The invention of radio communication was preceded by many decades of establishing theoretical underpinnings, discovery and experimental investigation of radio waves, and engineering and technical developments related to their transmission and detection. These developments allowed Guglielmo Marconi to turn radio waves into a wireless communication system.
The idea that the wires needed for electrical telegraph could be eliminated, creating a wireless telegraph, had been around for a while before the establishment of radio-based communication. Inventors attempted to build systems based on electric conduction, electromagnetic induction, or on other theoretical ideas. Several inventors/experimenters came across the phenomenon of radio waves before its existence was proven; it was written off as electromagnetic induction at the time.
The discovery of electromagnetic waves, including radio waves, by Heinrich Hertz in the 1880s came after theoretical development on the connection between electricity and magnetism that started in the early 1800s. This work culminated in a theory of electromagnetic radiation developed by James Clerk Maxwell by 1873, which Hertz demonstrated experimentally. Hertz considered electromagnetic waves to be of little practical value. Other experimenters, such as Oliver Lodge and Jagadish Chandra Bose, explored the physical properties of electromagnetic waves, and they developed electric devices and methods to improve the transmission and detection of electromagnetic waves. But they did not apparently see the value in developing a communication system based on electromagnetic waves.
In the mid-1890s, building on techniques physicists were using to study electromagnetic waves, Guglielmo Marconi developed the first apparatus for long-distance radio communication. On 23 December 1900, the Canadian-born American inventor Reginald A. Fessenden became the first person to send audio (wireless telephony) by means of electromagnetic waves, successfully transmitting over a distance of about a mile (1.6 kilometers,) and six years later on Christmas Eve 1906 he became the first person to make a public wireless broadcast.
By 1910, these various wireless systems had come to be called "radio".
Wireless communication theories and methods previous to radio
Before the discovery of electromagnetic waves and the development of radio communication, there were many wireless telegraph systems proposed and tested. In April 1872 William Henry Ward received U.S. patent 126,356 for a wireless telegraphy system where he theorized that convection currents in the atmosphere could carry signals like a telegraph wire. A few months after Ward received his patent, Mahlon Loomis of West Virginia received U.S. patent 129,971 for a similar "wireless telegraph" in July 1872. The patented system claimed to utilize atmospheric electricity to eliminate the overhead wire used by the existing telegraph systems. It did not contain diagrams or specific methods and it did not refer to or incorporate any known scientific theory.
In the United States, Thomas Edison, in the mid-1880s, patented an electromagnetic induction system he called "grasshopper telegraphy", which allowed telegraphic signals to jump the short distance between a running train and telegraph wires running parallel to the tracks. In the United Kingdom, William Preece was able to develop an electromagnetic induction telegraph system that, with antenna wires many kilometers long, could transmit across gaps of about 5 kilometres (3.1 miles). Inventor Nathan Stubblefield, between 1885 and 1892, also worked on an induction transmission system.
A form of wireless telephony is recorded in four patents for the photophone, invented jointly by Alexander Graham Bell and Charles Sumner Tainter in 1880. The photophone allowed for the transmission of sound on a beam of light, and on 3 June 1880, Bell and Tainter transmitted the world's first wireless telephone message on their newly invented form of light telecommunication.
In the early 1890s Nikola Tesla began his research into high-frequency electricity. Tesla was aware of Hertz's experiments with electromagnetic waves from 1889 on but doubted they existed, and agreed with the prevailing scientific thought at that time that they probably only travel in straight lines, making them useless for long range transmission.
Instead of using radio waves, Tesla's efforts were focused on building a conduction-based power distribution system, although he noted in 1893 that his system could also incorporate communication. His laboratory work and later large-scale experiments at Colorado Springs led him to the conclusion that he could build a conduction-based worldwide wireless system that would use the Earth itself (via injecting very large amounts of an electric current into the ground) as the means to conduct the signal very long distances (across the Earth), overcoming the perceived limitations of other systems. He went on to try to implement his ideas of power transmission and wireless telecommunication in his very large but unsuccessful Wardenclyffe Tower project.
Development of electromagnetism
Various scientists proposed that electricity and magnetism were linked. Around 1800 Alessandro Volta developed the first means of producing an electric current. In 1802 Gian Domenico Romagnosi may have suggested a relationship between electricity and magnetism but his reports went unnoticed. In 1820 Hans Christian Ørsted performed a simple and today widely known experiment on electric current and magnetism. He demonstrated that a wire carrying a current could deflect a magnetized compass needle. Ørsted's work influenced André-Marie Ampère to produce a theory of electromagnetism. Several scientists speculated that light might be connected with electricity or magnetism.
In 1831, Michael Faraday began a series of experiments in which he discovered electromagnetic induction. The relation was mathematically modelled by Faraday's law, which subsequently became one of the four Maxwell equations. Faraday proposed that electromagnetic forces extended into the empty space around the conductor, but did not complete his work involving that proposal. In 1846 Michael Faraday speculated that light was a wave disturbance in a "force field".
Expanding upon a series of experiments by Felix Savary, between 1842 and 1850 Joseph Henry performed experiments detecting inductive magnetic effects over a distance of 200 feet (61 m). He was the first (1838–42) to produce high frequency AC electrical oscillations, and to point out and experimentally demonstrate that the discharge of a capacitor under certain conditions is oscillatory, or, as he puts it, consists "of a principal discharge in one direction and then several reflex actions backward and forward, each more feeble than the preceding until equilibrium is attained". This view was also later adopted by Helmholtz, the mathematical demonstration of this fact was first given by Lord Kelvin in his paper on "Transient Electric Currents".
Maxwell and the theoretical prediction of electromagnetic waves
Between 1861 and 1865, based on the earlier experimental work of Faraday and other scientists and on his own modification to Ampere's law, James Clerk Maxwell developed his theory of electromagnetism, which predicted the existence of electromagnetic waves. In 1864 Maxwell described the theoretical basis of the propagation of electromagnetic waves in his paper to the Royal Society, "A Dynamical Theory of the Electromagnetic Field." This theory united all previously unrelated observations, experiments and equations of electricity, magnetism, and optics into a consistent theory. His set of equations—Maxwell's equations—demonstrated that electricity, magnetism, and light are all manifestations of the same phenomenon, the electromagnetic field. Subsequently, all other classic laws or equations of these disciplines were special cases of Maxwell's equations. Maxwell's work in electromagnetism has been called the "second great unification in physics", after Newton's unification of gravity in the 17th century.
Oliver Heaviside later reformulated Maxwell's original equations into the set of four vector equations that are generally known today as Maxwell's equations. Neither Maxwell nor Heaviside transmitted or received radio waves; however, their equations for electromagnetic fields established principles for radio design, and remain the standard expression of classical electromagnetism.
Of Maxwell's work, Albert Einstein wrote:
"Imagine [Maxwell's] feelings when the differential equations he had formulated proved to him that electromagnetic fields spread in the form of polarised waves, and at the speed of light! To few men in the world has such an experience been vouchsafed... it took physicists some decades to grasp the full significance of Maxwell's discovery, so bold was the leap that his genius forced upon the conceptions of his fellow-workers."
Other physicists were equally impressed with Maxwell's work, such as Richard Feynman who commented:
"From a long view of the history of the world—seen from, say, ten thousand years from now—there can be little doubt that the most significant event of the 19th century will be judged as Maxwell's discovery of the laws of electromagnetism. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade."
Experiments and proposals
Berend Wilhelm Feddersen, a German physicist, in 1859, as a private scholar in Leipzig, succeeded in experiments with the Leyden jar to prove that electric sparks were composed of damped oscillations.
In 1870 the German physicist Wilhelm von Bezold discovered and demonstrated the fact that the advancing and reflected oscillations produced in conductors by a capacitor discharge gave rise to interference phenomena. Professors Elihu Thomson and E. J. Houston in 1876 made a number of experiments and observations on high frequency oscillatory discharges. In 1883 George FitzGerald suggested at a British Association meeting that electromagnetic waves could be generated by the discharge of a capacitor, but the suggestion was not followed up, possibly because no means was known for detecting the waves.
Hertz experimentally verifies Maxwell's theory
When German physicist Heinrich Rudolf Hertz was looking for a subject for his doctoral dissertation in 1879, instructor Hermann von Helmholtz suggested he try to prove Maxwell's theory of electromagnetism. Hertz initially couldn't see any way to test the theory but his observation, in the autumn of 1886, of discharging a Leyden jar into a large coil and producing a spark in an adjacent coil gave him the idea of how to build a test apparatus. Using a Ruhmkorff coil to create sparks across a gap (a spark gap transmitter) and observing the sparks created between the gap in a nearby metal loop antenna, between 1886 and 1888 Hertz would conduct a series of scientific experiments that would validate Maxwell's theory. Hertz published his results in a series of papers between 1887 and 1890, and again in complete book form in 1893.
The first of the papers published, "On Very Rapid Electric Oscillations", gives an account of the chronological course of his investigation, as far as it was carried out up to the end of the year 1886 and the beginning of 1887.
For the first time, electromagnetic radio waves ("Hertzian waves") were intentionally and unequivocally proven to have been transmitted through free space by a spark-gap device, and detected over a short distance.
Hertz was able to have some control over the frequencies of his radiated waves by altering the inductance and capacitance of his transmitting and receiving antennas. He focused the electromagnetic waves using a corner reflector and a parabolic reflector, to demonstrate that radio behaved the same as light, as Maxwell's electromagnetic theory had predicted more than 20 years earlier.
Hertz did not devise a system for practical utilization of electromagnetic waves, nor did he describe any potential applications of the technology. Hertz was asked by his students at the University of Bonn what use there might be for these waves. He replied, "It's of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right, we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there."
Many physicists quickly realized that Hertzian waves could be used (instead of light) in systems akin to optical telegraph: for example, Richard Threlfall and John Perry suggested that in 1890, Alexander Pelham Trotter in 1891 and Frederick Thomas Trouton in 1892, however they all thought about it in terms of short flashes as opposed to telegraphic dots and dashes. In what may have been a little noticed article titled 'Some possibilities of electricity' in the February 1892 in The Fortnightly Review, Sir William Crookes described wireless telegraphy as having been accomplished a year earlier, although the method and type is not described. The American physicist Amos Emerson Dolbear brought similar attention to the idea a year later. Hertz's health deteriorated after a severe infection in 1892 and he died in 1894, so the art of radio wave communication was left to others to implement into a practical form.
Pre-Hertz radio wave detection
During 1789–91, Luigi Galvani noticed that a spark generated nearby caused a convulsion in a frog's leg being touched by a scalpel. In different experiments, he noticed contractions in frogs' legs caused by lightning and a luminous discharge from a charged Leyden jar that disappeared over time and was renewed whenever a spark occurred nearby.
Joseph Henry observed magnetised needles from lightning in the early 1840s.
In 1852 Samuel Alfred Varley noticed a remarkable fall in the resistance of masses of metallic filings under the action of atmospheric electrical discharges.