Applications
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Chapter 22: Electromagnetic Waves Applications |
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One of the fundamental needs of human civilization is telecommunication: the ability to communicate rapidly over long distances. Electromagnetic waves are ideal for this pupose because they travel at the speed of light and because they can be focused and directed.
People have been using electromagnetic waves to communicate for thousands of years. Signal fires are sources of visible electromagnetic waves (light) that can be seen over long distances. More recently, we have learned to produce radio waves and use them to carry information around the world. The history of telecommunications is a wonderful story, filled with examples of physics working for us. Here, we can only scratch the surface.
Perhaps the oldest known example of telecommunication dates to 1084 BC. At that time, a series of signal fires was used to relay a crucial piece of information across ancient Greece. The war was over. Troy ahd been taken, and the Greek armies were coming home. One of the recipients of this news was Clytemnestra, the wife of Agamemnon who commanded the armies. This allowed her time to prepare a most unpleasant homecoming, the murder of her husband by her lover Aegisthus.
In about 300 BC, also in Greece, telecommunication took a big step forward. Firelight was still the electromagnetic wave of choice, but the idea of code was invented. For the first time, alphabetic signals were agreed upon, and complex messages could be spelled out. The code was simple. A grid of 4 by 6 spaces was set up, one corresponding to each of 24 letters. An urn holding a small fire was moved sequentially from one grid space to another. to spell the message.
There is a great deal more of course, but let's take a big step forward, to the late 18th century and the invention of the telegraph. The first telegraphs, invented in France and in England were still essentially signal fires. Telescopes and more complicated signaling systems were used, but fire remained the central element. The various locations named "telegraph hill" around the US and Great Britain date from this era.
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| A historic telegraph key, courtesy of Neal McEwen, K5RW and his Telegraphy page |
This system worked, but was needlessly complicated. Again, the system was improved by the introduction of a code. Samuel F. B. Morse was a professor of Art at The University of the City of New York, which is now known as NYU. In the mid 1830's Morse created several crucial elements that revolutionized telegraphy. In 1835 Morse created the first version of his famous code, in which letters and numbers are replaced by a series of dots and dashes. In 1837 he was granted a patent for an actual telegraph system. A few years later, working with Alfred Vail, he refined the code (using the shortest sequences for the most common letters) and introduced the telegraph key, which made it possible to enter code rapidly. In 1843, Morse received a grant from the US government to set up the first intercity telegraph system, linking Baltimore MD to Washington D.C. The sytem was completed in one year. In 1844 the first message was sent. it was "What hath God wrought?"
Soon, the telegraph became the principal method for sending news over long distances. The Associated Press and Reuters news agencies were both formed in the late 1840's. These institutions are still among the most important international news services. Applications of the telegraph soon included personal communications and financial transactions. The Western Union company was founded by Ezra Cornell as the New York and Mississippi Valley Printing Telegraph Company in 1851, changing its name in 1856. Cornell made millions, and endowed the university in Ithaca, NY that still bears his name.
Telegraphs carry messages electromagnetically, however, the signals must run through wires. An even better possibility would be to use electromagnetic waves that travel through the air. Such waves were first predicted by James Clerk Maxwell a brilliant mathematical physicist born in Scotland in 1831. To get a sense of Maxwell's greatness consider a few highlights from his career. He published his first scientific paper at the age of 14, and took his first faculty position at the University of Aberdeen at 24. He was named a Fellow of The Royal Society (F. R. S.) at age 30. At the age of 42 he published his "Treatise on Electricity and Magnetism", in which he unified the theories of electricity and magnetism, and in which he predicted the existence of electromagnetic waves. Unfortunately, Maxwell died before he could see his prediction verified (he was only 48).
The first demonstration of electromangetic waves was carried out by another brilliant but short lived physicist. Henrich Hertz was born in 1853, received his Ph.D. at the age of 23. In 1888 he was a professor at the Karlsruhe Polytechnic in Berlin, Germany. While teaching
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| A basic spark generator |
Hertz's students asked him what he thought could be done with his remarkable invention. He replied "It's of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right -- we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there." This was typical of Hertz, who was known to be modest and unpretentious. When he died at the age of 37 one eulogist described him as "...a noble man who had the singular good fortune to find many admirers, but none to hate or envy him; those who came into personal contact with him were struck by his modesty and charmed by his amiability.".
Luckily, Hertz published a description of his work in a scientific journal, which was noticed by a young man who was vacationing in the Alps. Guglielmo Marconi was born in 1874 to an Italian father and an Irish mother. His father, Giuseppe Marconi was the son of a wealthy landowner; his mother, Annie Jameson, was the daughter of Andrew Jameson of the Jameson Irish Whiskey Company. They eloped when Annie was Seventeen years old.
After the an account of Hertz's work in 1894 Marconi immediately realized the possibilities of using the electromagnetic wave in a "wireless telegraph." Working with his older brother Alphonso as an assistant he began building experimental transmitters and receivers, and testing them over increasingly large distances on his family's estate near Bologna, Italy. Over short distances, they would acknowledge successful tranmissions by waving a flag. As the experiments progressed to longer distances, they substituted a shotgun blast. After only one year they were able to send signals over distances of two miles.
Marconi tried to interest the Italian government in his work, but had no success, so he moved to England where he was aided by his cousin Henry Jameson Davis. He was awarded the first patent for his invention by the British government in 1896. In 1897 Marconi demonstrated the first ship-to-shore communication, and in 1898 the first transmission across the English Channel. It is interesting to note that only the next year, in 1899 a Marconi system was used in the first ever broadcast of a sporting event: the America's cup yacht race.
The next few years Marconi worked feverishly on his new invention. In 1900 he was granted the now famous Patent 7777, for a system allowing different messages to be broadcast simultaneously on different radio frequencies. That patent was fought for many years, and was overturned in 1943, with the rights going among others, to Nikola Tesla. Marconi's most memorable achievement came in 1901 with the first successful transatlantic communication. In 1909, Marconi shared the Nobel Prize in Physics with Carl Braun "In recognition of their contributions to the development of wireless telegraphy.".
There are many more chapters in the story of telecommunications. From the first commercial radio broadcasts to the development of television, transatlantic cables, optical fibers, and satellite communications. The World Wide Web is among the most recent developments. Those stories will have to wait for another day.
For more information, you can try the following links
Telegraphy 
Radio
Morse
Maxwell
Heinrich Hertz
Marconi
Tesla
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