photothyristor. A light-activated silicon controlled rectifier (LASCR). Light falling on the gate-cathode junction triggers the LASCR into conduction.

phototransistor. A junction transistor in which emitter-collector current flows when light shines on the reverse-biased collector-base junction. A small window in the housing allows light to shine on the junction and control the current. Phototransistors produce a current gain and are often used in place of photodiodes.

photovoltaic cell. A solid-state electrical component that produces a voltage when it is exposed to light.

phugoid oscillation. Oscillations of an aircraft about its pitch, or lateral, axis (along its longitudinal axis). These oscillations consist of long-term, low-amplitude climbs and dives, with little or no change in the angle of attack.
physics. The branch of natural science that deals with the interaction between matter and energy in the fields of: acoustics, optics, heat, mechanics, electricity, magnetism, radiation, atomic structure, and nuclear phenomena.
pi (π). The mathematical constant 3.1415927… .
Pi, usually written with the Greek letter π, is an irrational number, a number that cannot be expressed as an even ratio of any two integers. Pi is the ratio of the length of the circumference of a circle to the length of its diameter.
piano hinge. A continuous hinge consisting of hinge sections fastened to both the fixed and movable bodies. A piece of hard spring-steel wire passes through holes in the two sections to act as the hinge pin.
PIBAL (pilot balloon observation). A method of winds-aloft observation by visually tracking a pilot balloon.
pickling. The treatment of a metal surface with an acid to remove surface corrosion.
pico. The metric prefix meaning one millionth of a millionth (1 x 10-12). A picofarad, for example, is one millionth of a millionth of a farad. Pico was at one time called micro-micro.
pictorial diagram. A diagram of a wiring or plumbing installation in which sketches of the actual component, rather than symbols, are used to represent components.
piezoelectric. The characteristic of certain materials that causes them to produce a voltage when they are bent or twisted or when pressure is applied to them.
piezoelectric crystal. A thin square of quartz or Rochelle salt that has the characteristic of producing a voltage across its opposite faces when it is physically distorted. The physical resonant frequency of the crystal makes it useful as a component in an oscillator to maintain a constant electrical frequency. When the crystal is shocked with a pulse of electrical energy, it vibrates at its resonant frequency. As it vibrates, it
produces a voltage across its faces that varies at the same frequency as the crystal vibrates.

piezoelectricity. Electricity produced when certain crystalline materials, such as quartz or Rochelle salt, are deformed (bent or twisted) by pressure.
piezoelectric transducer. An electrical device that uses a crystal to change mechanical movement into an electrical signal. Crystal phonograph pickups and crystal microphones are examples of piezoelectric transducers.