The Roman naturalist Pliny the Elder (23–79 AD) acknowledged the similarity between the attraction of the magnet and the attraction of pieces of paper by amber rubbed by silk or wool. This parallel suggested some link between the two phenomena. In 1820, Hans Christian OerstedOersted, Hans Christian (1777–1851) proved it, observing that an electric currentcurrent created a fieldfield that moved a compass needle. André-Marie Ampère (1775–1836) showed that a coil of copper wire, if free to turn, behaved as a compass needle. Michael FaradayFaraday, Michael (1791–1867) studied the symmetric effect: he obtained an electric currentcurrent, electric with a magnet moving in and out of a coil, inventing the first generator. Many investigators built electrostatic machines that used friction to charge different objects. Until the eighteenth century, it was not known that the electrification by friction and current transport through conductors involved the same charge carriers. The hypothesis of one single electrical fluid was made by Benjamin Franklin (1706–1790). André-Marie Ampère (1775–1836) suggested that the properties of the magnet were derived from “molecular” currents inside it. Alessandro Volta (1745–1827) created the first electric batterybattery by piling several pairs of discs of two different metals, separated by cloth or cardboard moistened in brine.