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.  G-16