May 1967 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Before there were vacuum
tube and semiconductor diodes for use as RF signal detectors, there was the
coherer. This article
by Henry Davis in the May 1967 Popular Electronics magazine discusses the invention
of the coherer by Dr. Edouard Branley. Dr. Branley's work was, as is often the
case for the ultimate inventors of a new apparatus or process, based on predecessors
who over time pieced together the puzzle of electromagnetic energy and its effects on
objects located some distance from the source. Coherers rely on relatively strong, broadband
energy like that emitted from the early spark gap transmitters in order to function;
both being extremely crude methods compared to modern techniques. The important thing
is that the system worked and provided a springboard for rapid advances in wireless communications
as the world quickly recognized the importance of the newfangled science. Wireless would
reduce (but never has completely replaced) the reliance on telegraph and telephone the
way telegraph replaced the Pony Express as a means of rapid communications. In fact, the much-vaunted
Pony Express only ran (pun intended) for a little over a year before being obsoleted
by telegraph. Oh, and very few
were ever used by riders; most were horses. BTW, you can buy a
coherer on eBay if you want to experiment or just have a
conversation piece for your desk.
The Early History of Radio Communications and
the Men and Devices That Made it Possible
By Henry B. Davis
While most history books date the birth of radio from Marconi's invention of wireless,
this is only a half-truth. Like many scientific discoveries, the radio phenomenon was
known as a "paper theory" many years before its actual existence was proven.
As early as 1845, Michael Faraday observed that the characteristics of light and electricity
were basically similar. It was this observation that spurred James Clerk Maxwell, a brilliant
British physicist, to dig deeper into the phenomenon.
In his paper "On
A Dynamical Theory Of The Electromagnetic Field," written in 1864, Maxwell noted
that a change in a magnetic field could bring about a change in an electrical field,
and vice versa. This led to his conclusion that electromagnetic energy could be. propagated
into space from a wire conductor, and that the energy traveled at the speed of light.
He failed, however, to present physical proof of his theory.
In the years that followed, a great deal of scientific thought was given to Maxwell's
theory. But it was 1887 before Heinrich Hertz, a German physicist, demonstrated a device
which proved that the Maxwell theory was correct.
The apparatus that Hertz used to prove the existence of electromagnetic
waves is shown here schematically. A crude oscillator, left, functioned as the transmitter
for the demonstration, while the receiver consisted of a loop wire bent into a ring (center).
To generate electromagnetic energy (really radio waves), Hertz used a spark transmitter,
operating around 4 meters (75 MHz). His "receiver" consisted of a length of wire with
a small metal ball at each end; the wire was bent to form a ring with a small air gap
between the metal balls. When the transmitter switch was thrown, the spark generated
electromagnetic energy and this energy was induced into the wire "receiver," causing
a spark to jump the gap between the receiver's metal balls. Thus, a spark produced by
the transmitter induced a spark in the "receiver." No physical contact between the transmitter
and the "receiver" existed.
Although the distance between the two units was limited to a few feet, it was soon
learned that this range could be increased to about 50 feet simply by limiting the size
of the "receiver" wire to the wavelength of the oscillator's frequency and carefully
adjusting the gap between the metal balls at the wire's ends.
Hertz's demonstration encouraged new interest in electrical waves - which came to
be called "Hertzian" waves. Attempts to conduct radio waves through earth and water were
carried out. Then experiments were made with large coils of wire to try and find a method
of transmitting and receiving electromagnetic radiation by induction alone. But it was
the introduction of the first sensitive radio wave detector - the "Coherer" - that made
it possible to use radio waves as a means of demonstrating intelligent communications.
Back in 1850 the French scientist, Pierre Guitard, had discovered that dust particles
in the air cohered, or collected together, when electrified. Later, in 1879, David E.
Hughes, an American electrician and the inventor of the carbon microphone, while investigating
the resistance properties of loose carbon granules, discovered that the granules cohered,
going from a high-resistance to a low-resistance state, when a current was passed through
Dr. Edouard Branley, another French physicist, perhaps borrowing from Hughes' elementary
discovery, built the first "Coherer." His Coherer consisted of a glass tube partially
filled with iron filings and plugged with corks through which wire electrodes had been
forced. In operation, the iron filings cohered when a strong radio signal was impressed
across the electrodes. Branley did not use his instrument for the reception of radio
waves, but he did find that the Coherer had to be tapped manually to decohere the filings
in order to return the unit to a high-resistance condition.
A British physicist, Sir Oliver Lodge, was the first to use the Coherer in place of
Hertz's wire loop, for the detection and pen recording of Morse code signals.
Because the Coherer had to be decohered after detecting each pulse of electromagnetic
energy, it was suitable only for a Morse code type of communications set-up. Sir Oliver,
understanding this to be the case, used a "trembler" to decohere the iron filings.
This setup was independently designed by Sir Oliver Lodge and Guglielmo
Marconi. The transmitter employed an antenna which replaced the oscillator plates used
in Hertz's device. The receiver consisted of Branley's Coherer, a Popov decohering device,
and a receiving antenna. Operation was essentially the same as in Hertz's setup, but
there was an enormous increase in sensitivity.
In 1895, when the Russian physicist, Aleksandr Stepanovitch Popov, employed the armature
of an electric bell to decohere the particles, practical transmissions of pulses at a
reasonable rate of speed became possible. The bell did away with the need for necessarily
slow manual decohering. But an even more significant achievement attributed to Popov
is the fact that he was the first person to consider using an antenna with the Coherer
circuit. The addition of the receiving antenna increased the radio range to more than
At this point, Marconi enters the picture. Sir William Crookes, in the British publication
Fortnightly Review, predicted in 1892 that wireless [radio] telegraphy would replace
all other means of rapid communications. It is likely that this prediction inspired Guglielmo
Marconi, the Italian inventor, to make the dream come true.
Marconi took the crude Coherer Branley had designed and made improvements on it. He
replaced the corks with silver plugs. And by using a mixture of silver and nickel filings
in place of iron filings and evacuating the air from the tube, Marconi succeeded in producing
a device many times more sensitive than the original Coherer.
With his own improved version of the Coherer, the Popov method of decohering, and
a receiving antenna, Marconi attained results that can be described as only slightly
less than spectacular. Signals of from 2 to 9 miles were observed almost immediately,
and by early 1901 the figure had increased to 200 miles. About the same time, at the
suggestion of Sir Oliver Lodge, Marconi incorporated an "oscillation transformer" in
his radio system which permitted the system to be tuned to a given resonant frequency.
Marconi's crowning achievement, however, came about when, on December 12, 1901, he
succeeded in proving that radio waves could be intercepted around the curvature of the
earth. On that day, he received a signal transmitted from England-some 2000 miles from
where he waited on the coast of Newfoundland, Canada.
It is not really clear when radio was actually born. But it certainly was not in existence
before Hertz demonstrated his apparatus, and just as certainly it came about not later
than Sir Oliver Lodge's demonstration. Both of these events took place prior to Marconi's
historic adventure into the new technology.
The achievements of these early pioneers were monumental considering the fact that
most of the work was accomplished before the advent of the electronic amplifier. Just
how incredible these achievements were can be fully realized only by building and using
a Coherer yourself. The diagrams and text on the following page provide all the necessary
Posted November 7, 2018