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Brothers and colleagues: Jacques (left) and Pierre (right) Curie, discoverers of the piezoelectric effect.
Microphones, quartz watches, and inkjet printers all rely on an unusual phenomenon known as the piezoelectric effect found in various crystals, ceramics, and even bone. It was discovered by none other than French physicist Pierre Curie, working with his older brother Jacques, who found that putting pressure on these materials created electricity (the name comes from piezein — Greek for “squeeze”).
Born in Paris in 1859 to a physician named Eugene Curie, Pierre’s early education was decidedly unorthodox: his father opted for private tutors for his son, believing it to be the best approach given the boy’s temperament and keen intellect. Pierre showed an early aptitude for mathematics, and at 16 entered the Sorbonne for his university studies. He successfully earned the equivalent of a master’s degree by 18, but was forced to postpone his doctoral studies. During this time, he earned a meager living as a lab instructor.
Pierre started conducting chemistry experiments at the age of 20 with Jacques, focusing on the structure of crystals. They were especially interested in the pyroelectric effect, in which a change in temperature in a crystalline material generates an electric potential. This effect had been known since the mid-18th century, thanks to the work of Carl Linnaeus and Franz Aepinus, and subsequent scientists had hypothesized that there could be a relationship between the properties of mechanical stress and electrical potential. But experimental confirmation proved elusive.
The brothers Curie thought there would be a direct correlation between the potential generated by temperature changes and the mechanical strain that gave rise to piezoelectricity. They expected that a piezoelectric effect would arise in materials with certain crystal asymmetries. Armed with the crudest of materials — tinfoil, glue, wire, magnets, and a simple jeweler’s saw — they tested various types of crystals, including quartz, topaz, cane sugar, Rochelle salt, and tourmaline. As a result, the Curies found that when such materials were compressed, the mechanical strain did indeed result in an electric potential. The strongest piezeoelectric effects were found in quartz and Rochelle salt. The brothers put their discovery immediately to good use by inventing the piezoelectric quartz electrometer.
There was a twist to the piezoelectric saga still to come. The following year, mathematician Gabriel Lippman demonstrated that there should be a converse piezoelectric effect, whereby applying an electric field to a crystal should cause that material to deform in response. The brothers rushed to test Lippman’s theory, and their experiments showed the mathematician was correct. Piezoelectricity could indeed work in the other direction.
After the initial flurry of excitement died down, piezoelectric research faded into the background for the next 30 years or so, in part because the theory was so mathematically complex. But incremental progress was still being made. In 1910, Woldemar Voigt published the definitive treatise on the subject, Lehrbuch der Kristallphysik, a massive tome describing the 20-odd classes of natural crystal with piezoelectric properties. More importantly, it rigorously defined the 18 possible macroscopic piezoelectric coefficients in crystal solids.
This set the stage for subsequent development of practical applications for such materials, beginning with sonar in 1917, when Paul Langevin developed an ultrasonic transducer for use on submarines using thin quartz crystals. Many automobiles today have ultrasonic transducers to assist drivers in measuring the distance between the rear bumper and any obstacles in its path.
Pierre moved on to investigating magnetism, uncovering an intriguing effect of temperature on paramagnetism now known as Curie’s law. Another discovery was the Curie point: the critical temperature at which ferromagnetic materials cease to be ferromagnetic. He even flirted with paranormal spiritualism as the 19th century drew to a close, attending séances with famed medium Eusapia Palladino, approaching them as a scientific experiment with detailed observational notes, in hopes that such study would shed light on magnetism. “I must admit that those spiritual phenomena intensely interest me,” he wrote to his fiancée, Marie Sklodowska, in 1894. “I think in them are questions that deal with physics.”
Pierre married Marie the following year, when he also finally completed his doctorate, thanks to her encouraging him to use his magnetism work as a doctoral thesis. He became a professor of physics and chemistry at Paris in 1895. (Jacques became a professor of mineralogy at the University of Montpellier.) His new wife replaced his brother as his scientific partner. The two discovered radium (and later, polonium), sharing the 1903 Nobel Prize in Physics with Henri Becquerel. The piezoelectric quartz electrometer invented by Pierre and Jacques all those years before proved an essential instrument in their ongoing work.
Towards the end of his life, Pierre showed early signs of over-exposure to radium. In fact, his clothes were often so radioactive he had to postpone experiments by several hours because it interfered with his instruments. The unit of radioactivity is called the curie in his and Marie’s honor. But he was spared a gruesome death by radiation sickness. Instead, he was killed in a freak accident, run down by a wagon on the Place Dauphine as he was crossing the busy street.
Marie always felt Pierre did not get the respect and support he deserved from his scientific colleagues. He did not engage in academic politics, preferring to focus on his research. He was rejected for a professorship in mineralogy and denied membership in the French Academy in 1903, the same year he won the Nobel Prize. His early work on piezoelectricity was not, perhaps, his most significant discovery over his illustrious career, but as he observed in an 1894 letter to Marie: “[In science] we can aspire to accomplish something…. every discovery, however small, is a permanent gain.”
Curie, Jacques and Curie, Pierre (1880). “Development, via compression, of electric polarization in hemihedral crystals with inclined faces,” Bulletin de la Societe de Minerologique de France, 3: 90-93.
Curie, Jacques, and Curie, Pierre (1881). “Contractions and expansions produced by voltages in hemihedral crystals with inclined faces,” Comptes Rendus 93: 1137-1140.
Hurwic, Anna. Pierre Curie, Translated by Lilananda Dasa and Joseph Cudnik. Paris: Flammarion, 1995.
Lippman, G. (1881). “Principal of the conservation of electricity,” Annales de Chemie et de Physique 24: 145.
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