Editing Spark-gap transmitter (section)

===Charging circuit and spark rate===
The circuit which charges the capacitors, along with the spark gap itself, determines the ''spark rate'' of the transmitter, the number of sparks and resulting damped wave pulses it produces per second, which determines the tone of the signal heard in the receiver. The spark rate should not be confused with the ''frequency'' of the transmitter, which is the number of sinusoidal oscillations per second in each damped wave. Since the transmitter produces one pulse of radio waves per spark, the output power of the transmitter was proportional to the spark rate, so higher rates were favored. Spark transmitters generally used one of three types of power circuits:<ref name="CodellaSparkRadio"/><ref name="Kennedy"/><ref name="Sarkar">{{cite book
| last1= Sarkar
| first1= T. K.
| last2= Mailloux
| first2= Robert
| last3= Oliner
| first3= Arthur A.
| title= History of Wireless
| publisher= John Wiley and Sons
| date= 2006
| url= https://archive.org/stream/HistoryOfWireless#page/n260/mode/2up
| isbn= 978-0471783015
}}</ref>{{rp|p.359–362}}

====Induction coil====
An [[induction coil]] (Ruhmkorff coil) was used in low-power transmitters, usually less than 500 watts, often battery-powered.  An induction coil is a type of transformer powered by DC, in which a vibrating arm switch contact on the coil called an [[interrupter]] repeatedly breaks the circuit that provides current to the primary winding, causing the coil to generate pulses of high voltage. When the primary current to the coil is turned on, the primary winding creates a magnetic field in the iron core which pulls the springy interrupter arm away from its contact, opening the switch and cutting off the primary current. Then the magnetic field collapses, creating a pulse of high voltage in the secondary winding, and the interrupter arm springs back to close the contact again, and the cycle repeats. Each pulse of high voltage charged up the capacitor until the spark gap fired, resulting in one spark per pulse. Interrupters were limited to low spark rates of 20–100&nbsp;Hz, sounding like a low buzz in the receiver. In powerful induction coil transmitters, instead of a vibrating interrupter, a [[Induction coil#Mercury and electrolytic interrupters|mercury turbine interrupter]] was used. This could break the current at rates up to several thousand hertz, and the rate could be adjusted to produce the best tone.

====AC transformer====
In higher power transmitters powered by AC, a [[transformer]] steps the input voltage up to the high voltage needed. The sinusoidal voltage from the transformer is applied directly to the capacitor, so the voltage on the capacitor varies from a high positive voltage, to zero, to a high negative voltage. The spark gap is adjusted so sparks only occur near the maximum voltage, at peaks of the AC [[sine wave]], when the capacitor was fully charged. Since the AC sine wave has two peaks per cycle, ideally two sparks occurred during each cycle, so the spark rate was equal to twice the frequency of the AC power<ref name="Hyder" /> (often multiple sparks occurred during the peak of each half cycle). The spark rate of transmitters powered by 50 or 60&nbsp;Hz mains power was thus 100 or 120&nbsp;Hz. However higher audio frequencies cut through interference better, so in many transmitters the transformer was powered by a [[motor–generator|motor–alternator]] set, an [[electric motor]] with its shaft turning an [[alternator]], that produced AC at a higher frequency, usually 500&nbsp;Hz, resulting in a spark rate of 1000&nbsp;Hz.<ref name="Hyder" />

====Quenched spark gap====
The speed at which signals may be transmitted is naturally limited by the time taken for the spark to be extinguished. If, as described above, the conductive plasma does not, during the zero points of the alternating current, cool enough to extinguish the spark, a 'persistent spark' is maintained until the stored energy is dissipated, permitting practical operation only up to around 60 signals per second.{{citation needed|date=September 2024}} If active measures are taken to break the arc (either by blowing air through the spark or by lengthening the spark gap), a much shorter "quenched spark" may be obtained.{{citation needed|date=September 2024}} A simple quenched spark system still permits several oscillations of the capacitor circuit in the time taken for the spark to be quenched. With the spark circuit broken, the transmission frequency is solely determined by the antenna resonant circuit, which permits simpler tuning.

====Rotary spark gap====
In a transmitter with a "rotary" spark gap ''(below)'', the capacitor was charged by AC from a high-voltage transformer as above, and discharged by a spark gap consisting of electrodes spaced around a wheel which was spun by an electric motor, which produced sparks as they passed by a stationary electrode.<ref name="CodellaSparkRadio"/><ref name="Hyder" /> The spark rate was equal to the rotations per second times the number of spark electrodes on the wheel. It could produce spark rates up to several thousand hertz, and the rate could be adjusted by changing the speed of the motor. The rotation of the wheel was usually synchronized to the AC [[sine wave]] so the moving electrode passed by the stationary one at the peak of the sine wave, initiating the spark when the capacitor was fully charged, which produced a musical tone in the receiver. When tuned correctly in this manner, the need for external cooling or quenching airflow was eliminated, as was the loss of power directly from the charging circuit (parallel to the capacitor) through the spark.