IGNITION, SPARK: When the mixture of air and fuel in an internal-combustion engine is ignited by an electric spark (as in a gasoline engine).

IMPEDANCE: The total opposition offered to the flow of an alternating current. It may consist of any combination of resistance, inductive reactance, and capacitive reactance.

IMPELLER: An encased, rotating element provided with vanes, which draw in fluid at the center and expel it at a high velocity at the outer edge.

IMPINGEMENT: The striking or hitting against an object with a clash or sharp collision, as air impinging upon the rotor of a turbine or motor.

IMPULSE LINES: Piping that connects a sensing element to the point at which it is desired to sense pressure, flow, temperature, etc.

INDICATED HORSEPOWER (ihp): The power transmitted to the pistons by the gas in the cylinders.

INDICATED THERMAL EFFICIENCY: The ratio of indicated horsepower to equivalent power input in the form of heat from fuel.

INDICATOR: An instrument for recording the variation of cylinder pressure during the cycle.

INDICATOR CARD: A graphical record of the cylinder pressures made by an indicator.

INDIRECT DRIVE: A drive mechanism coupled to the driven member by gears or belts.

INDUCTION: The act or process of pro-ducing voltages by the relative motion of a magnetic field across a conductor.

INERT: Inactive, such as gases that will not burn or support combustion.

INERTIA: The tendency of a body at rest to remain at rest, and a body in motion to continue to move at a constant speed along a straight line, unless the body is acted upon in either case by an unbalanced force.

INHIBITOR: Any substance which retards or prevents such chemical reactions as corrosion or oxidation.

INJECTION NOZZLE: A device which protrudes into the combustion chamber and delivers fuel to the cylinder.

INJECTION SYSTEM: A system designed to deliver fuel to the cylinder at the proper time and in the proper quantity under various engine loads and speeds.

IN-LINE ENGINE: An engine in which the cylinders are arranged in one straight line.

IN PHASE: Applied to the condition that exists when two waves of the same frequency pass through their maximum and minimum values of like polarity at the same instant.

INPUT SIGNAL: A pressure or flow of fluid that is directed into an input port to control an element or logic function.

INSULATION: A material used to retard heat transfer. A dielectric material which prevents the flow of electricity from an electric com-ponent.

INTAKE SYSTEM: Combination of com-ponents designed to supply air required for combustion.

INTEGRAL: Essential to completeness, as an integral part. (The valve stem is an integral part of the valve.)

INTERCOOLER: A device which cools a gas between the compression stages of a multiple stage compressor.

INTERFACE: Surface or area between abutting parts usually of different materials.

INTERNAL THREAD: A thread on the inside of a member (for example, the thread in-side a nut).

INVERSELY: Inverted or reversed in position or relationship.

INVERSE PROPORTION: The relation that exists between two quantities when an increase in one of them produces a corresponding decrease in the other.

ISOCHRONOUS GOVERNOR: A device that maintains the speed of the engine truly constant, regardless of the load.

ISOMETRIC DRAWING: A type of pictorial drawing. See ISOMETRIC PROJECTION.

ISOMETRIC PROJECTION: A set of three or more views of an object that appears rotated, giving the appearance of viewing the object from one corner. All lines are shown in their true length but angles are not accurately represented.

JACKBOX: A receptacle, usually secured to a bulkhead, in which telephone jacks are mounted.

JACKET: An outer case such as a water jacket or an insulative covering.

JACKET WATER: Water used as a coolant in the cooling system of an engine (usually chemically treated distilled water).

JACKING: Mechanically rotating an engine or reduction gear at very low speed.

JAM NUT: A second nut used on a bolt or stud to lock the holding nut. See LOCK NUT.

JOB ORDER: An order issued by a repair activity to its own subdivision to perform a repair job in response to a work request.

JOURNAL: Serves as the point of support and center of rotation for the shaft. That part of a shaft that is prepared to accept a bearing (connecting rod, main bearing).

JUMPER: Any connecting pipe, hose, or wire normally used in emergencies aboard ship to bypass damaged sections of a pipe, hose, or wire. See BYPASS.