Wireless telegraphy is electronic signaling through the ground, bodies of water, or the air, which does not require the direct metallic connection, from transmitter to receiver, that was needed by the original electric telegraphs. The term covers a number of related technologies developed beginning in the mid-1800s, including earth conduction, electrostatic induction, electromagnetic induction, and, most importantly, electromagnetic radiation (radio). In most implementations, Morse code was used for communication.
Radio proved to be by far the most efficient of these methods, so, beginning around 1900, most references to "wireless" actually mean radio transmissions, and for those purposes "wireless telegraph" was eventually supplanted by the more precise term "radiotelegraph". The term "radioteletype" emerged to describe non-Morse text transmission.
Multiple technologies fall under the term "wireless telegraphy", which sometimes creates confusion, as it is not always clearly stated exactly which form of "wireless" technology is being employed. For each of these technologies, signals are created by electrical currents, which, depending on the frequencies employed, produce different forms of radiation. However, often more than one type of radiation is being produced, which can make it difficult to determine which one is responsible for an observed effect. Among early experimenters, there was often significant uncertainty about exactly how they were producing their results.
Ground and water conduction
The earliest experiments with wireless telegraph transmissions date back to the beginnings of the electric telegraph. The original electric telegraphs employed both sending and return wires, in order to provide a complete electrical circuit for the message transmission. However, in 1837, Carl August von Steinheil of Munich, Germany found that, by connecting the terminal end of the sending wire to metal plates buried in the ground, the return wire could be eliminated, and only a single wire was needed for telegraphing. At the time, a common belief was that, with the single wire configuration, the return current was now traveling through the ground back to the sending point in order to complete the electrical circuit. This turned out to be incorrect, as the transmitted current was actually being absorbed into the earth at the receiving point, but it did lead to speculation that it might be possible to someday also eliminate the sending wire, and telegraph through the ground without using any wires at all.
Other attempts were made to send telegraphic signals through bodies of water, for example, in order to span river crossings. Prominent experimenters along these lines included Samuel F. B. Morse in the United States and James Bowman Lindsay in Great Britain; in 1854 Lindsay demonstrated transmission across the Firth of Tay from Dundee to Woodhaven (now part of Newport-on-Tay), a distance of nearly 2 miles [3 kilometers]. However, because of the very high resistance to electrical currents, earth conductivity transmissions were found to be limited to only a few meters, and even the somewhat greater distances possible through water had little practical use.
Electrostatic induction and electromagnetic induction
Both electrostatic and electromagnetic induction were used to develop wireless telegraph systems that saw limited commercial application. In the United States, Thomas Edison, in the mid-1880s, patented an electrostatic 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. This system was successful technically but not economically, as there turned out to be little interest by train travelers in an on-board telegraph service.
The most successful creator of an electromagnetic induction system was William Preece in Great Britain, who began tests in 1882. By 1892 he was able to telegraph about 5 kilometers across the Bristol Channel. However, his induction system required extensive lengths of wire, many kilometers long, at both the sending and receiving ends, which made it impractical for use on ships or small islands, and the relatively short distances spanned meant it had few advantages over underwater cables.
Electromagnetic radiation (radio)
During the 1880s, German Heinrich Hertz demonstrated the production and reception of electromagnetic radiation (radio waves) in a series of groundbreaking experiments. This led to numerous experimenters working at using radio signals for wireless communication, initially with limited success. However, by 1897, Guglielmo Marconi had made a series of demonstrations in Great Britain which showed the practicality of using radio for signaling for far greater distances than had been achieved by any other means, which helped expand research worldwide.
By the 1920s, there was a worldwide network of commercial and government radiotelegraphic stations, plus extensive use of radiotelegraphy aboard ships for navigational and commercial communication plus passenger messages. One sophisticated implementation of wireless telegraphy was telex using radio signals, developed in the 1940s, which for many years provided the only reliable form of communication between many distant countries. The most advanced standard, CCITT R.44, automated both the routing and encoding of messages, which were transmitted using short wave radio.
While radiotelegraphy had long been part of maritime safety, the requirement to monitor a Morse distress channel eventually was made obsolete in the Global Maritime Distress and Safety System. Radiotelegraphy still has limited use in some military and covert communications, and in amateur radio.