DOT MATRIX PRINTER rate of change in the effective length of the path of travel between the source and the point of observation. DOT MATRIX PRINTER A type of printer that creates text char- acters with a series of closely spaced dots. DOUBLE SIDED BOARD A printed board with a conductive pat- tern on both sides. DOWN CONDUCTOR, LIGHTNING The conductor connecting the air terminal or overhead ground wire to the earth electrode sub- system. DOWN THE SLAT Vernacular expression for a successful flight of a missile down the test range and within the left and right range lim- its (parallel lines previously plotted on the range plotting board) established by range safety personnel. DOWNTIME That portion of calendar time when the item cannot perform its intended function. DOWNWASH Aerodynamic interference from the wings of an air- craft or missile that can strike the aft tail surfaces in such as way as