a us army signal corps
radio operator in 1943 in new guinea transmitting by radiotelegraphy
wireless telegraphy or radiotelegraphy is transmission of telegraph signals by radio waves; before about 1910 when radio became dominant, the term wireless telegraphy was also used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, and ground conduction telegraph systems.
radiotelegraphy was the first means of radio communication; the first practical radio transmitters and receivers invented in 1894–5 by guglielmo marconi used radiotelegraphy. it continued to be the only type of radio transmission during the first three decades of radio, called the "wireless telegraphy era" up until world war i, when the development of amplitude modulation (am) radiotelephony allowed sound (audio) to be transmitted by radio. in radiotelegraphy, information is transmitted by pulses of radio waves of two different lengths called "dots" and "dashes", which spell out text messages, usually in morse code. in a manual system, the sending operator manipulates a switch called a telegraph key which turns the transmitter on and off, producing the pulses of radio waves. at the receiver, the pulses are audible in the receiver's speaker as beeps, which are translated back to text by an operator who knows morse code.
radiotelegraphy was used for long-distance person-to-person commercial, diplomatic, and military text communication throughout the first half of the 20th century. it became a strategically important capability during the two world wars since a nation without long-distance radiotelegraph stations could be isolated from the rest of the world by an enemy cutting its submarine telegraph cables. beginning about 1908, powerful transoceanic radiotelegraphy stations transmitted commercial telegram traffic between countries at rates up to 200 words per minute. radiotelegraphy was transmitted by several different modulation methods during its history. the primitive spark gap transmitters used until 1920 transmitted damped waves, which had very large bandwidth and tended to interfere with other transmissions. this type of emission was banned by 1930. the vacuum tube (valve) transmitters which came into use after 1920 transmitted code by pulses of unmodulated sinusoidal carrier wave called continuous waves (cw), which is still used today. to make cw transmissions audible, the receiver requires a circuit called a beat frequency oscillator (bfo). the third type of modulation, frequency shift keying (fsk) was used mainly by radioteletypes. morse code radiotelegraphy was gradually replaced by radioteletype networks (rtty) in most high volume applications by world war ii. today it is nearly obsolete, the only remaining users are the radio amateur community and some limited training by the military for emergency use.