"The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is the same, only without the cat. "
-- Albert Einstein
Samuel Finley Breese Morse (1791-1872) was an American inventor and painter.
After a successful career painting in oils (first painting historical scenes
and then portraits), Morse built the first American telegraph around 1835 (the
telegraph was also being developed independently in Europe).
A telegraph sends electrical signals over a long distance, through wires. In
1830, Joseph Henry (1797-1878) made the first long-distance telegraphic device
- he sent an electronic current for over a mile on wire that activated an electromagnet,
causing a bell to ring.
Morse patented a working telegraph machine in 1837, with help from his business
partners Leonard Gale and Alfred Vail. Morse used a dots-and-spaces code for
the letters of the alphabet and the numbers (Morse Code was later improved to
use dots, dashes and spaces: for example E is dot, T is dash, A is dot-dash,
N is dash-dot, O is dash-dash, I is dot-dot, S is dot-dot-dot, etc.). By 1838,
Morse could send 10 words per minute. Congress provided funds for building a
telegraph line between Washington D.C. and Baltimore, Maryland, in 1843 Morse
sent the first telegraphic message (from Washington D.C. to Baltimore) on May
24, 1844; the message was: "What hath God wrought?" (What has God
worked?)
Heinrich Hertz's Wireless Experiment (1887)
In the 1880s many were seeking experimental evidence to establish the equivalence
of light and electromagnetic propagation. James Clerk Maxwell's mathematical
theory of 1873 had predicted that electromagnetic disturbances should propagate
through space at the speed of light and should exhibit the wave-like characteristics
of light propagation.
In 1883 Hertz became a lecturer in theoretical physics at the University of
Kiel and two years later he was appointed professor of physics at Karlsruhe
Polytechnic. In 1887 Hertz designed a brilliant set of experiments tested Maxwell's
hypothesis. He used an oscillator made of polished brass knobs, each connected
to an induction coil and separated by a tiny gap over which sparks could leap.
Hertz reasoned that, if Maxwell's predictions were correct, electromagnetic
waves would be transmitted during each series of sparks. To confirm this, Hertz
made a simple receiver of looped wire. At the ends of the loop were small knobs
separated by a tiny gap. The receiver was placed several yards from the oscillator.
According to theory, if electromagnetic waves were spreading from the oscillator sparks, they would induce a current in the loop that would send sparks across the gap. This occurred when Hertz turned on the oscillator, producing the first transmission and reception of electromagnetic waves. Hertz also noted that electrical conductors reflect the waves and that they can be focused by concave reflectors. He found that nonconductors allow most of the waves to pass through. Another of his discoveries was the photoelectric effect.
Marconi's pictures and sounds in flash
The coherer consists of a glass tube having very fine metallic filings enclosed between two silver plugs forming a V gap, and is attached to an ivory stem with a square end. The coherer is situated about one-sixteenth of an inch from the de-coherence tapper hammer.
On receiving a wireless signal, the metal filings in the coherer become conductive, permitting a weak current to pass through the coherer and the telegraph relay. This brings into action the Morse recorder, which registers a dot or dash as the case may be. The tapper then taps the coherer, causing the filings to de-cohere within the coherer and thus it becomes non-conductive again. The tapper then ceases to function and the coherer is ready for the next wireless signal.
Professor A. S. Popov's Receiver (July 1892)
ETHERNET
The diagram ... was drawn by Dr. Robert M. Metcalfe in 1976 to present Ethernet ... to the National Computer Conference in June of that year.
In 1980, the original IEEE 802.3 Ethernet spec defined a mechanism called carrier-sense multiple access with collision detection (CSMA/CD). This scheme ensures that all stations are granted access on a first-come, first-served basis. Since Ethernet was intended only to carry data, no provisions were made for quality of service or prioritization. CSMA/CD simply ensures that the same access rules apply equally to all network nodes. Ethernet designers originally set workstations as being 2 kilometers apart, and this would help to detect when a collision might occur. This distance limitation results from the relationship between the times it takes to transmit a minimum-sized Ethernet frame (64 bytes) and the ability to detect a collision (a limit known as the roundtrip propagation delay). When a collision occurs the MAC layer sends a jam signal telling the transmitting stations to stop and try the transmission again.