Interpole. A field pole in a compound-wound DC generator used to minimize armature reaction. Interpoles are located between each of the regular field poles, and their coils are in series with the armature winding so all of the armature current flows through them. The magnetic field produced by the interpole coils cancels the distortion caused by the armature field and allows the brushed to remain in the neutral plane where there is no potential difference between the commutator segments. Keeping the brushes in the neutral plane minimizes sparking. 

Inverted engine. An in-line or V-engine in which the cylinders are mounted below the crankshaft. 

Iridium. A very hard, brittle, highly corrosion-resistant, whitish-yellow, metallic chemical element. Iridium is used for the fine-wire electrodes in spark plugs that must operate in engines using fuel with an exceptionally high lead content. iso-octane. An organic compound used as the high reference fuel for rating the antidetonation characteristics of aviation gasoline (CH$_{3}$)$_{2}$CHCH$_{2}$C(CH$_{3}$)$_{3}$. 

Isothermal change. A physical change that takes place within a material in which heat energy is added to or taken from the material as needed to keep its temperature constant. 

Jet fuel. Fuel designed and produced to be used in aircraft gas turbine engines. 

Jet propulsion. A method of propulsion by accelerating a relatively small mass of air through a large change in velocity. 

Jeweler’s file.  A small, fine-cut, metalworking file used by jewelry manufacturers. 

Joule. A measure of energy. In terms of electrical energy, one joule is equal to one watt-second. 

Journal (bearing). A hardened and polished surface on a rotating shaft that rides in a plain bearing. 

Kerosene.  A light, almost colorless, hydrocarbon liquid obtained from crude oil through the fractional distillation process. Kerosene is the base for turbine engine fuel. 

Kevlar. The registered trade name by DuPont for a patented aramid fiber. 

Kinematic viscosity. The ratio of the absolute viscosity of a fluid to its density. Kinematic viscosity is measured in centistokes. 

Labyrinth seal.  A type of air and/or seal used around the main –shaft bearings in a gas turbine engine. The seal consists of a series of rotating blades that almost contact the seal land. A small amount of air flows between the seal and the land to prevent oil flowing past the seal. 

Land (piston) The portion of a piston between the ring grooves. 

Land (splined shaft).  The portion of a splined shaft between the grooves. 

Laser tachometer. A highly accurate tachometer that shines a laser beam on a rotating element that has reflective tape or a contrasting mark. The reflected laser beam is converted into electrical pulses which are counted and displayed on a monitoring instrument. 

Last-chance oil filter. A small filter installed in the oil line to the bearing jet in a gas turbine engine. This filter traps any contaminants that have passed the main filter and holds them until the engine is disassembled for overhaul. 

LCD. Liquid crystal display. A digital display that consists of two sheets of glass separated by a sealed-in, normally transparent liquid crystal material. The outer surface of each glass sheet has a transparent conductive coating with the viewing side etched into character-forming segments with leads going to the edges of the display. A voltage applied between the front and back coatings disrupts the orderly arrangement of molecules and causes the liquid to darken so that light cannot pass through it. The segment to which the voltage is applied appears as black against a reflected background. 

Leading edge. The thick edge at the front of a propeller blade. 

Lean die-out.  A condition in which the fire in a gas turbine engine goes out because the fuel-air mixture ratio is too lean to sustain combustion. 

Lean mixture. A fuel-air mixture that contains more than 15 parts of air to 1 part of fuel, by weight. 

Line boring. A method of assuring concentricity of bored holes. A boring bar extends through all of the holes and cuts the inside diameters so they all have the same center. 

Link rod.  The rod in a radial engine that connects one of the piston wrist pins to a knuckle pin on the master rod. Also called articulating rods. 

