I’ve always been interested in crystals: their outer beauty hints at a certain kind of inner beauty, caused by the orderly arrangement of molecules at the atomic level. When I was a kid, I made crystals from sugar, salt, alum, and copper sulfate, but never tried Rochelle salts. Rochelle salts are interesting because they are piezoelectric: in response to mechanical deformation, they generate an electrical current. Similarly, when stimulated with an electric current, they deform. This makes them useful for all sorts of cool applications such as microphone pickups.

Cooking up piezo crystals at home – Hack a Day

In an attempt to give you some value added, beyond just the link, I did some searching on Google Books. Somewhat interestingly, it turned up the Nov 11, 1946 issue of Life magazine, which ran an article on Rochelle salt crystals, which had the tag line:

ROCHELLE SALT CRYSTALS

Pretty girls in sunsuits grow a unique mineral that can turn pressure into electric current

Perhaps of greater interest to the experimenter is this article from Popular Science, November 1945 which talks about the history of microphones in general, and includes a bunch of experiments (somewhat tersely described) which can be done.