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The spark gap transmitter was the first practical way to send radio signals. Before the invention of the vacuum tube (or thermionic valve ) in 1907, the only way to produce large amounts of radio frequency (RF) energy was to use the pulse created when a high voltage caused air to ionize, allowing current to flow across a spark gap. If the spark gap is connected to a LC circuit, the energy produced is concentrated near the resonant frequency of the circuit. If the spark is interrupted many times per second (say by some mechanical device) and the LC circuit is coupled to an antenna, a buzzing sound will be heard in a radio receiver tuned to the same frequency some distance away. By switching the voltage across the gap on and off with a telegraph key, it becomes possible to communicate using Morse code.
The person deserving credit for the invention of the spark gap transmitter is hotly disputed among historians of radio, but Guglielmo Marconi was the first to develop wireless telegraphy into a practical communications tool. One of the earliest uses for wireless telegraphy was communication with ships at sea. Its primary use was sending telegrams ("MarconiGrams") on behalf of passengers. Its importance for emergency communications became clear after the sinking of the Titanic in 1912. The First International Conference on the Safety of Life at Sea was convened in 1913 and produced a treaty requiring radio on international passenger ships, with radio rooms to be manned 24 hours a day.
A typical high-power spark gap was a rotating commutator with six to twelve contacts per wheel, 9 inches to a foot wide, driven by about 2000 volts DC. As the gaps made and broke contact, the radio wave was audible as a tone in a crystal set. The telegraph key often directly made and broke the 2000 volt supply. One side of the spark gap was directly connected to the antenna. Early radio amateurs built low power spark gap transmitters using the spark plug magneto from Ford Model T automobiles.
The invention of the vacuum tube allowed RF energy to be produced at essentially a single frequency, allowing continuous wave (CW) telegraphy. CW was more efficient because receivers could be tuned more narrowly. Also many more stations could operate in the same frequency band. It took some time before high power vacuum tubes were developed. Thus vacuum tube receivers became commonplace before spark-gap transmitters were replaced by continuous wave transmitters.
The very wide bandwidth of spark gap transmitters caused overcrowding of radio bands and eventually led to spark gap wireless being banned in the 1920s. However spark gap transmitter technology is still used to generate RF energy in TIG welding systems used to weld aluminum and other metals.
External links
Brief history of spark (http://www.vistech.net/users/w1fji/spark.html)
See also radio, telegraph
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