More specifically, the first battery-operated sex toy was a voltaic pile, which was the primitive form of the battery. Unfortunately, this particular voltaic pile was made by Johann Ritter, who put it to unnatural, but entertaining, uses.
Ultraviolet light is super useful in tanning people who want their skin to look like leather and for killing vampires, but it's not something you can see or feel. So how did people figure out it existed, in the first place? An ingenious — and quite simple — experiment.