Editing Spark-gap transmitter (section)

===Rotary gap transmitters===
A second type of spark gap that had a similar quenching effect<ref name="Hyder" /> was the "rotary gap", invented by Tesla in 1896<ref name="Patent20981">British patent GB189620981 Henry Harris Lake for Nikola Tesla ''[https://worldwide.espacenet.com/publicationDetails/biblio?II=6&ND=3&adjacent=true&locale=en_EP&FT=D&date=18961121&CC=GB&NR=189620981A&KC=A# Improvements relating to the production, regulation, and utilization of electric currents of high frequency, and apparatus therefore]'' filed: 22 September 1896, granted: 21 November 1896</ref><ref name="Morse1">{{cite book
| last1= Morse
| first1= A. H.
| title= Radio: Beam and Broadcast
| publisher= Ernst Benn, Ltd.
| date= 1925
| location= London
| pages= 25, 138–148
| url= https://archive.org/stream/radiobeamandbroa029214mbp#page/n29
}}</ref> and applied to radio transmitters by [[Reginald Fessenden]] and others.<ref name="Sarkar" />{{rp|p.359–362}}<ref name="Beauchamp3">{{cite book
| last1= Beauchamp
| first1= Ken
| title= History of Telegraphy
| publisher= IET
| date= 2001
| pages= 192–194
| url= https://books.google.com/books?id=k3XCkncd83AC&pg=PA193
| isbn= 978-0852967928
}}</ref> It consisted of multiple electrodes equally spaced around a disk rotor spun at high speed by a motor, which created sparks as they passed by a stationary electrode.<ref name="CodellaSparkRadio"/><ref name="Jansky1"/> By using the correct motor speed, the rapidly separating electrodes extinguished the spark after the energy had been transferred to the secondary.<ref name="Hyder" /><ref name="CodellaSparkRadio"/><ref name="Sarkar" />{{rp|p.359–362}}<ref name="Beauchamp3"/> The rotating wheel also kept the electrodes cooler, important in high-power transmitters.

<gallery mode="packed" heights="130">
File:Rotary spark gap 1919.jpg|A typical rotary spark gap used in low-power transmitters
File:Rotary spark gap transmitter.jpg|Small rotary spark transmitter, 1918
File:Murdock 1 kW rotary gap spark transmitter 1914.jpg|1 kilowatt rotary spark transmitter, 1914.
File:Fessenden synchronous spark transmitter.jpg|[[Reginald Fessenden|Fessenden]]'s 35&nbsp;kW synchronous rotary spark transmitter, built 1905 at Brant Rock, Massachusetts, with which he achieved the first 2 way transatlantic communication in 1906 on 88&nbsp;kHz.
File:Navy NAA spark transmitter Arlington 1913.jpg|US Navy 100&nbsp;kW rotary gap transmitter built by Fessenden in 1913 at Arlington, Virginia. It transmitted on 113&nbsp;kHz to Europe, and broadcast the US's first radio time signal.
</gallery>

There were two types of rotary spark transmitter:<ref name="Hyder" /><ref name="Sarkar" />{{rp|p.359–362}}<ref name="CodellaSparkRadio"/><ref name="Beauchamp3"/><ref name="Belrose"/>
*''Nonsynchronous'': In the earlier rotary gaps, the motor was not synchronized with the frequency of the AC transformer, so the spark occurred at random times in the AC cycle of the voltage applied to the capacitor. The problem with this was the interval between the sparks was not constant.<ref name="Sarkar" />{{rp|p.359–362}} The voltage on the capacitor when a moving electrode approached the stationary electrode varied randomly between zero and the peak AC voltage. The exact time when the spark started varied depending on the gap length the spark could jump, which depended on the voltage. The resulting random phase variation of successive damped waves resulted in a signal that had a "hissing" or "rasping" sound in the receiver.<ref name="Kennedy"/>
*''Synchronous'': In this type, invented by Fessenden around 1904, the rotor was turned by a [[synchronous motor]] in synchronism with the cycles of the AC voltage to the transformer, so the spark occurred at the same points of the voltage sine wave each cycle. Usually it was designed so there was one spark each half cycle, adjusted so the spark occurred at the peak voltage when the capacitor was fully charged.<ref name="Kennedy"/> Thus the spark had a steady frequency equal to a multiple of the AC line frequency, which created [[harmonic (music)|harmonics]] with the line frequency. The synchronous gap was said to produce a more musical, easily heard tone in the receiver, which cut through interference better.<ref name="Kennedy"/>
To reduce interference caused by the "noisy" signals of the burgeoning numbers of spark transmitters, the 1912 US Congress "Act to Regulate Radio Communication" required that "''the logarithmic decrement per oscillation in the wave trains emitted by the transmitter shall not exceed two tenths''"<ref name="Jansky1"/><ref name="CodellaSparkRadio"/><ref name="RadioAct1912">{{cite web
| title= An act to regulate radio communication
| work= Public 264 S. 6412 approved 13 August 1912
| publisher= United States Congress
| date= 1912
| url= https://babel.hathitrust.org/cgi/pt?id=uc1.b3420424;view=1up;seq=8
|pages = 6–14
| access-date= 14 April 2019}} included in ''Radio Communication Laws of the United States'', July 27, 1914 edition, Department of Commerce, United States government printing office</ref> (this is equivalent to a [[Q factor]] of 15 or greater). Virtually the only spark transmitters which could satisfy this condition were the quenched-spark and rotary gap types above,<ref name="Jansky1"/> and they dominated wireless telegraphy for the rest of the spark era.
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==== Marconi's timed spark system ====
In 1912 in his high-power stations Marconi developed a refinement of the rotary discharger called the "timed spark" system, which generated what was probably the nearest to a [[continuous wave]] that sparks could produce.<ref name="Bucher2">{{cite book
| last1= Bucher
| first1= Elmer E.
| title= Practical Wireless Telegraphy
| publisher= Wireless Press, Inc.
| date= 1917
| location= New York
| pages= 274–275
| url= https://archive.org/stream/wirelesstele00buchrich#page/274/mode/2up
}}</ref><ref name="Coursey">{{cite journal
| last1= Coursey
| first1= Phillip R.
| title= The Marconi Timed-Spark Continuous-Wave Transmitter
| journal= Wireless World
| volume= 7
| issue= 78
| pages= 310–316
| date= September 1919
| url= https://www.americanradiohistory.com/Archive-Wireless-World/10s/Wireless-World-1919-09.pdf
| access-date= 19 August 2018}}</ref><ref name="Sarkar" />{{rp|p.399}} He used several identical resonant circuits in parallel, with the capacitors charged by a DC [[dynamo]].<ref name="Goldsmith">{{cite book
| last1= Goldsmith
| first1= Alfred N.
| title= Radio Telephony
| publisher= Wireless Press, Inc.
| date= 1918
| location= New York
| pages= 73–75
| url= https://archive.org/stream/radiotelephony00goldgoog#page/n86/mode/2up
}}</ref> These were discharged sequentially by multiple rotary discharger wheels on the same shaft to create overlapping damped waves shifted progressively in time, which were added together in the oscillation transformer so the output was a [[superposition principle|superposition]] of damped waves. The speed of the discharger wheel was controlled so that the time between sparks was equal to an integer multiple of the wave period. Therefore, oscillations of the successive wave trains were [[in phase]] and reinforced each other. The result was essentially a continuous sinusoidal wave, whose amplitude varied with a ripple at the spark rate. This system was necessary to give Marconi's transoceanic stations a narrow enough bandwidth that they didn't interfere with other transmitters on the narrow [[very low frequency|VLF]] band. Timed spark transmitters achieved the longest transmission range of any spark transmitters, but these behemoths represented the end of spark technology.<ref name="Sarkar" />{{rp|p.399}}

{{multiple image
| align= center
| direction= horizontal
| header =
| image1= Marconi transatlantic wireless station at Carnarvon, Wales 1917.jpg
| width1= 302
| caption1= Transmitter building, showing the 36 feedlines feeding power to the 3,600 ft. flattop wire antenna.
| image2= Marconi Carnarvon 300kW transatlantic spark transmitter 1919 - tuning coil.png
| width2= 235
| caption2= 5 ft diameter primary coil of oscillation transformer, consisting of 3 turns of specialized [[litz wire]] one foot thick
| image3= Marconi Carnarvon 300kW transatlantic spark transmitter 1919 - rotary spark dischargers.png
| width3= 260
| caption3= The three 5 ft rotary spark discharger wheels of the "timed spark" system.
| footer= Marconi 300&nbsp;kW transatlantic timed spark transmitter built 1916 at [[Caernarfon|Carnarvon]], [[Wales]], one of the most powerful spark transmitters ever built. During World War I it transmitted telegram traffic at 200 words per minute on 21.5&nbsp;kHz to receivers in Belmar, New Jersey.<ref name="WirelessWorld">{{cite journal
| title= Great Wireless Stations: Carnarvon
| journal= Wireless World
| volume= 7
| issue= 78
| pages= 301–307
| date= September 1919
| url= https://www.americanradiohistory.com/Archive-Wireless-World/10s/Wireless-World-1919-09.pdf
| access-date= 19 August 2018}}</ref> The roar of the spark could reportedly be heard a kilometer away. On 22 September 1918 it transmitted the first wireless message from Britain to Australia, a distance of 15,200&nbsp;km (9,439&nbsp;miles).<ref name="MacKinnon">{{cite web
| last= MacKinnon
| first= Colin
| title= The first direct wireless messages from England to Australia
| work= Australian Amateur Radio History
| publisher= VK2DYM's military radio and radar information site
| date= 2004
| url= https://www.qsl.net/vk2dym/radio/Marconi.htm
| access-date= 4 May 2018}}</ref> In 1921 it was replaced by [[Alexanderson alternator]] transmitters.
}}