Wireless Telegraphy — A State of the Art in 1935
(source SFR)Warning.
These articles date from the 1930s. At that time, scientists felt the need to introduce a medium, the "Ether," to ensure the propagation of electromagnetic waves in the "vacuum." This notion has of course disappeared today. The following should therefore be read by placing oneself in this context.
The origins of radioelectricity lie in the work on electrical or magnetic actions at a distance, carried out at the beginning of the 19th century by the Dane Œrsted (1777-1851), the Englishman Faraday (1788-1827), and the Frenchman André-Marie Ampère (1775-1836). The illustrious name of the French physicist Fresnel, the author of the theory of light undulations in the ether, should be associated with these names.
THE INVENTION OF WIRELESS TELEGRAPHY
James Clark Maxwell (1831-1879)
Heinrich Rudolf Hertz (1857-1894)
Around the middle of the century, the Englishman Maxwell, a student of Faraday, hypothesized that electrical phenomena were due to the movements of Fresnel's fluid, and, in calculations forever famous, specifying what Ampère had had the audacity to glimpse, he demonstrated the identical origin of light vibrations and electrical manifestations. It was a German, Hertz, who died in 1894 at the age of thirty-six, who would experimentally crown the work of these two geniuses. In optics, Fresnel's work was complete; in electricity, Maxwell's, of unprecedented boldness, was mainly theoretical. Hertz succeeded, in his laboratory, in demonstrating the electrical waves of the ether: he measured their speed, their length, and reproduced with them all the phenomena of reflection and refraction of light waves, generating at a point in space, by means of electrical oscillations, the long waves of the ether, which History, grateful, has called Hertzian waves, detecting their passage at another point in his laboratory, he was truly the creator of radioelectricity.
Wireless telegraphy uses, as we have seen, the propagation, through space, of Hertzian waves. These waves form around a conducting wire, the site of alternating movements of electrons, and conversely, their passage determines, in the conductors they encounter, oscillations of the electrical charges of these conductors. The problem is therefore reduced to the following two cases: producing alternating currents of electrons in wires and detecting these movements, even when they are very attenuated.
The first part of the problem seems easy to solve: it is the most difficult. It is known that alternating currents are commonly used for lighting, that is, back-and-forth movements of electricity that heat and make incandescent thin wires enclosed in lamps. The ether, which bathes the distribution lines like all bodies, is agitated, "stirred" by these currents: waves, electrical waves, therefore propagate in all directions... And the problem is solved?... Alas! no. Industrial currents change direction about fifty times per second... This is sufficient for lighting... For Wireless Telegraphy, it is much too slow. It was only since 1917 that it became possible to directly produce alternating currents of sufficiently high frequency by means of machines. When the frequency is too low, everything happens as if, having plunged our hand into a basin full of water, we moved it very, very slowly: the surface would barely be traversed by a few featureless ripples that die out immediately. It was therefore necessary to find something else. Imagine a reservoir full of compressed air with a weak point, for example, an opening closed by a stopper. Gradually increase the pressure in the reservoir; a moment comes when the stopper is expelled. Immediately, the compressed air rushes out, the atmosphere reacts by inertia, while yielding under the shock; sound waves are immediately formed, resulting in a violent noise. These waves, moreover, damp very quickly: the perceived sound lasts only a very short time. To produce rapid and violent electrical oscillations, Hertz resorted to a similar device, using the phenomenon of the oscillating discharge of a capacitor, studied for the first time by Federsen.
Suppose that electrical charges are accumulated on an isolated conducting body, but placed very close, a few millimeters away, from another conductor, a metal ball, for example, connected to buried wires. A moment arrives when the pressure, that is, the voltage of the electricity on the first conductor, is such that a spark jumps towards the ball connected to the earth and the charges rush into the ground by virtue of this conducting spark due to the incandescent metal particles it carries. By inertia, the enormous terrestrial charge reacts and sends back the new arrivals. Rapid oscillations thus occur between the conductor, the ball, and the ground. Similarly, a rubber ball thrown to the ground bounces, falls back, bounces again until, little by little, its movement dampens and it remains motionless. But, while its movements are slow, those of the electrical discharges suddenly triggered, then sent back, are of extreme vivacity. There are hundreds of thousands of oscillations in a second and the movement dampens almost instantaneously... Powerful waves have, during this time, developed in the ether and spread into space.
The first part of the problem seems easy to solve: it is the most difficult. It is known that alternating currents are commonly used for lighting, that is, back-and-forth movements of electricity that heat and make incandescent thin wires enclosed in lamps. The ether, which bathes the distribution lines like all bodies, is agitated, "stirred" by these currents: waves, electrical waves, therefore propagate in all directions... And the problem is solved?... Alas! no. Industrial currents change direction about fifty times per second... This is sufficient for lighting... For Wireless Telegraphy, it is much too slow. It was only since 1917 that it became possible to directly produce alternating currents of sufficiently high frequency by means of machines. When the frequency is too low, everything happens as if, having plunged our hand into a basin full of water, we moved it very, very slowly: the surface would barely be traversed by a few featureless ripples that die out immediately. It was therefore necessary to find something else. Imagine a reservoir full of compressed air with a weak point, for example, an opening closed by a stopper. Gradually increase the pressure in the reservoir; a moment comes when the stopper is expelled. Immediately, the compressed air rushes out, the atmosphere reacts by inertia, while yielding under the shock; sound waves are immediately formed, resulting in a violent noise. These waves, moreover, damp very quickly: the perceived sound lasts only a very short time. To produce rapid and violent electrical oscillations, Hertz resorted to a similar device, using the phenomenon of the oscillating discharge of a capacitor, studied for the first time by Federsen.
Suppose that electrical charges are accumulated on an isolated conducting body, but placed very close, a few millimeters away, from another conductor, a metal ball, for example, connected to buried wires. A moment arrives when the pressure, that is, the voltage of the electricity on the first conductor, is such that a spark jumps towards the ball connected to the earth and the charges rush into the ground by virtue of this conducting spark due to the incandescent metal particles it carries. By inertia, the enormous terrestrial charge reacts and sends back the new arrivals. Rapid oscillations thus occur between the conductor, the ball, and the ground. Similarly, a rubber ball thrown to the ground bounces, falls back, bounces again until, little by little, its movement dampens and it remains motionless. But, while its movements are slow, those of the electrical discharges suddenly triggered, then sent back, are of extreme vivacity. There are hundreds of thousands of oscillations in a second and the movement dampens almost instantaneously... Powerful waves have, during this time, developed in the ether and spread into space.
E. Branly
It was with a similar device, known as the "Hertz exciter or oscillator," that the young German scientist produced waves of the ether, which he detected by means that we will not examine here. These means were somewhat rudimentary; but even with today's more perfected devices, it would have been impossible to detect the passage of the first Hertzian waves at a distance of more than a few hundred meters. The "Hertz exciter" did not yet produce movements energetic enough in the ether to make themselves felt far away.
Around 1890, Dr. Branly noticed that the electrical conductivity of metal filings was modified when electrical sparks were made to jump in the vicinity of these filings. In a horizontal glass tube, as thick as a finger, he placed a little filing of an almost inoxidable metal, nickel, silver, gold, forming a thin layer between the walls of the tube, which he closed with small polished steel stoppers, and he joined these two stoppers to the two poles of a very weak battery; under these conditions, he found that no current passed through this tube, which is explained by the nature of the imperfect contact between the very fine filings and the steel stoppers. But, if, by any means, a spark was made to jump at some distance from the tube, even behind an obstacle such as a wall, the nature of the contacts of the filings was modified, and the tube allowed the current to pass. It was then sufficient to give it a light tap with the finger to break the conductivity of the device again: the current, in passing, had somehow welded particles of filings against the walls of the stoppers, and the shock on the tube broke these small welds. The device was very sensitive and convenient to use.
In 1892, Nikola Tesla, an engineer of Dalmatian origin (ex Croatia), invented high-frequency transformers and expounded the entire theory of the generation and reception of electrical waves.
Among the inventors of wireless telegraphy, the English physicist Oliver Lodge, who, in 1894, automated the operation of Branly's coherer and used it to detect effects at a distance, should also be mentioned. He inserted into the battery circuit connected to the terminals of the filings tube a small bell whose hammer, instead of striking a bell, could butt against the walls of the radioconductor. At each strike, the tube became non-conductive again, then, being under the influence of a train of waves triggered by a spark, it "cohered" again and allowed the current to pass. But then the bell came into play, its hammer struck the tube which became resistant again until the passage of the next wave train.
The Russian professor Popoff sought to use Branly's tube to announce the approach of storms. The electrical discharges of distant clouds
obviously cause the formation of ether waves. These ether waves encountering a vertical wire, a lightning rod conductor
for example, produce in this conductor, as we have seen, electrical oscillations. Popoff connected the base of the conductor to
a Branly tube mounted with Lodge's device and thus detected distant discharges.
Marquis Guglielmo Marconi
It was reserved for Guglielmo Marconi, then a twenty-two-year-old student in Bologna, to conceive the idea of bringing together all these elements and telegraphing — wirelessly — dots and dashes, according to the Morse code. The first success was obtained in the garden of the family home in 1896.
From 1899, the year Marconi succeeded in communicating between Dover and Calais, research multiplied in all countries.
Today, electrical waves encircle the world.
We will not recall here the numerous improvements that were successively made to radioelectric technology and we will conclude this summary history by mentioning the discovery made in 1906 by the American scientist Lee de Forest, based on the work of Professor Fleming (1904), that of the three-electrode tube. This discovery was subsequently to prove extraordinarily fruitful and it dominates all radioelectricity today. It is what has made it possible to perfect radiocommunications considerably and has made possible the most recent applications, radiotelephony, broadcasting, television, etc.
Sources and references
[1] Société Française de Radioélectrique, "VINGT-ANNÉES DE TSF", 1935
