permanent set (mechanical deformation). A permanent deformation in a material that has been strained beyond its elastic limit. According to Hooke’s law, a material deforms, or strains, in proportion to the amount of stress put on it, and when the stress is removed, the material will return to its original shape and size. This is true until the elastic limit of the material is reached. If the elastic limit of the material is exceeded, the material will not return to its original condition; rather it will take a permanent set.
permeability (μ). A measure of the ease with which lines of magnetic flux can pass through a material. The permeability of a material is found by dividing the flux density (B), in lines per square inch, by the magnetizing force (H), in ampere-turns per inch, needed to produce this flux density.
Air is used as the reference, and it is assigned a permeability of one. Pure iron has a permeability of about 200,000.
persistence (cathode-ray oscilloscope characteristic). A measure of the length of time the phosphor dot on the surface of a cathode-ray tube will glow after the beam of electrons passes on. A long-persistence tube leaves a definite trace of the pattern of the beam after the beam has moved on. In a short-persistence tube, the electron beam leaves very little trace of its passage.
person. 14 CFR Part 1: “An individual, firm, partnership, corporation, company, association, joint-stock association, or governmental entity. It includes a trustee, receiver, assignee, or similar representative of any of them.”
perspective view. A view of an object in which parallel lines do not appear parallel, but converge at a vanishing point off of the drawing.
petrolatum-zinc dust compound (abrasive compound). A special abrasive compound, used inside an aluminum terminal that is being swaged onto an aluminum electrical wire. As the terminal is compressed, the zinc dust abrades the oxide from the wires, and the petrolatum prevents oxygen reaching the wire, so no more oxides can form.
petroleum. The family of flammable materials obtained from crude oil by the process of distillation. Butane, propane, gasoline, naphtha, kerosine, diesel fuel, furnace oil, lubricating oil, marine fuel, road oil, tar, and asphalt are all petroleum products.
petroleum fractions. The various components of a hydrocarbon fuel which are separated by boiling them off at different temperatures in the process of fractional distillation.
P-factor (aerodynamics). The asymmetrical thrust produced by a propeller when the airplane is flying at a high angle of attack. The downward moving propeller blade has a greater angle of attack than the blade moving upward, and the thrust produced by the half of the propeller disk having the downward moving blade is greater than the thrust produced by the other half.
pH. A measure of acidity or alkalinity of a solution. Solutions with pH values less than 7 are acidic. A solution with a pH of 7 is neutral, and solutions with pH values greater than 7 are alkaline.
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