Nitrogen or air sparks are very powerful light sources that produce flashes that last only a few nanoseconds. This line-powered circuit generates a continuous series of very small sparks across electrodes with a 0.05-inch gap.
Source: http://discovercircuits.com/DJ-Circuits/spark4.htm

The Lightning Bulb circuit uses a quadrac (see Fig. 1)–a device that combines a triac and a diac trigger in a single package–to control the supplied current. Figure 1A shows the schematic symbol for the quadrac, while figure 1B shows the pinout for the unit used in our circuit. Note that quadracs are increasingly difficult [...]

In this configuration, the primary winding and the feedback winding are arranged such that a sustaining oscillation is ensured once the supply is switched on. The waveform’s duty cycle is asymmetrical, but it is not very important in this application. Please note that if the oscillations do not occur at the ‘switch-on’ time, the transformer [...]