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

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СКАЧАТЬ 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 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 physician scientists of the times, (see above), 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, Gilbert 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.

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