2,637 Years of Physics from Thales of Miletos to the Modern Era. Sheldon Cohen

      Benjamin Franklin (1706-1790)

      the great American politician, statesman, inventor, and natural philosopher, used a metal conductor to discharge the inner and outer layers of tin foil in a Leyden jar creating the clearly visible and audible spark. He was one of those who speculated about the similarity of the visible and audible spark to lightening. Could lightening be an electric discharge similar to the spark produced within the Leyden jar?

       To prove this hypothesis he flew a kite with a metal tip in a thunderstorm. He used wet hemp line, a conductor of electricity, to fly the kite and attached a metal key to the end of the line. From the key he attached a non-conducting silk line that he held in his hand. When he held his other hand near the key he drew sparks from it, thus proving that lightening was an electrical phenomenon with the same properties as the electric spark produced with the Leyden jar. Franklin was fortunate: the next two men who tried duplicating Franklin’s experiments died of electric shock.

       The lightening experiments brought great fame to Franklin. Much of his success as a diplomat in France was due to his reputation as a natural philosopher. For the French knew that they were receiving one of the world’s leading scientific figures, and not just an American patriot.

       Franklin showed that the electrical experiments performed in a laboratory are related to the natural events of our world---lightening. Therefore, future natural philosophic studies of nature could not be divorced from electricity.

       Franklin postulated that electricity was a single fluid that existed in all matter and explained the effects of electricity by a shortage or excess of this fluid. The word fluid, used by natural philosophers, was the primitive description of a force of nature that future experimenters would gradually shed light upon…

      Luigi Galvani (1737-1798),

      an Italian anatomist and physician discovered that when the lower legs of a dissected frog were in proximity to an electrical source, such as a Leyden jar, the frog’s muscles twitched into what he described as a “convulsive state.” Galvani had uncovered an interesting phenomenon. Whereas a spark could touch a muscle and cause it to contract, Galvani demonstrated that the muscle would contract merely by being in the proximity of an electric source. Up to this point, an electric current was only demonstrated in an instantaneous fashion. Once an electric spark transferred its current, there was no more; no further current flowed. He also demonstrated that by drawing a spark from an electrical machine at a distance from a muscle, and simultaneously touching metal to the frog’s sciatic nerve, the leg muscles twitched as if cramped. Further, he showed that touching a muscle with two different metals caused the muscle to twitch and the twitching would continue multiple times. Clearly, these muscles were being affected by electricity, but in this case the electricity would manifest itself not as one instantaneous jolt and then no more, but rather in a continuous fashion. How could this be? Galvani concluded that it was the muscle itself that generated the electric current responsible for this continuous twitching, and therefore animals utilized electricity as part of their physiological processes. He named this “animal electricity,” and thought that this was a different force than the natural electricity of lightening or the static electricity produced within the Leyden jar. The phenomenon of electricity was expanding in many directions.

       These experiments demonstrated the electrical nature of nerve muscle function and opened the door to the study of neurophysiology.

      Electrical forces in animal tissue? Did electrical energy permeate living as well as inanimate objects? Was electricity a universal force explaining life itself? These questions would “spark” much thinking and open new doors.

      Alessandro Volta (1745-1827)

      from Como, Italy was a student of languages and had a broad liberal arts education. Electricity became his hobby and he developed the battery in 1793. Rather than accept Galvani’s notion of the muscle source for the continuous electricity, he suspected that the two dissimilar strips of metal were really the electrical generating medium. Following this hunch, he discovered that electricity was produced when two different metal strips were placed within a salt solution that had no connection with animal tissue. This simple act would produce electricity that continued as long as the chemical reaction continued. The electricity was then drawn off continuously through a wire. For the first time electricity was produced by chemical means. This was a far more powerful force that could be obtained by electrostatic machines and resulted in enhanced research efforts.

       In honor of Volta’s brilliant work, the volt now describes the unit of electrostatic potential.

      It was shortly after Volta’s discovery that

      William Nicholson (1753-1815) working together with

      Anthony Carlisle (1768-1840),

      both from England, passed an electric current through water and discovered that they could break down the water into their component parts: hydrogen and oxygen. For the first time an electric current brought about a chemical reaction. They named the process hydrolysis---taking apart by electricity. Electrical insight was expanding exponentially.

      Joseph Priestly (1733-1804)

      of England, who emigrated to America and became a friend of Benjamin Franklin, determined that as the distance between two charged bodies of equal sign are increased by a factor, the repellant force is reduced by the square of that factor. This means that if two negatively charged pieces of metal are very close there is a strong force repelling them apart, but if they were moved, for example, two inches apart, the repelling force would be reduced by two squared—two times two or four times. Although this assumption was correct, no one demonstrated this fact experimentally. Priestly, a chemist, is better known for his discovery of oxygen.

       As a result of the work by scientists mentioned above as well as many others, by the last quarter of the 18^th century certain principles about electric phenomenon were established:

       There are two signs of electrical charge:

       Charges of the same kinds repel each other.

       Charges of the opposite kinds attract each other.

       Electricity was considered a kind of fluid, although there was controversy as to whether there were two types of fluid that could be added or taken away from different materials, or a single fluid that could be transferred in part or whole from one material to the next.

       There are conductors of electricity through which an electric charge can freely move.

       There were insulators through which no charge could move.

       Electricity is stable and unchanging.

       When Thales of Miletos polished amber with cat fur, and the amber was able to pick up bits of straw or feathers, there was no natural philosophic explanation for this finding. As mentioned before, what Thales had uncovered was static electricity, but this terminology would not enter the scientific lexicon for many centuries, or until further advances shed more light on the subject.

      Petrus Peregrinus (1220-?)

      was a French physician, who performed the first serious study of magnetism in 1269. This is probably a reflection of the fact that there was so little understood in the field of medicine, that restless minds of some physicians forced them to investigate other subjects, if for no other reason but to stimulate their minds. Working with the magnetic stone known as lodestone he described the polarity of magnets. He found that the magnetic North Pole of one magnet would be attracted to the magnetic South Pole of another. He also demonstrated that North Poles repulsed North Poles and South Poles repulsed South Poles. In short, the concept of the attraction of opposite poles and the repulsion of like poles was attributed to Peregrinus.

      William Gilbert (1544-1603)

      was born