the dawn of amateur radio in the u[1].k.(收音机雏形的诞生)-第2章
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wire behaving like a magnet when a current is passed through it。
A development of Oersted's demonstration with the compass needle
was used to construct the world's first system of signaling by the use of
electricity。
In 1837 Charles Wheatstone and William Cooke took out a patent for
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the world's first Five…needle Telegraph； which was installed between
Paddington railway station in west London and West Drayton station a few
miles away。 The five copper wires required for this system were embedded
in blocks of wood。
Electrolysis； the chemical decomposition of a substance into its
constituent elements by the action of an electric current； was discovered
by the English chemists Carlisle and William Nicholson （1753…1815）。 If
an electric current is passed through water it is broken down into the two
elements of which it is composed hydrogen and oxygen。 The process is
used extensively in modern industry for electroplating。 Michael Faraday
（1791…1867） who was employed as a chemist at the Royal Institution； was
responsible for introducing many of the technical terms connected with
electrolysis； like electrolyte for the liquid through which the electric
current is passed； and anode and cathode for the positive and negative
electrodes respectively。 He also established the laws of the process itself。
But most people remember his name in connection with his practical
demonstration of electromagnetic induction。
In France Andre…Marie Ampere （1775…1836） carried out a complete
mathematical study of the laws which govern the interaction between
wires carrying electric currents。
In Germany in 1826 a Bavarian schoolmaster Georg Ohm （1789…
1854） had defined the relationship between electric pressure （voltage）；
current （flow rate） and resistance in a circuit （Ohm's law） but 16 years had
to elapse before he received recognition for his work。
Scientists were now convinced that since the flow of an electric
current in a wire or a coil of wire caused it to acquire magnetic properties；
the opposite might also prove to be true： a magnet could possibly be used
to generate a flow of electricity。
Michael Faraday had worked on this problem for ten years when
finally； in 1830； he gave his famous lecture in which he demonstrated； for
the first time in history； the principle of electromagnetic induction。 He had
constructed powerful electromagnets consisting of coils of wire。 When he
caused the magnetic lines of force surrounding one coil to rise and fall by
interrupting or varying the flow of current； a similar current was induced
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in a neighbouring coil closely coupled to the first。
The colossal importance of Faraday's discovery was that it paved the
way for the generation of electricity by mechanical means。 However； as
can be seen from the drawing； the basic generator produces an alternating
flow of current。（A。C。）
Rotating a coil of wire steadily through a complete revolution in the
steady magnetic field between the north and south poles of a magnet
results in an electromotive force （E。M。F。） at its terminals which rises in
value； falls back to zero； reverses in a negative direction； reaches a peak
and again returns to zero。 This completes one cycle or sine wave。 （1Hz in
S。I。units）。
In recent years other methods have been developed for generating
electrical power in relatively small quantities for special applications。
Semiconductors； which combine heat insulation with good electrical
conduction； are used for thermoelectric generators to power isolated
weather stations； artificial satellites； undersea cables and marker buoys。
Specially developed diode valves are used as thermionic generators with
an efficiency； at present； of only 20％ but the heat taken away from the
anode is used to raise steam for conventional power generation。
Sir Humphry Davy （1778…1829） one of Britain's leading chemists of
the 18th century； is best remembered for his safety lamp for miners which
cut down the risk of methane gas explosions in mines。 It was Davy who
first demonstrated that electricity could be used to produce light。 He
connected two carbon rods to a heavy duty storage battery。 When he
touched the tips of the rods together a very bright white light was
produced。 As he drew the rods apart； the arc light persisted until the tips
had burnt away to the critical gap which extinguished the light。 As a
researcher and lecturer at the Royal Institution Davy worked closely with
Michael Faraday who first joined the institution as his manservant and
later became his secretary。 Davy's crowning honour in the scientific world
came in 1820； when he was elected President of the Royal Society。
In the U。S。A。 the prolific inventor Thomas Alva Edison （1847…1831）
who had invented the incandescent carbon filament bulb； built a number of
electricity generators in the vicinity of the Niagara Falls。 These used the
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power of the falling water to drive hydraulic turbines which were coupled
to the dynamos。 These generators were fitted with a spinning switch or
commutator （one of the neatest gadgets Edison ever invented） to make the
current flow in unidirectional pulses （D。C。） In 1876 all electrical
equipment was powered by direct current。
Today mains electricity plays a vital part in our everyday lives and its
applications are widespread and staggering in their immensity。 But we
must not forget that popular demand for this convenient form of power
arose only about 100 years ago； mainly for illumination。
Recent experiments in superconductivity； using ceramic instead metal
conductors have given us an exciting glimpse into what might be achieved
for improving efficiency in the distribution of electric power。
Historians of the future may well characterise the 20th century as ‘the
century of electricity & electronics'。 But Edison's D。C。 generators could
not in themselves； have achieved the spectacular progress that has been
made。 All over the world we depend totally on a system of transmitting
mains electricity over long distances which was originally created by an
amazing inventor whose scientific discoveries changed； and are still
changing； the whole world。 His name was scarcely known to the general
public； especially in Europe； where he was born。
Who was this unknown pioneer？ Some people reckon that it was this
astonishing visionary who invented wireless； remote control； robotics and
a form of X…ray photography using high frequency radio waves。 A patent
which he took out in the U。S。A。 in 1890 ultimately led to the design of the
humble ignition coil which energises billions and billions of sp
