lapping (machining operation). A method of producing a close fit between two metal parts by rubbing them together with a very fine abrasive lapping compound between them. The fit of a lapped joint is checked by removing all the lapping compound and lightly coating one of the surfaces with Prussian blue transfer dye. The parts are reassembled and rubbed together. If the parts fit as they should, the Prussian blue will transfer evenly to the part that was not covered. If not enough blue transfers, more lapping is required.
lapping compound. A very fine abrasive paste used to wear away a metal surface to make it smooth, and to assure a perfect fit between two machined surfaces. Lapping compound is similar to valve-grinding compound.
laps (metal defects). A type of metal defect produced in the rolling mill. Ridges or bulges on the surface of the metal are folded over and forced down into the metal as it is being rolled. Laps decrease the strength of the metal.
lapse rate (meteorology). The rate of decrease of an atmospheric variable with height. Lapse rates often refer to a decrease in temperature with height.
lap winding (electrical armature). A method of connecting the armature coils in a DC generator or motor. The ends of each coil on the armature are connected to the next adjacent commutator segment, and the coils lap over each other. Lap winding differs from wave winding in which the ends of the coils are attached to segments on the opposite side of the commutator.
large aircraft. 14 CFR Part 1: “Aircraft of more than 12,500 pounds, maximum certificated takeoff weight.”
large calorie (C). A unit of heat energy. One large calorie, also called a kilocalorie, is the amount of heat energy needed to raise the temperature of one kilogram of water one degree Celsius. One large calorie is equal to 1,000 small calories.
LASCR (light-activated silicon controlled rectifier). A semiconductor device having two P-sections and two N-sections. Light falling on the LASCR produces gate current that triggers it into a state of conduction.

laser. A coined word meaning Light Amplification by Stimulated Emission of Radiation. A laser changes incoherent light, light whose vibrations do not have any consistent phase relationship, into coherent light, light whose vibrations are all in phase.