Editing Spark-gap transmitter (section)

===Operation cycle===
The transmitter works in a rapid repeating cycle in which the capacitor is charged to a high voltage by the transformer and discharged through the coil by a spark across the spark gap.<ref name="CodellaSparkRadio"/><ref name="Nahin4">Nahin, Paul J. (2001) ''[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA45&dq=spark The Science of Radio: with MATLAB and Electronics Workbench demonstrations, 2nd Ed.]'', p. 38-43</ref> The impulsive spark excites the resonant circuit to "ring" like a bell, producing a brief oscillating current which is radiated as electromagnetic waves by the antenna.<ref name="CodellaSparkRadio"/> The transmitter repeats this cycle at a rapid rate, so the spark appeared continuous, and the radio signal sounded like a whine or buzz in a [[radio receiver]].

[[File:Massie spark gap.jpg|thumb|right|Demonstration of the restored 1907 [[Massie Wireless Station]] spark gap transmitter 
{{Listen
| filename    = Massie Wireless Station "PJ" Spark Sound.ogg
| title       = Audio of Massie spark gap transmission
| description = [[Morse code]] of "[[CQ (call)|CQ DE]] PJ"
| pos         = left
| plain       = yes}}
]]
#The cycle begins when current from the transformer charges up the capacitor, storing positive electric charge on one of its plates and negative charge on the other.  While the capacitor is charging the spark gap is in its nonconductive state, preventing the charge from escaping through the coil.
#When the voltage on the capacitor reaches the [[breakdown voltage]] of the spark gap, the air in the gap [[ionization|ionizes]], starting an [[electric spark]], reducing its [[Electrical resistance|resistance]] to a very low level (usually less than one [[ohm]]). This closes the circuit between the capacitor and the coil.
#The charge on the capacitor discharges as a current through the coil and spark gap. Due to the [[inductance]] of the coil when the capacitor voltage reaches zero the current doesn't stop but keeps flowing, charging the capacitor plates with an opposite polarity, until the charge is stored in the capacitor again, on the opposite plates. Then the process repeats, with the charge flowing in the opposite direction through the coil. This continues, resulting in oscillating currents flowing rapidly back and forth between the plates of the capacitor through the coil and spark gap.
#The resonant circuit is connected to the antenna, so these oscillating currents also flow in the antenna, charging and discharging it. The current creates an oscillating [[magnetic field]] around the antenna, while the voltage creates an oscillating [[electric field]]. These oscillating fields radiate away from the antenna into space as an [[electromagnetic wave]]; a radio wave.
#The energy in the resonant circuit is limited to the amount of energy originally stored in the capacitor. The radiated radio waves, along with the heat generated by the spark, uses up this energy, causing the oscillations to decrease quickly in [[amplitude]] to zero. When the oscillating electric current in the primary circuit has decreased to a point where it is insufficient to keep the air in the spark gap ionized, the spark stops, opening the resonant circuit, and stopping the oscillations. In a transmitter with two resonant circuits, the oscillations in the secondary circuit and antenna may continue some time after the spark has terminated. Then the transformer begins charging the capacitor again, and the whole cycle repeats.

The cycle is very rapid, taking less than a millisecond. With each spark, this cycle produces a radio signal consisting of an oscillating [[sinusoidal]] wave that increases rapidly to a high [[amplitude]] and decreases [[exponential decay|exponentially]] to zero, called a [[Damped wave (radio transmission)|damped wave]].<ref name="CodellaSparkRadio"/> The [[frequency]] <math>f</math> of the oscillations, which is the frequency of the emitted radio waves, is equal to the [[resonant frequency]] of the resonant circuit, determined by the [[capacitance]] <math>C</math> of the capacitor and the [[inductance]] <math>L</math> of the coil:
:<math>f = \frac {1}{2 \pi} \sqrt { \frac {1}{LC}} \,</math>
The transmitter repeats this cycle rapidly, so the output is a repeating string of damped waves. This is equivalent to a radio signal [[amplitude modulation|amplitude modulated]] with a steady frequency, so it could be [[demodulation|demodulated]] in a radio receiver by a [[Rectifier|rectifying]] [[amplitude modulation|AM]] [[detector (radio)|detector]], such as the [[crystal detector]] or [[Fleming valve]] used during the wireless telegraphy era. The [[frequency]] of repetition (spark rate) is in the [[audio frequency|audio]] range, typically 50 to 1000 sparks per second, so in a receiver's [[earphone]]s the signal sounds like a steady tone, whine, or buzz.<ref name="Kennedy"/>

In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a [[switch]] called a [[telegraph key]] in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in [[Morse code]]. As long as the key is pressed the spark gap fires repetitively, creating a string of pulses of radio waves, so in a receiver the keypress sounds like a buzz; the entire Morse code message sounds like a sequence of buzzes separated by pauses. In low-power transmitters the key directly breaks the primary circuit of the supply transformer, while in high-power transmitters the key operates a heavy duty [[relay]] that breaks the primary circuit.