Potassi Et Sodii Tartras

There are a number of different varieties of piezoelectric  filters. While crystals have been used as rectifiers since the 1870s, it wasn't until the 1920s that crystals were discovered to also resonate acoustically at radio frequencies. The design concept for utilizing quartz crystals as a filtering component was first established by Dr. Walter Guyton Cady at GE in 1922, (or Alexander Nicholson) but later work by Walter P. Mason into the 1930s is what really allowed engineers to integrate crystals into communications hardware. But it all really begins in about 1672 in an apothecary.

Cady's early experiments at GE employed Rochelle salt crystals as transducers. Rochelle salt is also known as potassium sodium tartrate (KNaC4H4O6+4H2O). It's a "double salt." These are salts containing more than one cation or anion. You can read up on ions here.  They form when more than one salt is dissolved in a liquid but form a single mineral when they crystallize. It was first prepared by Pierre Seignette, of La Rochelle, France The 1880 Medical text the National Dispensary states that it's an antilithic, emetro-cathartic, and febrifuge. None of those reasons are why Seignette was making it.

The confusion begins with which Seignette made the discovery and which family members were involved. In many versions of the history Pierre's brother Jehan was also an apothecary. In some their father was also an apothecary named Jehan. In one their father is the inventory and an apothecary and named Elie. Elie could also be their mother. The version I believe is the one from the 1817 book A History of Inventions and Discoveries. There the father (Pierre) invents Rochelle Salt, and the son (Elie) continues the business.  But there are two theories on how it's use was discovered. In one version he was trying to develop salts  based on a hunch that it could be used as a laxative. In the other he just ate some and observed the results...

The popular laxative of the time was folia sennae. It was dangerous because Senna can also cause renal failure. When Elie or Pierre developed their salt they named it “Sel Polychreste.” Rochelle salts sold well. Pierre Seignette died in 1719, wholly unaware of the long tail attached to his discovery. In 1731 the formula was cracked by Claude Joseph Geoffroy Jr. who published his method in the Royal Society but credited Seignette with the discovery.  Fast forward a century, and in 1824 David Brewster was making observations on pyroelectricity in various crystals including Rochelle Salt. Pierre and Paul-Jacques Curie performed similar studies in the 1880s. The piezoelectric qualities of “Sel Polychreste” remained known but not exploited until Dr. Walter Guyton Cady found a use for it. More here.