van der Pol, Balthazar (1889–1959)
Balthazar van der Pol was a Dutch electrical engineer who began the modern experimental study of dynamical systems in the 1920s and '30s. Van der Pol discovered that electrical circuits employing vacuum tubes display stable oscillations, now called limit cycles, but that when these circuits are driven with a signal whose frequency is near that of the limit cycle, the periodic response shifts its frequency to that of the driving signal. The resulting waveform, however, can be quite complicated and contain a rich structure of harmonics and subharmonics. In 1927, van der Pol and his colleague van der Mark reported that an "irregular noise" was heard at certain driving frequencies between the natural entrainment frequencies. It's now clear that, without realizing it, they had described one of the first experimental instances of chaos.