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BACKLASH, Acoustic: A direct and distinct echo from a nearby smooth surface, and often noticeable when a loud-speaker is placed in front of a hard smooth wall.
BACKLASH, Mechanical: Another name for lost motion, especially in relation to the projector shutter blades.
BALANCE RESISTANCE: A resistance made to take the electrical load from a projector lamp when the latter is switched off. The use of a balance resistance in a projector without automatic speed-control results in the motor continuing its normal running speed, which in some cases would be considerably and suddenly increased were the voltage used by the lamp suddenly diverted to the motor. The balance resistance is also a necessity with certain generators.
BALLAST LAMP: An expensive and incorrect method of reducing current to the amount required for a specific purpose. A transformer or resistance should be used instead. The ballast lamp, usually of high wattage, is connected in series with the equipment.
BINDING POST: An American term used to denote a terminal.
BLATTNER PHONE: A system for recording sound by variable magnetism upon a length of steel tape or wire.
BLOOPING PATCH: The black patch formed after painting over the sound track at the point of a newly-made join. Failure to carry out this necessary part of splicing results in a noisy "plop" each time the join passes through the sound-head.
BLOWING A LAMP OR VALVE: The actual burning of the filaments, due to age, excess voltage or perhaps carelessness.
BLOWING POINT: The point at which a certain amperage will cause the weakest part of a circuit to melt, such as a fuse wire or the filaments of a lamp.
B.O.T. UNIT: Board of Trade Unit of electrical energy equal to one Kilowatt-hour. (Kilowatt is 1,000 watts.)
B.Th.U.: British thermal unit. The amount of heat required to raise the temperature of i lb. of water by i degree F.
BY-PASS CONDENSER: In an amplifier, a condenser used to separate A.C. and D.C. in a circuit. Usually of the fixed capacity type, and so arranged that it "by-passes" certain frequencies which would otherwise pass through the circuit across which it is placed.
CANDLE POWER: A unit of illumination, based upon the amount of light produced by a single standard candle, and measured at a distance of one foot from the centre of the flame. Illumination is thus measured in terms of foot candles.
CAPACITANCE OR CAPACITY: In a condenser the amount or measure of an electrical charge it is capable of holding. Capacity is measured, in relation to the initial voltage supplied, by microfarads.
CAPACITY: In a fuse or circuit, the amount of current it is designed to carry.
CELLULOID: The hard transparent base of standard film, highly inflammable.
CHECK SPEAKER: The monitor speaker, used in the projection room for the benefit of operators, enabling them to judge the sound within the auditorium.
CHANGE-OVER: Changing from one projector to another during a show.
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