may be a rational number, or even an irrational one (the latter case is known as the "quasiperiodic" regime). When the periodic and quasiperiodic regimes overlap, the behavior of the circuit may become aperiodic, meaning that the pattern of the oscillations never repeats. This aperiodicity correspond to the behavior of the circuit becoming chaotic (see chaos theory). The forced neon bulb oscillator was the first system in which chaotic behavior was observed. Van der Pol and van der Mark wrote, concerning their experiments with demultiplication, that Often an irregular noise is heard in the telephone receivers before the frequency jumps to the next lower value. However this is a subsidiary phenomenon, the main effect being the regular frequency demultiplication. Any periodic oscillation would have produced a musical tone; only aperiodic, chaotic oscillations would produce an "irregular noise". This is thought to have been the first observation of chaos, although van der Pol and van der Mark didn't realize its significance at the time. See also Relaxation oscillator Schmitt trigger 555 timer Negative resistance Notes References S. O. Pearson and H. St. G. Anson, Demonstration of Some Electrical Properties of Neon-filled Lamps, Proceedings of the Physical Society of London, vol.34, no. 1 (December 1921), pp. 175–176 S. O. Pearson and H. St. G. Anson, The Neon Tube as a Means of Producing Intermittent Currents, Proceedings of the Physical Society of London, vol. 34, no. 1 (December 1921), pp. 204–212 Analog circuits