discharges will not occur. Some entertainment coils have a sharp "spark point" projecting from the torus to start discharges. Types The term "Tesla coil" is applied to a number of high-voltage resonant transformer circuits. Excitation Tesla coil circuits can be classified by the type of "excitation" they use, what type of circuit is used to apply current to the primary winding of the resonant transformer: Spark-excited or Spark Gap Tesla Coil (SGTC): This type uses a spark gap to close the primary circuit, exciting oscillations in the resonant transformer. Spark gaps have disadvantages due to the high primary currents they must handle. They produce a very loud noise while operating, noxious ozone gas, and high temperatures which may require a cooling system. The energy dissipated in the spark also reduces the Q factor and the output voltage. Tesla's coils were all spark-excited. Static spark gap: This is the most common type, which was described in detail in the previous section. It is used in most entertainment coils. An AC voltage from a high-voltage supply transformer charges a capacitor, which discharges through the spark gap. The spark rate is not adjustable but is determined by the 50 or 60 Hz line frequency. Multiple sparks may occur on each half-cycle, so the pulses of output voltage may not be equally-spaced. Static triggered spark gap: Commercial and industrial circuits often apply a DC voltage from a power supply to charge the capacitor, and use high-voltage pulses generated by an oscillator applied to a triggering electrode to trigger the spark. This allows control of the spark rate and exciting voltage. Commercial spark gaps are often enclosed in an insulating gas atmosphere such as sulfur hexafluoride, reducing the length and thus the energy loss in the spark. Rotary spark gap: These use a spark gap consisting of electrodes around the periphery of a wheel rotated at high speed by a motor, which create sparks when they pass by a stationary electrode. Tesla used this type on his big coils, and they are used today on large entertainment coils. The rapid separation speed of the electrodes quenches the spark quickly, allowing "first notch" quenching, making possible higher voltages. The wheel is usually driven by a synchronous motor, so the sparks are synchronized with the AC line frequency, the spark occurring at the same point on the AC waveform on each cycle, so the primary pulses are