A device which produces alternating current (AC) by converting the engine's turning (mechanical) energy into alternating electrical current at all engine speeds. The AC must be rectified (converted from AC to DC) before reaching the car's electrical system . The alternator is driven by a belt at the front of the engine. Alternators replaced the direct-current (DC) generators used up to the 1960's because they were less efficient especially at idle. The electrically demanding options like air conditioning forced the use of alternators over generators.