How could these results be explained? Charles-Frangois Dufay (1698-1739) concluded that there were two types of electricity. He named them “vitreous” (meaning glass, precious stones) and “resinous” (amber, sealing wax, silk). Friction separates the two types. When they are combined they neutralize each other. Jean-Antoine Nollet modeled these types as two fluids, each composed of particles that repelled each other. Charging amber gave it an excess of resinous fluid. Charging glass with silk gave it an excess of vitreous fluid. When the two were touched together the fluids combined with each other leaving the objects uncharged.

Benjamin Franklin believed there was only one fluid. When glass was rubbed the fluid filled the glass. When amber was rubbed the fluid left the amber. He called an object with an excess of fluid “positive” and one with too little fluid “negative.” When they were touched the fluid flowed from the glass to the amber, leaving each with its proper amount of fluid. The flow was likened to water in a river. The “electrical tension” (difference in potential) and “electrical current” were analogous to the difference of water levels between two points and of the amount of water transferred.