Potassium sodium tartrate tetrahydrate is a
double salt of
tartaric acid first prepared (in about 1675) by an
apothecary, Pierre Seignette, of
La Rochelle,
France. Potassium sodium
tartrate and
monopotassium phosphate were the first materials discovered to exhibit
piezoelectricity. This property led to its extensive use in "crystal" gramophone (phono) pick-ups, microphones and earpieces during the post-War consumer electronics boom of the mid-20th Century. Such
transducers had an exceptionally high output with typical pick-up cartridge outputs as much as 2 volts or more. Rochelle salt is
deliquescent so any transducers based on the material deteriorated if stored in damp conditions.