Polytechn
Polytechnic. Including many of the ar
ic to Portal ts and sciences; as a polytecftnic school. Polyvinyls. (Plast.) The family. includes polyvinyl chloride, p.v. acetate, p.v. acetals, and p.v. alcohol : the first being a thermoplastic material composed of polymers of vinyl chloride. It has outstanding resistance to water, alcohol, acids nad alkalies, and therefore it is found in film and sheet vapor barriers, shower curtains, silo covers and upholstery. (SEE PLASTIcs.)
Poplar. (Woodwkg.) A common tree whose wood is soft, light in weight, and from white to a lightgreenish-yellow. in color. It is easy to work and is much used in the arts.
Poppet. (Mech.) (I) The headstock of a lathe. (2) A lathe center. The term is not generally bused.
Poppet valves. (Engin.) The disk and stem type of valves as used in an automobile motor.
Poppy heads. (Arch.) The ornaments which form the tops of the ends of benches or pews.
Porcelain. China or chinaware. A translucent kind of ceramic ware, usually blaZed.
Porch. (Arch.) A covered structure, outside of a building, and with separate roof, forming an entrance to the building.
Porosity. The tendency to permit air, gas or fluid under pressure to ooze through metal or other material. A tire is porous when many small holes allow the escape of air. A cylinder is porous if the metal is not dense enough to prevent leakage through its walls.
Porpoising. (Aero.) An undulatory movement of a seaplane consisting of a combination of a vertical oscillation and an oscillation about its transverse axis, which occurs at certain stages of planing. Port~ (Auto.) An opening through which fuel may be admitted to the combustion chamber of the engine or one from which exhaust gases are released.
Portable crane. (Shopwk.) A hoisting device carried by a frame mounted on wheels. It is particularly useful for quick moving about in a shop for the handling of parts too heavy for a man to lift.
Portable firebox boiler. (Engin.) Widely used t)'pe of steel heating boiler, fitted with water-jacketed firebox. May be of welded or riveted construction, and have fire tubes or water tubes. Designated portable because it is shipped complete, in one piece, ready for installation.
Portal. (Arch.) A door, also a gate Portico to Pottasium
way or entrancceway.usually applied to imposing structures. Portico.a space with a roof supported by columns usually a porch brfore th e entarance to a building
portiere. (Furn.) A curtain for use In a doorway.
Portland cement. (Masonry) The building cement of common use, made by heating clay and lime substance. The vitrified product, when ground, forms a very strong hydraulic cement.
Positive. (Photog.) A photographic image on paper, film. or glass corresponding to the original copy. Reverse of negative.
Positive carbon. (Elec.) In directcurrent arc lamps the positive carbon has formed in it a crater. This carbon is usually the top one.
Positive-driven-type super-charger. (Aero.) A supercharger driven at a fixed speed ratio from the engine shaft by gears or other positive means.
Positive feed. (Mach.) Direct feed motion by means of gears, without friction clutches or belts.
Positive group. (Elec.) A number of storage-battery plates, welded to a common terminal, which forms the positive element for a single cell.
Positive mold. (Plast.) A mold designed to trap all the molding material to prevent its escape when it closes.
Positive motion. (Mech.) Motion obtained by use of gears, linkages,levers etc in which there is ni loss by sliping or friction.
Positive plate.The plate of a storage cell,usually brownish in color. from which the curr~nt flows to the negative plate dunng the process of discharging. .
Positive terminal. (Elec.) The potnt of connection of a circuit or battery from which electricity flows when a circuit is completed.
Post drill. {Mach.) A drilling machine constructed for attachment to a post or column. Often used in small shops, service stations, etc. Post-office bridge. (Elec.) A type of Wheatstone bridge, where the galvanometer and known resistances are all enclosed in a box. The balance is obtained by removing brass connecting plugs which insert certain krlown resistances into the circuit. The name comes from the rows of plugs inserted in the face of the box.
Potassium. (Chern.) A soft, wax-like, silver metal, which rapidly oxidizes in moist air; melting point 63.5 deg. C.; specific gravity 0.8621. The metal itself has no
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