POLI 100K, Railroads and American Politics: Topic 5,
The Evolution of Railroad Technology
The Evolution of Radio
- The Electromagnetic Spectrum
The Electromagnetic Spectrum: Wavelength vs.
Frequency in Hertz (Cycles)
The Wavelengths of Visible Light
- Violet: 0.4 - 0.446 mm
- Blue: 0.446 - 0.500 mm
- Green: 0.500 - 0.578 mm
- Yellow: 0.578 - 0.592 mm
- Orange: 0.592 - 0.620 mm
- Red: 0.620 - 0.7 mm
- Many discoveries in the field of electricity were necessary
before the concept of radio was possible. One of the most important of
these was the work of Michael Faraday. In 1821 Faraday plotted the magnetic
field around a conductor carrying an electric current. In 1831 he followed
this accomplishment with the discovery of
Electromagnetic Induction. Faraday
demonstrated the induction of one electric current by another.
Michael Faraday (1791 - 1867)
- The history of radio really begins with the publication in 1873 of
Treatise of Electricity and Magnetism
by James Clerk Maxwell. Maxwell built upon the work of Faraday but his
insights were extraordinary. Around 1865 Maxwell developed his
electromagnetic theory of light. Maxwell saw light as consisting of
transverse waves of electric and magnetic force and had come to this
conclusion by explaining electromagnetic induction mathematically. He
calculated that the velocity of the induced electric waves was the same as
the speed of light. He then realized
that there was no set lmit to the wave length (frequency) of these
waves and he predicted the existence of other electromagnetic waves.
His theory also suggested the ability to create electromagnetic waves
artificially.
All these insights were combined in Maxwell's famous four equations --
Gauss' Law for Electrostatics; Gauss' Law for Magnetostatics; Faraday's
Law; and Ampere's Law. The Maxwell equations allowed one to calculate
and predict the relationship between electricity and magnetism. It was
not until the development of quantum mechanics and the theory of relativity
that the reasons for the relationships developed by Maxwell were
fully understood.
James Clerk Maxwell (1831 - 1879)
Maxwell's Equations
- Heinrich Hertz was the first person to demonstrate experimentally
the production and detection of Maxwell's waves. In 1887, using the spark
of an induction coil, Hertz succeeded in producing and detecting
electromagnetic waves. He showed experimentally that these waves possessed
many of the properties of light, i.e., measurable velocity and wave length,
reflection, refraction, and polarization. Hertz' demonstration of the
existence of electromagnetic waves was originally of purely theoretical
interest as confirmation of Maxwell's theory of the electromagnetic
nature of electricity and light, but his discoveries led directly to the
development of radio.
Heinrich Hertz (1857 - 1894)
- Guglielmo Marconi is generally credited with the invention of
radio -- wireless transmission of a message. Around 1895 he developed
an improved coherer -- a glass tube
loosely filled with zinc and silver filings. The device was originally
invented by Joseph Lodge and was used to detect radio waves. The coherer
would become conductive in the presence of a strong radio wave. The
coherer was the earliest true radio receiver. Marconi hooked the
coherer to a crude antenna with its lower end grounded. He also
improved the spark oscillator -- the earliest true radio
transmitter -- and hooked it to an antenna. He used a telegraph key
to turn the spark oscillator on and off thereby sending out a message
to the antenna hooked to the coherer. The coherer actuated a telegraphic
instrument through a relay.
In 1896 Marconi succeeded in sending a message over a distance of 1 mile
and by 1897 was able to send messages to a ship at sea 18 miles distant.
In 1901 he sent messages over a distance of 200 miles and by 1902 across
the Atlantic ocean.
Guglielmo Marconi (1874 - 1937)
The Basic Spark Transmitter
The Basic Receiver
- The early spark equipment could only be used to send and receive
morse code.
For voice to be transmitted the vacuum tube was necessary
because voice required amplification and much more sophisticated
detection. The original vacuum tube was developed by Thomas Edison. He
discovered that a current will flow between the hot filament of an incandescent
lamp and another electrode placed in the lamp and that this current will
flow in only one direction. In 1904 John Fleming developed the
diode, or two-element tube. This tube
was used as a detector, rectifier, and limiter.
A key advance in the history of radio and the beginnings of the science
of electronics ("computers" are an
application of electronics), was the creation of
the triode tube by Lee De Forest in
1906. Although some claim that Reginald Fessenden was the real inventor
and De Forest stole the design, nevertheless De Forest patented the design
in 1906. The breakthrough was the addition of a grid between the
filament and the plate. This made the triode an
amplifier and made voice transmission
and reception possible.
Lee De Forest (1873 - 1961)
1904 Radio Installation (Location Unknown)
1905 Radio Installation in Puerto Rico
- Edwin Howard Armstrong is the man who took De Forest's triode and
turned it into a practical amplifier and
oscillator with his invention of the
regenerative circuit in 1913. One
year later in 1914 Armstrong invented the
superheterodyne circuit that made
modern tube radios practical devices. A superheterodyne receiver is based
upon the fact that it is possible to mix two oscillating currents of
different frequencies to produce a "beat" current whose frequency is
equal to the difference between the two. This was a fundamental discovery
and broadcast radio was a reality by the early 1920s. Armstrong would later
go on to invent FM radio transmission in 1933.
Edwin Howard Armstrong (1890 - 1954)
