O-ring. A type of seal used in fluid-power system components. An O-ring is a ring-shaped seal with a round cross section, made of a resilient material. An O-ring fits into a groove to form a seal between two parts.
O-rings are two-way seals; they seal in either direction. They can be used as either packings between moving parts or gaskets between parts having no relative motion.
ornithopter. A type of flying machine that has been proposed since the days of Leonardo daVinci (around 1500 AD), but has never been built in a practical form. An ornithopter is supposed to support itself in the air by moving the air with flapping wings.
orographic. A term pertaining to mountains or anything caused by mountains. Examples are orographic clouds, orographic lift, and orographic precipitation.
oronasal oxygen mask. A type of oxygen mask that covers only the mouth and nose of the wearer. An oronasal mask is distinguished from the full face mask, which covers the entire face as well as the eyes of the person wearing it.
orthographic projection (graphics). A method of showing all the sides of an object by projecting straight lines from the object to the drawing surface. An orthographic projection of a solid object requires six separate views: top, bottom, right and left sides, front, and back.
oscillate. To swing back and forth. A pendulum is useful as a timing device because it oscillates with a definite period, or length of time required for each cycle of oscillation.
oscillator (electronic circuit). An electronic circuit that converts direct current into alternating current. The component values in an oscillator can be chosen to produce AC with any desired frequency and with any desired waveform.
oscillograph. An electronic instrument that produces a permanent record of the values of electrical current as it changes with time. Transducers produce electrical signals that relate to the mechanical movement being measured. These signals are recorded on a time base in such a way that they produce a graphic record of the way the mechanical movement changes with reference to time.
oscilloscope. An electrical measuring instrument that displays, on the face of a cathode-ray tube, the way an electrical value changes with time. A high-velocity beam of electrons strikes a phosphorescent coating on the inside of the cathode-ray tube face and produces a visible glow. The beam is swept from side to side by a time-controlled oscillator and deflected up and down by the electrical value being measured.
osmium. A bluish-white, hard, metallic chemical element. Osmium’s symbol is Os, its atomic number is 76, and its atomic weight is 190.2. Osmium is used as a hardener for platinum to increase its wearing ability.
Otto thermodynamic cycle. The constant-volume cycle of energy transformation used in a reciprocating engine. Fuel and air are drawn into the cylinder as the piston moves to the bottom of its stroke. The fuel-air mixture is then compressed as the piston moves upward in the cylinder, and when the piston is near the top of its stroke, the mixture is ignited and burns. The burning mixture heats and expands the air inside the cylinder and forces the piston down, performing useful work. The piston then moves back up, forcing the burned gases out of the cylinder. The pressure inside the cylinder varies throughout the cycle, but all events take place in the same location.