focused on developing new instruments and experiments to investigate such occult phenomena. A notable experimentalist of the day was Francis Hauksbee (1666-1713), an instrument maker and demonstrator who was appointed curator of experiments at the Royal Society by its new president, Isaac Newton. Through modifications of equipment and refinements of experiments, Hauksbee created a device that could generate much larger effects -- a long cylindrical glass tube that could be charged by rubbing. The great advances in experimenting with electricity came during the 1700s from increased ability to control it. Pictured here (Physico-Mechanical Experiments, 1702) is a device Hauksbee made with a hollow glass sphere that could be mechanically rubbed at high speed, producing larger and more stable charges. Notice the illustrations of his series of experiments with the attraction of hairs to the globe, indicating the directions of attraction and supporting his hypothesis of lines rather than vortices.
At the same time, Stephen Gray (1666-1736) was pursuing a long series of experiments, promoting a different hypothesis. In producing charge on a long glass tube, he discovered in 1729 that he could communicate the electrical effect to other objects by direct connection. Using string, he could charge an object over 50 feet from the rubbed tube, but oddly enough some other substances, such as silk thread, would not carry charge. Brass wire would transmit charge even better. These experiments with charged strings and glass tubes revealed the properties of conduction, insulation, and transmission. (The names of 'conductors' and 'insulators' came from Jean Desaguliers, who had succeeded Hauksbee as Newton's choice for experimental demonstrator of the Royal Society.) The plate to the right is one of Gray's most famous experiments, in which he showed that a boy suspended by (insulating) silk cords could be charged (with the glass tube) and then as a (conducting) body could (electrostatically) attract small objects. Dramatic experiments such as these became quite well-known -- this plate is in fact from a German publication in 1744 describing Gray's work.
Electrical demonstrations became all the rage in the salons of intellectuals excited by the new physics, as revealed by these contemporary illustrations:
In Paris, Charles Du Fay (1698-1739) developed the idea of grounding by noticing how the object to be electrified had to be supported by or rest on insulators. He and his assistant Jean Nollet (1700-1770) collaborated also in pursuing properties of repulsion, which had generally been ignored in pursuits of the attractive electric force. From these experiments, in 1733 Du Fay derived an hypothesis of two fluids of electricity, attracting opposites and repelling like kinds. Du Fay's and Nollet's demonstrations also led to experimental and theoretical attention to the electric shock and spark produced when one touched a charged objecct. Pictured at left, a suspended woman is being charged in a salon demonstration by a large friction wheel. At right, the Abbé Nollet in his Paris school is charging up one of his students (suspended by silken ropes) to demonstrate his attraction (to small bits of paper) and conduction of sparks (from his nose to the young lady's hand). He taught the royal children, and published a widely popular text on experimental physics [Leçons de physique expérimentale].
In a famous episode, shown at left, the German experimentalist Georg Richmann, working at the Russian court in St. Petersburg, was killed in 1753 while trying to capture the charge of lightning. This tragedy was widely illustrated in the contemporary press as a story of heroism in the pursuit of science.
Besides creating static electricity with friction machines, by 1746 experimentalists learned how to capture and store it. The "Leyden jar" was named for the Dutch university where the professor Pieter von Musschenbroek (1692-1761) refined the device. It was a condensor that let one store charge in the water (or conducting foil) in a bottle and then discharge a powerful spark. Continuous advances in isolating, producing, and controlling electrical phenomena, however, also produced a proliferation of hypotheses to explain the puzzling and complicated effects. No theory escaped contradictions, and none united the field.
Du Fay and Ben Franklin, for example, each had a theory of two electricities but disagreed over the nature of electricity. For Nollet, electricity was one subtle material emitted and set in motion by rubbing. It streams out from bodies in strong flow from points, which causes surrounding electrical matter to flow more diffusely back to them. It was the nature of its movement that created electrical effects, which seemed a promising as a way to explain many experimental phenomena of attraction, repulsion, and discharge; and it provided for the possibility for a quantitative theory. Explaining much, it did however fail to explain how the glass Leyden jar could transport charge. Picking up on the latest equipment and experiments from Europe, Franklin's series of investigations suggested to him a different theory. He agreed that electricity is a single emitted subtle substance, put in motion by the rubbing. This subtle matter is attracted to ordinary matter, and thus denser objects could hold more electricity. Electricity repels itself, but as it does not exist in free space, the two impulses must be in equilibrium, with each body having its stable amount of electrical matter. He explained the Leyden jar as the insulating glass holding in more electric substance (having been charged) but the electrical repulsion acting at a distance through the walls. Charging sets up a disequilibrium of positive and negative states; discharge is return to normal (Experiments and Observations on Electricity, 1751). Action-at-a-distance here is analagous to Newton's gravitational attraction of all matter. Explaining the exciting and important Leyden jar phenomena made this theory compelling, but it had its failures as well. Bodies were supposed to have either more or less than the equilibrium amount of electricity. That could not explain what Du Fay had explained with his two electrical substances -- that bodies can be charged positively or negatively, with consequent attraction or repulsion between them. If two insulated people were charged in the same way, they repelled each other. But if one was charged from the glass tube and one from the rubbing cloth, a spark would pass between them, leaving both neutralized again.
The experimental demonstrations of positive and negative charge with forces of attraction and repulsion and a conservation of charge led to attempts to quantify and mathematicize the phenomena. In the 1760s and '70s, various experiments suggested that electric force obeys an inverse-square law in attraction and repulsoin. With a stunning display of experimental precision, Charles Augustin Coulomb (1736-1806), a French military engineer and member of the Académie des Sciences, measured in 1785 the force of electrostatic charge. He invented a torsion balance, in which the attraction of the small charged balls is compensated by the torsion of the wire (with the value read off the knob at the top). He was able to demonstrate that the force follows an inverse-square law, with a striking parallel to Newton's gravitation equation. This was a triumph for experimental and mathematical Newtonian physics, and it lent support to a mathematical theory of a single fluid of electricity.
At the end of the century, Alessandro Volta (1745-1827) provided the practical innovation that allowed the controlled, quantitative study of electricity to advance rapidly. He had noted that two different metals brought into contact acquired different electric potentials (placed on the tongue, they give an acid or alkaline sensation, which he showed to be electrical). He suggested that conductors can be categorized as of two types -- metals that had different potentials in contact, and liquids that would have the same potential as a metal placed in them. Different metals showed different values, and Volta determined a scale of relations that let him create a chain of metals that would accumulate the difference of potential.
He called this a "pile" -- literally a pile of metal discs separated by cloth soaked in acid. This pile, the first metal-acid wet battery, generated an electrical current -- making available to experimentalists for the first time a strong sustained current (rather than the electrostatic machines weak output or the strong single discharge of a Leyden jar).
The plate to the right is from his noted lectures to the French Académie in 1801.
Volta's fame was even greater after his demonstrations to Napoleon -- who gave him honors, a new position, and noble title.
By the end of the century, there were also tantalizing experimental suggestions that electricity might be the force of life, a Newtonian entity that would explain "animal spirits". Deeply influential were the experimental results of the anatomist Luigi Galvani (1737-98), who reported in 1791 his startling work on the effects of electricity on muscle.
In the well-known account of his serendipitous discovery, a dissected frog was accidentally touched with a scalpel, near a charged electrostatic machine. The dead frog leg twitched. Noting the simultaneous spark, Galvani embarked on a series of electrical experiments that measured the effect of electricity on muscular contraction. He extended the idea of bodies conducting charge, showing that the frog could be made into a sort of Leyden jar. Most provocative was the fact that dead tissue could be given movement.
Galvani argued that his "animal electricity" was not the same as ordinary electricity, a conclusion that drew considerable controversy as others thought perhaps the nature of life was being revealed as a Newtonian phenomenon. An "electrical" nature of vitality was an idea that would have a long life in the next century, in popular medicine, literature, and materialist philosophy.