Editing Tesla coil (section)

===Operation cycle===
The circuit operates in a rapidly repeating cycle in which the supply transformer ''(T)'' charges the primary capacitor ''(C1)'' up, which then discharges in a spark through the spark gap, creating a brief pulse of oscillating current in the primary circuit which excites a high oscillating voltage across the secondary:<ref name="Anderson"/><ref name="Denicolai2"/><ref name="BurnettOperation"/><ref name="Gerekos5"/>
# Current from the supply transformer ''(T)'' charges the capacitor ''(C1)'' to a high voltage.
# When the voltage across the capacitor reaches the [[breakdown voltage]] of the spark gap ''(SG)'' a spark starts, reducing the spark gap resistance to a very low value. This completes the primary circuit and current from the capacitor flows through the primary coil ''(L1)''. The current flows rapidly back and forth between the plates of the capacitor through the coil, generating radio frequency oscillating current in the primary circuit at the circuit's [[resonant frequency]].
# The oscillating [[magnetic field]] of the primary winding induces an oscillating current in the secondary winding ''(L2)'', by [[Faraday's law of induction]]. Over a number of cycles, the energy in the primary circuit is transferred to the secondary. The total energy in the tuned circuits is limited to the energy originally stored in the capacitor ''C1'', so as the oscillating voltage in the secondary increases in amplitude ("ring up") the oscillations in the primary decrease to zero. Although the ends of the secondary coil are open, it also acts as a tuned circuit due to the capacitance ''(C2)'', the sum of the [[parasitic capacitance]] between the turns of the coil plus the capacitance of the toroid electrode ''E''. Current flows rapidly back and forth through the secondary coil between its ends. Because of the small capacitance, the oscillating voltage across the secondary coil which appears on the output terminal is much larger than the primary voltage.
# The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to the primary, causing the oscillating voltage in the secondary to decrease ("ring down"). This process repeats, the energy shifting rapidly back and forth between the primary and secondary tuned circuits. The oscillating currents in the primary and secondary gradually die out due to energy dissipated as heat in the spark gap and resistance of the coil.
# When the current through the spark gap is no longer sufficient to keep the air in the gap ionized, the spark stops ("quenches"), terminating the current in the primary circuit. The oscillating current in the secondary may continue for some time.
# The current from the supply transformer begins charging the capacitor ''C1'' again and the cycle repeats.

This entire cycle takes place very rapidly, the oscillations dying out in a time of the order of a millisecond. Each spark across the spark gap produces a pulse of damped sinusoidal high voltage at the output terminal of the coil. Each pulse dies out before the next spark occurs, so the coil generates a string of [[damped wave]]s, not a continuous sinusoidal voltage.<ref name="Anderson"/> The high voltage from the supply transformer that charges the capacitor is a 50 or 60&nbsp;Hz [[sine wave]]. Depending on how the spark gap is set, usually one or two sparks occur at the peak of each half-cycle of the mains current, so there are more than a hundred sparks per second. Thus the spark at the spark gap appears continuous, as do the high-voltage streamers from the top of the coil.

The supply transformer ''(T)'' secondary winding is connected across the primary tuned circuit. It might seem that the transformer would be a leakage path for the RF current, damping the oscillations. However its large [[inductance]] gives it a very high [[electrical impedance|impedance]] at the resonant frequency, so it acts as an open circuit to the oscillating current. If the supply transformer has inadequate [[short-circuit inductance]], radio frequency [[choke (electronics)|choke]]s are placed in its secondary leads to block the RF current.