Michael Faraday invented the electric generator to convert mechanical energy into electrical energy. It consists of a number of wire loops rotating freely in a magnetic field. When the velocity of the loop is perpendicular to the magnetic field, the current produced by the induced EMF (electromotive force, the potential difference given to charges, commonly by a battery) is greatest, reaching a maximum value . For an alternating current generator—also known as an alternator—the electric current inverts its direction each half-spin, as shown by the orange arrow.