rectification. The conversion of alternating current into direct current by means of a rectifier.
rectification (electrical welding). A condition in AC electric arc welding in which oxides on the surface of the metal act as a rectifier and prevent electrons flowing from the metal to the electrode during the half cycle when the electrode is positive.
rectifier (electrical circuit). An electrical circuit that changes alternating current into direct current. A rectifier circuit uses one or more rectifier devices, such as semiconductor diodes, vacuum-tube diodes, or metal-oxide rectifiers. These devices act as electron check valves, allowing electrons to flow during one alternation of the AC, but opposing their flow during the next alternation.

recurring Airworthiness Directive (aircraft maintenance document). An Airworthiness Directive that must be complied with on an aircraft or aircraft engine on a scheduled basis. For example, certain inspections must be performed on a part every so many hours until the part is replaced.
red brass. A copper alloy containing 85% copper, 5% zinc, 5% tin, and 5% lead.
red-line condition. A maximum allowable condition. This term is derived from the fact that the maximum allowable condition is normally marked on an indicating instrument with a red radial line. Airspeed, engine RPM, and various engine temperatures all have specified red-line conditions.
reducing flame (gas welding). See carburizing flame.
reduction factor (weight and balance). A constant which, when divided into a moment, results in an index. Reduction factors of 100, 1,000, and 10,000 are used to simplify weight and balance computations.
reduction gear train. A gear arrangement in which the output shaft turns more slowly than the input shaft. A reduction gear train is used to increase the torque produced by a motor.
Reed and Prince screw. A form of recessed head screw that can be driven with a power screwdriver. The cross cut into the head of a Reed and Prince screw has straight sides, and the bottom of the recess is a sharp
V.

reed valve. A simple check valve that allows fluid to flow in one direction, but blocks its flow in the opposite direction. The reed is a flat piece of flexible material, often spring steel, fastened by one of its edges to one side of a hole. Fluid flowing from the side of the hole opposite the reed forces it off its seat, and the fluid flows through the hole. When fluid tries to flow through the hole from the side on which the reed is mounted, it forces the reed tightly against the hole, and no fluid can flow.
Reed valves are used in some types of simple air compressors and pulse jet engines.
reference junction (thermocouple junction). One of the two junctions in a thermocouple system. The voltage produced by a thermocouple is determined by the difference in the temperature of the two junctions. By holding the temperature of the reference junction constant, the voltage produced by the system is determined by the temperature of the measuring junction.