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.
Johann Ritter was a brilliant young scientists with many major accomplishments. His most famous scientific contribution was the discovery of ultraviolet light. Less well-known is his invention of the first battery-operated sex toy. It was very stripped down. It was, in fact, just a voltaic pile and some wires.
The "voltaic pile" was invented by Allessandro Volta, who we can be pretty sure didn't have Ritter's use in mind when he put his invention together. Voltaic piles were made by stacking layers of zinc and copper. Between each metal layer was a thin electrolyte, often just rags soaked in salt water, or in something slightly acidic. The zinc dissolved in the acid, giving up two electrons in the process. The copper, being a generous element, donated two electrons to the zinc. The zinc promptly dissolved once more, yielding the electrons all over again. Pile it all up, and there's a surplus of positive charge at the copper end that just can't wait to make a connection the surplus of negative charge at the zinc end. Volta found that many things completed this connection and let the charge flow, including the human body.
Ritter was more interested in the effects of those charges on the human body. He made himself a test dummy. At first, that was comparatively normal; lots of scientists shocked themselves. Ritter shocked his skin. He shocked his tongue. He even shocked his penis.
Then things got weird. He started telling people, including his publisher, that he would "marry" his voltaic pile. He worked out the best way to shock his genitalia, dipping them (the genitalia, not the voltaic pile) in milk before applying the wires. He wrote that "swelling" would soon occur, and then came the climax. He started upping the strength of the shocks. When they became too painful, he took opium so he could withstand them. He temporarily paralyzed his arm. He temporarily numbed his tongue. He'd leave himself exhausted for days.
In part, it's a shame that Ritter didn't live long enough to improve the voltaic pile in the ways he clearly wanted to. His further work would have been a revelation to both science and erotica. Sadly, he fell victim to tuberculosis, and died at 33.
[Via Electrified Sheep]