He thereupon disconnected the power source, not knowing how much charge it had stored. The jar discharged through his assistant. This was probably the first large man made shock on record. Capacitors changed very little until the invention of electrolytics early in the twentieth century. These utilize high surface area aluminum plates in contact with a salt solution.
Aluminum forms a very thin insulating oxide layer.
The thin oxide layer allows the capacitor to store a large amount of charge . Upon charging, salt ions electrostatically absorb to the electrodes. Even so, the charge storage ability of ordinary electrolytics is not nearly large enough to purify water. Patents to this technology date from the nineteen fifties or earlier, with Donald Boos of Carborundum Company prominent among the early inventors. This technology was subsequently licensed in Japan, where it was manufactured profitably for use in computer memory circuits and other applications that require a large amount of charge storage.