Читать книгу The History of Physics from 2000BCE to 1945 - Sheldon J.D. Cohen - Страница 7

Scientists continued to do work on the attractive force discovered by the Greeks (static electricity). Charles Francois Dufay (1698-1739) was the gardener to the king of France. In his spare time, he produced two types of electric charges when he rubbed different substances together. Working with glass, he discovered that some charged pieces would either attract or repel other charged pieces of glass. He called these two types of effects resinous and vitreous. He theorized that matter contained two fluids in a definite balance that was disturbed during the act of rubbing; each body acquiring either an excess or a deficiency of the fluids. Subsequently the terminology of resinous and vitreous for this unknowm mysterious force became negative and positive. The use of the word fluid would remain for about one hundred-fifty years, or until the exact nature of this mysterious force was discovered.

In the mid 1700s, Pieter van Musschenbroek (1692-1761) a Dutch physicist from the University of Leyden, developed a means of storing static electricity. He half filled a glass container with water and sealed the top with a cork. He then pierced the cork with a nail that extended into the water. The exposed nail subjected to friction caused static electricity to be stored within the glass container. Discontinuing the friction, then touching the exposed nail, resulted in a shock. This was the earliest form of what would be known as the Leyden jar. It eventually consisted of a jar coated inside and out with tin foil. The outer tin foil connected to the earth; the inner tin foil connected to a brass rod projecting through the mouth of the jar. Charging one of the tin foils with an electrostatic machine would produce a severe shock if both tin foils were touched at the same time.

It was possible to force a large amount of electric charge into the Leyden jar. In fact, if the jar carried a sufficiently large amount, even approaching a nearby object could discharge it and a spark would force its way through the air to the other object. When it did this, there were those who saw a resemblance to lightening. From this primitive beginning, natural philosophers developed numerous devices that created electric current.

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 to duplicate 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 in a cramp. 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 effected 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 even an animal utilized electricity as part of their physiological processes. He named this “animal electricity,” and thought that this was a different force then the natural electricity of lightening or the static electricity produced within the Leyden jar.

These experiments demonstrated the electrical nature of nerve muscle function and opened up 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?

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 then 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 then could be obtained by electrostatic machines and resulted in enhanced research efforts.

The volt now describes the unit of electrostatic potential.

It was shortly after Volta’s discovery that William Nicholson 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.

Attractive or repulsive forces diminish as the electric charges are separated. 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.

Petrus Peregrinus (1220-?) a French physician, performed the first serious study of magnetism in 1269. Working with a 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 in Colchester, England to a family of some wealth. He went to St. John’s College, Cambridge, and over a nine-year period obtained a BA, MA, and M.D. He set up medical practice in London, and held a number of offices in the Royal College of Physicians including president. He became physician to Queen Elizabeth.

As was typical of the scientists of the times, he held many interests, and Gilbert furthered the study of magnetism when he proposed that the entire earth was a large magnet with a north and south pole. His book, De Magnete, published in 1600, became the standard upon which all work in magnetism was measured. It remains one of the classics of natural philosophic literature.

In addition, he advanced the work of Thales of Miletos, and Theophrastus when he discovered that more materials could be “electrified” when rubbed with a cloth. He called these materials “electricians” and he proposed the word “electricity,” from the Latin word, electrum, for amber. He also theorized that electricity was due to the existence of a fluid surrounding an electrified body and the act of electrification was due to the passage of this fluid into the electrician. He also suggested that this fluid transfer was made possible when the friction of rubbing created the heat necessary to promote this passage.

Although both the rubbed amber and magnetism were attractive forces, Gilbert was the first scientist to explain the difference between magnetism and the amber effect---static electricity. The term static would not enter the physicists vocabulary for two hundred more years or until electric currents were identified as moving charges of electricity.