For more information on Leyden jars, including photographs and instructions on how to build them, go this page at the Static Generator site:
http://www.alaska.net/~natnkell/leyden.htm
A static electricity web page is here: http://www.sciencemadesimple.com/static.html
In 1729 English chemist Stephen Gray transmitted electricity over a wire. He sent charges nearly 300 feet over brass wire and moistened thread. An electrostatic generator powered his experiments, one charge at a time. A few years later, Dutchman Pieter van Musschenbroek and German Ewald Georg von Kleist in 1746 independently developed the Leyden jar, a sort of battery or condenser for storing static electricity. Named for its Holland city of invention, the jar was a glass bottle lined inside and out with tin or lead. The glass sandwiched between the metal sheets stored electricity; a strong charge could be kept for a few days and transported. Over the years these jars were used in countless experiments, lectures, and demonstrations.
In 1753 an anonymous writer, possibly physician Charles Morrison, suggested in The Scot's Magazine that electricity might transmit messages. He thought up a scheme using separate wires to represent each letter. An electrostatic generator, he posited, could electrify each line in turn, attracting a bit of paper by static charge on the other end. By noting which paper letters were attracted one might spell out a message. Needing wires by the dozen, signals got transmitted a mile or two. People labored with telegraphs like this for many decades. Experiments continued slowly until 1800. Many inventors worked alone, misunderstood earlier discoveries, or spent time producing results already achieved. Poor equipment didn't help either.
Balky electrostatic generators produced static electricity by friction, often by spinning leather against glass. And while static electricity could make hair stand on end or throw sparks, it couldn't provide the energy to do truly useful things. Inventors and industry needed a reliable and continuous current.
In 1800 Alessandro Volta produced the first battery. A major development, Volta's battery provided sustained low powered electric current at high cost. Chemically based, as all batteries are, the battery improved quickly and became the electrical source for further experimenting. But while batteries got more reliable, they still couldn't produce the power needed to work machinery, light cities, or provide heat. And although batteries would work telegraph and telephone systems, and still do, transmitting speech required understanding two related elements, namely, electricity and magnetism.
In 1820 Danish physicist Christian Oersted discovered electromagnetism, the critical idea needed to develop electrical power and to communicate. In a famous experiment at his University of Copenhagen classroom, Oersted pushed a compass under a live electric wire. This caused its needle to turn from pointing north, as if acted on by a larger magnet. Oersted discovered that an electric current creates a magnetic field. But could a magnetic field create electricity? If so, a new source of power beckoned. And the principle of electromagnetism, if fully understood and applied, promised a new era of communication
For an excellent summary of Christian Oersted's life, visit:
http://www.longman.co.uk/tt_secsci/resources/scimon/mar_01/oersted.htm
In 1821 Michael Faraday reversed Oersted's experiment and in so doing discovered induction. He got a weak current to flow in a wire revolving around a permanent magnet. In other words, a magnetic field caused or induced an electric current to flow in a nearby wire. In so doing, Faraday had built the world's first electric generator. Mechanical energy could now be converted to electrical energy. Is that clear? This is a very important point.
The simple act of moving ones' hand caused current to move. Mechanical energy into electrical energy. Although many years away, a turbine powered dynamo would let the power of flowing water or burning coal produce electricity. Got a river or a dam? The water spins the turbines which turns the generators which produce electricity. The more water you have the more generators you can add and the more electricity you can produce. Mechanical energy into electrical energy.
(By comparison, a motor turns electrical energy into mechanical energy. Thanks to A. Almoian for pointing out this key difference and to Neal Kling for another correction.)
Click here for a clear, large diagram on turning mechanical energy into electrical energy.
And it's a good science fair idea!
I also have a page on easy to do electrical experiments for kids
Again, good science fair ideas.
Faraday worked through different electrical problems in the next ten years, eventually publishing his results on induction in 1831. By that year many people were producing electrical dynamos. But electromagnetism still needed understanding. Someone had to show how to use it for communicating.
For more information on Michael Faraday, visit the ENC at: http://www.enc.org/features/calendar/unit/0,1819,196,00.shtm (external link)
In 1830 the great American scientist Professor Joseph Henry transmitted the first practical electrical signal. A short time before Henry had invented the first efficient electromagnet. He also concluded similar thoughts about induction before Faraday but he didn't publish them first. Henry's place in electrical history however, has always been secure, in particular for showing that electromagnetism could do more than create current or pick up heavy weights -- it could communicate.
In a stunning demonstration in his Albany Academy classroom, Henry created the forerunner of the telegraph. In the demonstration, Henry first built an electromagnet by winding an iron bar with several feet of wire. A pivot mounted steel bar sat next to the magnet. A bell, in turn, stood next to the bar. From the electromagnet Henry strung a mile of wire around the inside of the classroom. He completed the circuit by connecting the ends of the wires at a battery. Guess what happened? The steel bar swung toward the magnet, of course, striking the bell at the same time. Breaking the connection released the bar and it was free to strike again. And while Henry did not pursue electrical signaling, he did help someone who did. And that man was Samuel Finley Breese Morse.
For more information on Joseph Henry, visit the Joseph Henry Papers Project at:
http://www.si.edu/archives/ihd/jhp/papers00.htm (external link)
From the December, 1963 American Heritage magazine, "a sketch of Henry's primitive telegraph, a dozen years before Morse, reveals the essential components: an electromagnet activated by a distant battery, and a pivoted iron bar that moves to ring a bell." See the two books listed to the left for more information.
In 1837 Samuel Morse invented the first workable telegraph, applied for its patent in 1838, and was finally granted it in 1848. Joseph Henry helped Morse build a telegraph relay or repeater that allowed long distance operation. The telegraph later helped unite the country and eventually the world. Not a professional inventor, Morse was nevertheless captivated by electrical experiments. In 1832 he heard of Faraday's recently published work on inductance, and was given an electromagnet at the same time to ponder over. An idea came to him and Morse quickly worked out details for his telegraph.
As depicted below, his system used a key (a switch) to make or break the electrical circuit, a battery to produce power, a single line joining one telegraph station to another and an electromagnetic receiver or sounder that upon being turned on and off, produced a clicking noise. He completed the package by devising the Morse code system of dots and dashes. A quick key tap broke the circuit momentarily, transmitting a short pulse to a distant sounder, interpreted by an operator as a dot. A more lengthy break produced a dash.
Telegraphy became big business as it replaced messengers, the Pony Express, clipper ships and every other slow paced means of communicating. The fact that service was limited to Western Union offices or large firms seemed hardly a problem. After all, communicating over long distances instantly was otherwise impossible. Yet as the telegraph was perfected, man's thoughts turned to speech over a wire.
In 1854 Charles Bourseul wrote about transmitting speech electrically in a well circulated article. In that important paper, the Belgian-born French inventor and engineer described a flexible disk that would make and break an electrical connection to reproduce sound. Bourseul never built an instrument or pursued his ideas further.
For more information on Bourseul and early communications in general, vist this German site:
http://www.fht-esslingen.de/telehistory/1870-.html (external link)
I have a page on easy to do electrical experiments for kids. And adults who want to understand the basics (internal link)
In 1861 Johann Phillip Reis completed the first non-working telephone. Tantalizingly close to reproducing speech, Reis's instrument conveyed certain sounds, poorly, but no more than that. A German physicist and school teacher, Reis's ingenuity was unquestioned. His transmitter and receiver used a cork, a knitting needle, a sausage skin, and a piece of platinum to transmit bits of music and certain other sounds. But intelligible speech could not be reproduced. The problem was simple, minute, and at the same time monumental. His telephone relied on its transmitter's diaphragm making and breaking contact with the electrical circuit, just as Bourseul suggested, and just as the telegraph worked. This approach, however, was completely wrong.
Reproducing speech practically relies on the transmitter making continuous contact with the electrical circuit. A transmitter varies the electrical current depending on how much acoustic pressure it gets. Turning the current off and on like a telegraph cannot begin to duplicate speech since speech, once flowing, is a fluctuating wave of continuous character; it is not a collection of off and on again pulses. The Reis instrument, in fact, worked only when sounds were so soft that the contact connecting the transmitter to the circuit remained unbroken. Speech may have traveled first over a Reis telephone however, it would have done so accidentally and against every principle he thought would make it work. And although accidental discovery is the stuff of invention, Reis did not realize his mistake, did not understand the principle behind voice transmission, did not develop his instrument further, nor did he ever claim to have invented the telephone.
The definitive book in English on Reis is:
Thompson, Silvanus P. Phillip Reis: Inventor of The Telephone. E.&F.N. Spon. London. 1883
For other views and explanations of the Reis instrument, visit Adventures in Cybersound:
http://www.acmi.net.au/AIC/REIS_BIO.html (external link)
In the early 1870s the world still did not have a working telephone. Inventors focused on telegraph improvements since these had a waiting market. A good, patentable idea might make an inventor millions. Developing a telephone, on the other hand, had no immediate market, if one at all. Elisha Gray, Alexander Graham Bell, as well as many others, were instead trying to develop a multiplexing telegraph, a device to send several messages over one wire at once. Such an instrument would greatly increase traffic without the telegraph company having to build more lines. As it turned out, for both men, the desire to invent one thing turned into a race to invent something altogether different. And that is truly the story of invention.
Alan J. Rogers' excellent introduction to electromagnetic waves, frequencies, and radio transmission. All applicable to telephony. Really well done. (19 pages, 164K in .pdf)
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Resources
[Britannica definition]"Telecommunications Systems: Telephone: THE TELEPHONE INSTRUMENT" Britannica Online. "In modern electret transmitters, developed in the 1970s, the carbon layer is replaced by a thin plastic sheet that has been given a conductive metallic coating on one side. The plastic separates that coating from another metal electrode and maintains an electric field between them. Vibrations caused by speech produce fluctuations in the electric field, which in turn produce small variations in voltage. The voltages are amplified for transmission over the telephone line."
"[Piezoelectric] crystals are used as transducers to convert mechanical or sound energy into electrical energy in such things as microphones, phonographs, and in sound and vibration detection systems."
"Piezoelectricity was first observed in 1880 when Pierre and Jacques Curie put a weight on a quartz crystal and detected a proportional electric charge on its surface. A year later the converse effect was demonstrated -- that is when a voltage is applied to a crystal, a displacement occurs which is proportional to the voltage."
"Reversing the polarity of the voltages reverses the direction of displacement. The term piezoelectricity is derived from the Greek word piezein meaning to press. Hence, a piezoelectric crystal is one capable of producing electricity when subjected to pressure."
An anonymous writer in the July, 1964 Lenkurt Demodulator
Analog and digital signals compared and contrasted
Analog transmission in telephone working. At the top of the illustration we depict direct current as a flat line. D.C. is the steady and continuous current your telephone company provides. The middle line shows what talking looks like. As in all things analog, it looks like a wave. The third line shows how talking varies that direct current. Your voice varies the telephone line's electrical resistance to represent speech. Click here for another diagram that complements this illustration.
Below is a simplified view of a digital signal. Current goes on and off. No wave thing. There was no chance the Reis telephone described above could transmit intelligible speech since it could not reproduce an analog wave. You can't do that making and breaking a circuit. A pulse in this case is not a wave! (internal link) It was not until the early 1960s that digital carrier techniques (internal link) simulated an analog wave with digital pulses. Even then this simulation was only possible by sampling the wave 8,000 times a second. (Producing CD quality sound means sampling an analog signal 44,000 times a second.) In these days all traffic in America between telephone switches is digital, but the majority of local loops are analog (internal link), still carrying your voice to the central office by varying the current.
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