By Rochelle Forrester
Ó All Rights Reserved
Publication Date 2006
The existence of radio waves was first suggested by James Clerk Maxwell in 1864. He suggested radio waves were a form of electro-magnetic radiation with a long wave length. Radio waves were shown to exist by Heinrich Hertz in 1888 when he conducted experiments that sent electrical waves, including radio waves, through space. Heinz used an oscillator and a resonator. The oscillator consisted of an electric battery that discharged electricity into a coil connected to two metal balls which were half an inch apart. When the discharge took place sparks jumped across the space between the two balls. The resonator was a circular piece of wire with a small gap in it and with two metal balls at the ends of the wire with the small gap separating the metal balls. When the electrical discharge occurred in the oscillator causing the spark to jump the half inch gap between the two balls in the oscillator, a spark jumped across the gap between the two balls in the resonator even though the resonator was not connected to the oscillator. The cause of the spark jumping across the gap in the resonator was electro-magnetic waves in space originating from the oscillator. Electro-magnetic waves of a very high frequency leave the oscillator and travel out in all directions and will create an electric current in any conductor they meet. These waves are radio waves, they are everywhere but are not normally detectable by people.
The detection of radio waves was made possible in 1890 when Professor Edouard Branley invented the coherer. The coherer was a glass tube containing metal filings and when radio waves hit the tube, it was able to detect the waves and turn them into usable electric current. The coherer was able to ring a bell and more usefully receive Morse signals through the air.
Marconi began experimenting with radio waves in 1894 and by 1899 he had sent radio waves across the English channel. In 1900 he invented the tuned circuit which allowed the sending of a constant series of waves of the same wave length to which a receiver could be tuned to receive those waves. This reduced interference from other radio waves and allowed the waves to be sent over longer distances. In 1901 Marconi sent radio waves from England to Canada showing that radio waves would follow the curvature of the Earth rather than fly off into space, as other electro-magnetic waves such as light do.
An important development for radio was the invention by Lee De Forest of the triode or audion tube. The triode was a vacuum tube in which a filament, metal grid and a positively charged metal plate were placed inside a glass tube containing a vacuum. The heated filament sends a flow of negatively charged electrons toward the positively charge metal plate. However the metal grid with an alternating current changing from negative to positive many times per second, lies between the filament and the positively charge metal plate. When the grid is negative, electrons emerging from the filament are repelled. When the grid changes to a positive state a great force of electrons goes through the mesh of the grid towards the positively charged metal plate. This has the result that one moment few electrons reach the positively charged metal plate, while at the next moment there is an enormous rush of electrons towards the positively charged metal plate. This means a weak radio wave can be turned into a greatly increased electric current which can be further strengthened by putting it through other triode vacuum tubes. The ability to increase the strength of the current which can be done both when the current is sent and received allows radio waves to be sent and received over much greater distances.
The triode was invented in 1906 and in the same year the first public radio broadcast of speech and music was made. A microphone was used to turn the sound waves of the music and speech into electric waves. The first trans-Atlantic speech was made in 1915 from a U.S. Navy radio station in Virginia to Paris. The superhet radio receiver which was better able to pick up weak signals and could be tuned into radio stations more effectively was invented in 1918. Short wave radio began to be used for long distance communication in the first half of the 1920’s.
The explanation for why radio waves followed the Earth’s surface was discovered in 1924. The Earth is surrounded by the ionosphere an area of electrified air about 80-200 miles above the Earths surface. The ionosphere reflects radio waves sent into the air back down to Earth which reflects the radio waves back into the air. The radio waves bounce between the ionosphere and the surface of the Earth as they travel around the world.
The transistor radio was developed in 1954 allowing much smaller and more portable radio receivers. It also resulted in radios appearing in cars and other motor vehicles.
Radio communication has had a wide variety of uses. A major early use involved ships at sea which could not communicate by telegraph. By 1903 50 merchant ships had radio telegraphy to keep in contact with shore stations. Navies adopted radio to enable more effective control of ships at sea and radio played a significant role in the Battle of Jutland. Many people were saved from the Titanic due to the radio operator on the ship informing nearby ships that the Titanic was sinking. The arrest of the famous murderer Dr Crippen was achieved when the captain of the ship Crippen was traveling on radioed his belief Crippen was on his ship to a shore based radio station. The station informed the police who arranged to be in port when Crippen’s ship arrived in Canada.
Early radio was a form of wireless telegraphy using Morse code. Radio reached its full potential when speech and music were broadcast. Politicians as diverse as Franklin D Roosevelt and Adolf Hitler used radio to get elected. Voters heard the results of elections on the radio. Sporting events taking place thousands of miles away from listeners were broadcast directly to listeners. Radio stations were initially funded by the manufacturers of radios but later used advertising to make profits. Ultimately the broadcasting of music became the major function of radio, closely followed by radio talk back which allowed the public to have their say on a wide variety of topics on radio. The transistor radio allowed people to have radio wherever they went and pop music stations kept the youth of western countries supplied with a constant stream of music.
Radio is only possible because the electro-magnetic spectrum includes waves of the radio wave length. Without waves of that wave length radio communication could not have happened. Long distance radio communication was also dependent upon the existence of the ionosphere without which radio waves could not travel around the world. Long distance radio is also dependent upon the triode vacuum tube or some other form of amplifier. If there was no amplifier radio could only have been used for reasonably short range communications.
Radio was also dependent upon the earlier invention of the microphone and loudspeaker. Without these inventions radio would have been limited to a form of wireless telegraphy using Morse code. There would have been no music, radio talk-back or sports commentaries. Radio was equally dependent on the human discovery of how to create and control electric currents and how to amplify them using valves and transistors. It could only appear after these discoveries had been made so it could not appear before the 20th century. The existence of radio waves, the ionosphere and the human ability to create and control an electric current and to invent microphones and loudspeakers shows how the structure of the universe or nature has a major effect on human social and cultural history.