Tap Changer : A mechanism usually fitted to the primary winding of a transformer, to alter the turns ratio of the transformer by small discrete amounts over a defined range
Young & Company Electric Power Products and Related Engineering Services, Glossary of Electrical Terms, http://www.youngco.com/young2.asp?ID=4&Type=3, CA., USA 2001. (Electr Sfty - Amer) {514}