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OHM: An electrical unit for measuring resistance.
OPTICS: The elements of an optical system, such as the lenses, prisms, and glass rods.
OVER-SHOOTING: A fault in projection, in which the picture-frame on the film tends to over-shoot the frame of the aperture. It is usually corrected by the operator using the Framer, or Framing Knob.
OVERLOADING: Using an electrical circuit for more current than it is designed to carry. Overloading will cause the weakest part of the circuit to over-heat and eventually burn or melt. This part should be the fuse.
PARALLEL WIRING: If two or more lamps are each connected across a circuit they are said to be in parallel. "Across a circuit" implies that both wires are connected to each lamp in turn.
PERFORATIONS: The holes in cine film, which engage with the teeth of the projector sprocket-wheels.
PHOTO-ELECTRIC CELL: A device which reacts electrically to light directed upon it, and is usually shaped like a small valve. It is used for many other purposes besides sound recording and reproduction. It is used to open and close doors, check trains in and out of stations, operate burglar alarms, count people, and switch the street lamps on and off.
PHOTOMETER: A device for measuring the intensity of light.
PICK-UP: The needle-holder and arm of an electric gramophone, which converts the impulses from the needle into electrical impulses.
PITCH: The "tone" in music, i.e., if the lower audio-frequencies dominate the higher frequencies, the tone will be low pitched. The tone is usually controlled by a handy knob on the amplifier, and which should be used by the operator to keep the pitch even, or in accord with the acoustics of the hall.
POTENTIAL: A specific pressure or voltage of an electric current.
POTENTIOMETER: The device actuated by the volume control. It consists of a number of tappings of varying potential, and the volume is thus controlled by varying the amount of voltage to the output valves.
PROJECTIONIST: A professional "operator." One who makes the projection of moving pictures his living.
POSITIVE PRINT: A copy taken from the negative film: the film used in projection. Also known as a release print. A reversal print is the original film which has been developed in such a way as to give a print directly.
RAIN: In projection, the vertical black lines which appear on the screen, and which are caused by scratches on the film. The scratches are quickly filled by dirt, and the resultant black lines are magnified with the rest of the picture.
RECTIFIER: Any apparatus or valve used to convert A.C. to D.C.
REEL: A 400' length of 16-m.m. film. In America, the spool on which the film is wound.
REFLECTOR: A highly polished disc with a concave surface. It is positioned within the lamphouse to collect and converge the rays of light from the lamp towards the aperture. The surface of a reflector may be either aspherical, parabolic or elliptical — something to remember when ordering replacements.
REFRACTION: A change in direction of light-rays after passing through a lens or other medium. Sound-waves may also be refracted.
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