Here is a podcast about the inventor " Alexander Watson "
Note: choose track number 42 .
Tuesday, 8 January 2013
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Hertz
Heinrich Rudolf Hertz
(22 February 1857 –
1 January 1894) was a German physicist who clarified and expanded James Clerk
Maxwell's electromagnetic theory of light, which was first
demonstrated by David Edward
Hughes using non-rigorous trial and
error procedures. Hertz is distinguished from Maxwell and
Hughes because he was the first to conclusively prove the existence of electromagnetic waves by engineering instruments to transmit and receive
radio pulses using experimental
procedures that ruled
out all other known wireless phenomena The
scientific unit of frequency — cycles per second — was named the "hertz" in his honor.
Early years
Hertz was born in Hamburg, then a sovereign state of the German Confederation, into a prosperous and cultured Hanseatic family. His father, David Gustav Hertz, was a writer and
later a senator. His mother was the former Anna Elisabeth Pfefferkorn. His
paternal great-grandfather, David Wolff Hertz (1757–1822), fourth son of
Benjamin Wolff Hertz, moved to Hamburg in 1793, where he made his living as a
jeweller; he and his wife Schöne Hertz (1760–1834) were buried in the former
Jewish cemetery in Ottensen. Their first son, Wolff Hertz (1790–1859), was
chairman of the Jewish community. Hertz' paternal grandfather, Heinrich David Hertz
(1797–1862), was a respected businessman, and his paternal grandmother, Betty
Oppenheim, was the daughter of the banker Salomon Oppenheim, from Cologne. Hertz's father and paternal grandparents converted from
Judaism to Christianity. His mother's family was Lutheran.
While studying at the Gelehrtenschule des Johanneums in Hamburg, he showed an aptitude for sciences as well as
languages, learning Arabic and Sanskrit. He studied sciences and engineering in the German cities of
Dresden, Munich and Berlin, where he studied under Gustav R. Kirchhoff and Hermann von Helmholtz.
In 1880, Hertz obtained his PhD from the University of Berlin; and remained for post-doctoral study under Hermann von Helmholtz.
In 1885, Hertz became a full professor at the University of Karlsruhe where he discovered electromagnetic waves.
The most dramatic prediction of Maxwell's theory of electromagnetism, published in 1865, was the
existence of electromagnetic waves moving at the speed of light, and the conclusion that light itself was just such a wave.
This challenged experimentalists to generate and detect electromagnetic
radiation using some form of electrical apparatus.
The first successful radio transmission was made by David Edward Hughes in 1879, but it would not be conclusively proven to have
been electromagnetic waves until the experiments of Heinrich Hertz in 1886. For
the Hertz radio wave transmitter, he used a high voltage induction coil, a condenser (capacitor, Leyden jar) and a spark gap—whose poles on either side are formed by spheres of 2 cm
radius—to cause a spark discharge between the spark gap’s poles oscillating at a
frequency determined by the values of the capacitor and the induction coil.
To prove there really was radiation emitted, it had to be detected. Hertz used a piece of copper
wire, 1 mm thick, bent into a circle of a diameter of 7.5 cm, with a small brass
sphere on one end, and the other end of the wire was pointed, with the point
near the sphere. He bought a screw mechanism so that the point could be moved
very close to the sphere in a controlled fashion. This "receiver" was designed
so that current oscillating back and forth in the wire would have a natural
period close to that of the "transmitter" described above. The presence of
oscillating charge in the receiver would be signaled by sparks across
the (tiny) gap between the point and the sphere (typically, this gap was
hundredths of a millimeter).
In more advanced experiments, Hertz measured the velocity of
electromagnetic radiation and found it to be the same as the light’s velocity.
He also showed that the nature of radio waves’ reflection and refraction was the
same as those of light and established beyond any doubt that light is a form of
electromagnetic radiation obeying the Maxwell equations.
Hertz's experiments triggered broad interest in radio
research that eventually produced commercially successful wireless telegraph, audio radio, and later television. In 1930 the International Electrotechnical Commission (IEC) honored Hertz by naming the unit of frequency—one
cycle per second—the "hertz"
Meteorology
He always had a deep interest in meteorology probably derived from his contacts with Wilhelm von Bezold (who was Hertz's professor in a laboratory course at the
Munich Polytechnic in the summer of 1878). Hertz, however, did not contribute
much to the field himself except some early articles as an assistant to
Helmholtz in Berlin, including research on the evaporation of liquids, a new kind of hygrometer, and a graphical means of determining the properties of
moist air when subjected to adiabatic changes.
Contact mechanics
Memorial of Heinrich Hertz on the campus of the Karlsruhe Institute of Technology
Main article: Contact mechanics
In 1886–1889, Hertz published two articles on what was to
become known as the field of contact mechanics. Hertz is well known for his contributions to the field of
electrodynamics (see below); however, most papers that look into the
fundamental nature of contact cite his two papers as a source for some important
ideas. Joseph Valentin Boussinesq published some critically important observations on Hertz's
work, nevertheless establishing this work on contact mechanics to be of immense
importance. His work basically summarises how two axi-symmetric objects placed in contact will behave under loading, he obtained results based upon the classical theory of
elasticity and continuum mechanics. The most significant failure of his theory was the neglect
of any nature of adhesion between the two solids, which proves to be important as the
materials composing the solids start to assume high elasticity. It was natural
to neglect adhesion in that age as there were no experimental methods of testing
for it.
To develop his theory Hertz used his observation of
elliptical Newton's rings formed upon placing a glass sphere upon a lens as the basis
of assuming that the pressure exerted by the sphere follows an elliptical distribution. He used the formation of Newton's rings again
while validating his theory with experiments in calculating the displacement which the sphere has into the lens. K. L. Johnson, K.
Kendall and A. D. Roberts (JKR) used this theory as a basis while calculating
the theoretical displacement or indentation depth in the presence of adhesion in their
landmark article "Surface energy and contact of elastic solids" published in
1971 in the Proceedings of the Royal Society (A324, 1558, 301–313). Hertz's
theory is recovered from their formulation if the adhesion of the materials is
assumed to be zero. Similar to this theory, however using different assumptions,
B. V. Derjaguin, V. M. Muller and Y. P. Toporov published another theory in
1975, which came to be known as the DMT theory in the research community, which
also recovered Hertz's formulations under the assumption of zero adhesion. This
DMT theory proved to be rather premature and needed several revisions before it
came to be accepted as another material contact theory in addition to the JKR
theory. Both the DMT and the JKR theories form the basis of contact mechanics
upon which all transition contact models are based and used in material
parameter prediction in nanoindentation and atomic force microscopy. So Hertz's research from his days as a lecturer, preceding
his great work on electromagnetism, which he himself considered with his
characteristic soberness to be trivial, has come down to the age of nanotechnology.
In 1886, Hertz developed the Hertz antenna receiver. This is a set of terminals
which is not electrically grounded for its operation. He also developed a transmitting type of dipole antenna, which was a center-fed driven element for transmitting
UHF radio waves. These antennas are the simplest practical
antennas from a theoretical point of view.
In 1887, Hertz experimented with radio waves in his
laboratory. These actions followed Michelson's 1881 experiment (precursor to the 1887 Michelson–Morley experiment), which did not detect the existence of aether drift. Hertz altered Maxwell's equations to take this view into account for electromagnetism. Hertz
used a Ruhmkorff coil-driven spark gap and one meter wire pair as a radiator.
Capacity spheres were present at the ends for circuit resonance adjustments. His
receiver, a precursor to the dipole antenna, was a simple half-wave dipole antenna for shortwaves. Hertz published his work in a book titled: Electric waves: being researches on the propagation of
electric action with finite velocity through space.
Through experimentation, he proved that transverse free space electromagnetic waves can travel over some distance. This had been predicted by
James Clerk Maxwell and Michael Faraday. With his apparatus configuration, the electric and magnetic
fields would radiate away from the wires as transverse waves. Hertz had positioned the oscillator about 12 meters from a zinc reflecting plate to produce standing waves. Each wave was about 4 meters. Using the ring detector, he
recorded how the magnitude and wave's component direction vary. Hertz measured
Maxwell's waves and demonstrated that the velocity of radio waves was equal to the velocity of light. The
electric field intensity and polarity was also measured by Hertz. (Hertz, 1887, 1888).
The Hertzian cone was first described by Hertz as a type of wave-front
propagation through various media. His experiments expanded the field of electromagnetic
transmission and his apparatus was developed further by others in the radio.
Hertz also found that radio waves could be transmitted through different types of materials,
and were reflected by others, leading in the distant future to radar.
Hertz helped establish the photoelectric effect (which was later explained by Albert Einstein) when he noticed that a charged object loses its charge more readily when illuminated by
ultraviolet light. In 1887, he made observations of the photoelectric effect and
of the production and reception of electromagnetic (EM) waves, published in the
journal Annalen der Physik. His receiver consisted of a coil with a spark gap, whereby a spark would be seen upon detection of EM waves.
He placed the apparatus in a darkened box to see the spark better. He observed
that the maximum spark length was reduced when in the box. A glass panel placed
between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length
would increase. He observed no decrease in spark length when he substituted
quartz for glass, as quartz does not absorb UV radiation. Hertz concluded his months of
investigation and reported the results obtained. He did not further pursue
investigation of this effect, nor did he make any attempt at explaining how the
observed phenomenon was brought about.
His discoveries would later be more fully understood by
others and be part of the new "wireless age". In bulk, Hertz' experiments explain reflection, refraction, polarization, interference, and velocity of electric waves.
In 1892, Hertz began experimenting and demonstrated that
cathode rays could penetrate very thin metal foil (such as aluminium). Philipp Lenard, a student of Heinrich Hertz, further researched this
"ray effect". He developed a version of the cathode tube and studied the
penetration by X-rays of various materials. Philipp Lenard, though, did not
realize that he was producing X-rays. Hermann von Helmholtz formulated
mathematical equations for X-rays. He postulated a dispersion theory before
Röntgen made his discovery and announcement. It was formed on the
basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not
work with actual X-rays.
Death at age 36
In 1892, an infection was diagnosed and Hertz underwent some
operations to correct the illness. He died of Wegener's granulomatosis at the age of 36 in Bonn, Germany in 1894, and was buried in Ohlsdorf, Hamburg at the
Jewish cemetery.
Hertz's wife, Elizabeth Hertz (maiden name: Elizabeth Doll),
did not remarry. Heinrich Hertz left two daughters, Joanna and Mathilde.
Subsequently, all three women left Germany in the 1930s and went to England,
after the rise of Adolf Hitler. Charles Susskind interviewed Mathilde Hertz in the 1960s
and he later published a book on Heinrich Hertz. Heinrich Hertz's daughters
never married and he does not have any descendants, according to the book by
Susskind.
History of radar
History of radar
The history of radar starts with
experiments by Heinrich
Hertz in the late 19th century that showed that
radio waves were reflected by metallic objects. This possibility was suggested
in James Clerk
Maxwell's seminal work on electromagnetism. However, it was
not until the early 20th century that systems were able to use these principles
were becoming widely available, and it was German inventor Christian
Hülsmeyer who first used them to build a simple ship
detection device intended to help avoid collisions in fog . Numerous similar
systems were developed over the next two decades.
The term RADAR was coined in 1940 by the
United
States Navy as an acronym for radio detection and ranging; this was a cover for
the highly secret technology. Thus, a true radar system must both detect and
provide range (distance) information for a target. Before 1934, no single system
gave this performance; some systems were omni-directional and provided ranging
information, while others provided rough directional information but not range.
A key development was the use of pulses that were timed to provide ranging,
which were sent from large antennas that provided accurate directional
information. Combining the two allowed for accurate plotting of
targets.
In the 1934–1939 period, eight nations
developed, independently and in great secrecy, systems of this type: the
United
States, Great
Britain, Germany, the USSR, Japan, the Netherlands, France, and Italy. In addition, Great Britain had shared
their basic information with four Commonwealth countries: Australia, Canada, New
Zealand, and South
Africa, and these countries had also developed
indigenous radar systems. During the war, Hungary was added to this list.
Progress during the war was rapid and of
great importance, probably one of the decisive factors for the victory of the
Allies. By the end of hostilities, the United
States, Great Britain, Germany, the USSR, and Japan had a wide diversity of
land- and sea-based radars as well as small airborne systems. After the war,
radar use was widened to numerous fields including: civil
aviation, marine navigation, radar
guns for police, meteorology and even medicine.
United States Army
As the Great Depression started, economic conditions led the U.S. Army Signal Corps to consolidate its widespread laboratory operations to
Fort Monmouth, New Jersey. On June 30, 1930, these were designated the Signal Corps Laboratories (SCL) and Lt. Colonel (Dr.) William R. Blair was appointed the SCL Director.
Among other activities, the SCL was made responsible for
research in the detection of aircraft by acoustical and infrared radiation means. Blair had performed his doctoral research
in the interaction of electromagnet waves with solid materials, and naturally
gave attention to this type of detection. Initially, attempts were made to
detect infrared radiation, either from the heat of aircraft engines or as
reflected from large searchlights with infrared filters, as well as from radio
signals generated by the engine ignition.
Some success was made in the infrared detection, but little
was accomplished using radio. In 1932, progress at the Naval Research Laboratory (NRL) on radio interference for aircraft detection was
passed on to the Army. While it does not appear that any of this information was
used by Blair, the SCL did undertake a systematic survey of what was then known
throughout the world about the methods of generating, modulating, and detecting
radio signals in the microwave region.
The SCL's first definitive efforts in radio-based target
detection started in 1934 when the Chief of the Army Signal Corps, after seeing
a microwave demonstration by RCA, suggested that radio-echo techniques be investigated. The
SCL called this technique radio position-finding (RPF). Based on the previous
investigations, the SCL first tried microwaves. During 1934 and 1935, tests of
microwave RPF equipment resulted in Doppler-shifted signals being obtained,
initially at only a few hundred feet distance and later greater than a mile.
These tests involved a bi-static arrangement, with the transmitter at one end of
the signal path and the receiver at the other, and the reflecting target passing
through or near the path.
In 1936, W. Delmar Hershberger, SCL’s Chief Engineer at that
time, started a modest project in pulsed microwave transmission. Lacking success
with microwaves, Hershberger visited the NRL (where he had earlier worked) and
saw a demonstration of their pulsed set. Back at the SCL, he and Robert H. Noyes
built an experimental apparatus using a 75 watt, 110 MHz (2.73 m) transmitter
with pulse modulation and a receiver patterned on the one at the NRL. A request
for project funding was turned down by the War Department, but $75,000 for support was diverted from a previous
appropriation for a communication project.
In October 1936, Paul E. Watson became the SCL Chief Engineer and led the project. A field
setup near the coast was made with the transmitter and receiver separated by a
mile. On December 14, 1936, the experimental set detected at up to 7 mi (11 km)
range aircraft flying in and out of New York City.
Work then began on a prototype system. Ralph I. Cole headed
receiver work and William S. Marks lead transmitter improvements. Separate
receivers and antennas were used for azimuth and elevation detection. Both receiving and the transmitting antennas used
large arrays of dipole wires on wooden frames. The system output was intended to
aim a searchlight. The first demonstration of the full set was made on the
night of May 26, 1937. A bomber was detected and then illuminated by the
searchlight. The observers included the Secretary of War, Henry A. Woodring; he was so impressed that the next day
orders were given for the full development of the system. Congress gave an
appropriation of $250,000.
The frequency was increased to 200 MHz (1.5 m). The
transmitter used 16 tubes in a ring oscillator circuit (developed at the NRL), producing about 75 kW peak
power. Major James C. Moore was assigned to head the complex electrical and
mechanical design of lobe switching antennas. Engineers from Western Electric and Westinghouse were brought in to assist in the overall development.
Designated SCR-268, a prototype was successfully demonstrated in late 1938 at
Fort Monroe, Virginia. Production of SCR-268 sets was started by Western
Electric in 1939, and it entered service in early 1941.
Even before the SCR-268 entered service, it had been greatly
improved. In a project led by Major (Dr.) Harold A. Zahl, two new configurations evolved – the SCR-270 (mobile) and the SCR-271 (fixed-site). Operation at 106 MHz
(2.83 m) was selected, and a single water-cooled tube provided 8 kW (100 kW
pulsed) output power. Westinghouse received a production contract, and started
deliveries near the end of 1940.
The Army deployed five of the first SCR-270 sets around the
island of Oahu in Hawaii. At 7:02 on the morning of December 7, 1941, one of these
radars detected a flight of aircraft at a range of 136 miles (219 km) due north.
The observation was passed on to an aircraft warning center where it was
misidentified as a flight of U.S. bombers known to be approaching from the
mainland. The alarm went unheeded, and at 7:48, the Japanese aircraft first
struck at Pearl Harbor.
World War II radar
At the start of World War
II in September 1939, both the United
Kingdom and Germany knew of each other's ongoing efforts in
radio
navigation and its countermeasures – the "Battle of
the beams". Also, both nations were generally aware
of, and intensely interested in, the other's developments in radio-based
detection and tracking, and engaged in an active campaign of espionage and false leaks about their respective
equipment. By the time of the Battle of
Britain, both sides were deploying range and
direction-finding units (radars) and control stations as part of integrated air
defense capability. However, the German Funkmessgerät (radio measuring
device) systems could not assist in an offensive role and was thus not supported
by Adolf
Hitler. Also, the Luftwaffe did not sufficiently appreciate the
importance of British Range and
Direction Finding (RDF) stations as part of RAF's air defense capability, contributing to
their failure.
While the United Kingdom and Germany led in
pre-war advances in the use of radio for detection and tracking of aircraft,
there were also developments in the United States, the Soviet Union, and Japan.
Wartime systems in all of these nations will be summarized. The acronym RADAR
(for RAdio Detection And Ranging) was coined by the U.S. Navy in 1940, and the
subsequent name "radar" was soon widely used.
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