Editing Spark-gap transmitter (section)

===Continuous waves===
Although their damping had been reduced as much as possible, spark transmitters still produced [[Damped wave (radio transmission)|damped wave]]s, which due to their large bandwidth caused interference between transmitters.<ref name="Terman"/><ref name="Aitken1985" />{{rp|p.72-79}} The spark also made a very loud noise when operating, produced corrosive [[ozone]] gas, eroded the spark electrodes, and could be a fire hazard.<ref name="Hyder" />  Despite its drawbacks, most wireless experts believed along with Marconi that the impulsive "whipcrack" of a spark was necessary to produce radio waves that would communicate long distances.<ref name="Sarkar"/>{{rp|p.374}}<ref name="McNicol" />{{rp|p.78}}

From the beginning, physicists knew that another type of waveform, [[continuous wave|continuous]] [[sinusoidal]] waves (CW), had theoretical advantages over damped waves for radio transmission.<ref name="Fitzgerald">George Fitzgerald as early as 1892 described a spark oscillator as similar to the oscillations produced when a cork pops out of a winebottle, and said what was needed was a continuous electromagnetic "whistle".  He realized that if the resistance of a tuned circuit were made zero or negative it would produce continuous oscillations, and tried to make an electronic oscillator by exciting a tuned circuit with negative resistance from a dynamo, what would today be called a parametric oscillator, but was unsuccessful.  G. Fitzgerald, ''On the Driving of Electromagnetic Vibrations by Electromagnetic and Electrostatic Engines'', read at the January 22, 1892 meeting of the Physical Society of London, in {{cite book
 |last        = Larmor
 |first       = Joseph, Ed.
 |title       = The Scientific Writings of the late George Francis Fitzgerald
 |publisher   = Longmans, Green and Co.
 |date        = 1902
 |location    = London
 |pages       = 277–281
 |url         = https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA277
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140707134922/https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA277
 |archive-date = 2014-07-07
}}</ref><ref name="Aitken2014" />{{rp|p.4–7, 32–33}} Because their energy is essentially concentrated at a single frequency, in addition to causing almost no interference to other transmitters on adjacent frequencies, continuous wave transmitters could transmit longer distances with a given output power.<ref name="Aitken1985" />{{rp|p.72-79}} They could also be [[modulation|modulated]] with an [[audio signal]] to carry sound.<ref name="Aitken1985" />{{rp|p.72-79}} The problem was no techniques were known for generating them. The efforts described above to reduce the damping of spark transmitters can be seen as attempts to make their output approach closer to the ideal of a continuous wave, but spark transmitters could not produce true continuous waves.<ref name="Aitken2014" />{{rp|p.4–7, 32–33}}

Beginning about 1904, continuous wave transmitters were developed using new principles, which competed with spark transmitters. Continuous waves were first generated by two short-lived technologies:<ref name="Aitken1985" />{{rp|p.72-79}}
*The [[arc converter]] (Poulsen arc) transmitter, invented by [[Valdemar Poulsen]] in 1904 used the [[negative resistance]] of a continuous [[electric arc]] in a [[hydrogen]] atmosphere to excite oscillations in a [[resonant circuit]].
*The [[Alexanderson alternator]] transmitter, developed between 1906 and 1915 by [[Reginald Fessenden]] and [[Ernst Alexanderson]], was a huge rotating alternating current generator ([[alternator]]) driven by an electric motor at a high enough speed that it produced [[radio frequency]] current in the [[very low frequency]] range.
These transmitters, which could produce power outputs of up to one [[megawatt]], slowly replaced the spark transmitter in high-power radiotelegraphy stations. However spark transmitters remained popular in two way communication stations because most continuous wave transmitters were not capable of a mode called "break in" or "listen in" operation.<ref name="Handy">{{cite book
  | last = Handy
  | first = Frances Edward
  | title = The Radio Amateur's Handbook, 1st Ed.
  | publisher = American Radio Relay league
  | date = 1926
  | location = Hartford, CN
  | pages = 123–124
  | language = 
  | url = https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/ARRL/The-Radio-Amateur's-Handbook-ARRL-1926-1st.pdf
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref>  With a spark transmitter, when the telegraph key was up between Morse symbols the carrier wave was turned off and the receiver was turned on, so the operator could listen for an incoming message. This allowed the receiving station, or a third station, to interrupt or "break in" to an ongoing transmission.  In contrast, these early CW transmitters had to operate continuously; the [[carrier wave]] was not turned off between Morse code symbols, words, or sentences but just detuned, so a local [[radio receiver|receiver]] could not operate as long as the transmitter was powered up. Therefore, these stations could not receive messages until the transmitter was turned off.