When a PhD student called Joseph Valasek discovered ferroelectricity exactly 100 years ago, few people realized the enormous impact it would have on science and technology. Amar S Bhalla and Avadh Saxena pick their favourite applications of this fundamental physics phenomenon. Great discoveries are sometimes made without anyone realizing quite how important they will be. C V Raman, for example, won the Nobel Prize for Physics in 1930 for discovering that light can change energy when it scatters, yet Raman spectroscopy did not become a valuable research tool until well after the laser was invented in 1960. Similarly, few could have imagined that Paul Dirac’s far-fetched yet bold proposal of antiparticles – for which he won the 1933 Nobel prize – would lead to positron emission tomography half a century later. But there is a lesser known – yet important – discovery that also went largely unrecognized at the time. It was made 100 years ago in 1920 by Joseph Valasek (1897–1993), who was then a graduate student working under the supervision of William Swann at the University of Minnesota, Minneapolis, US. Seeking to develop a seismograph to measure the vibrations from earthquakes, Valasek wondered if this could be done with piezoelectric crystals, which create an electric signal when squeezed. The most readily available piezoelectric he had at hand was a single-crystalline substance first synthesized in the 17th century by Pierre Seignette, a pharmacist from the French seaport of La Rochelle. Extracted from wine, it became known as Rochelle salt or Seignette salt and has the chemical formula potassium sodium tartrate tetrahydrate (KNaC₄H₄O₆·4H₂O). When Valasek placed a sample of this material in an electric field, E, he noticed that its resulting electric polarization, P, did something unusual. As he turned up the field, the polarization increased, with the graph of P versus E following an S-shaped curve. However, when the field was lowered again, the polarization was always higher than before albeit following the same kind of curve. In other words, the precise value of the polarization depended on whether the field was rising or falling: it was showing hysteresis (figure 1). So unusual was this observation that Swann presented it at the April 1920 meeting of the American Physical Society in Gaithersburg, Maryland, in a paper entitled “Piezoelectric and allied phenomena in Rochelle salt”. (As a lowly PhD student, Valasek did not even attend the meeting.) Swann and Valasek did not know what caused the hysteresis, but there were parallels with a discovery that had been made three decades earlier by the Scottish physicist James Alfred Ewing. He had seen a similar kind of behaviour in certain ferromagnets, noticing that the magnetic moment depends on how the magnetic field has changed. Valasek’s discovery therefore pointed to an entirely new class of materials, in which the electric dipole moment – and hence the polarization – depends on how the electric field has changed.