In a resonant transformer the high voltage is produced by resonance; the output voltage is not proportional to the turns ratio, as in an ordinary transformer. It can be calculated approximately from conservation of energy. At the beginning of the cycle, when the spark starts, all of the energy in the primary circuit formula_6 is stored in the primary capacitor formula_7. If formula_8 is the voltage at which the spark gap breaks down, which is usually close to the peak output voltage of the supply transformer "T", this energy is