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38 HANDBOOK OF PROJECTION FOR
PORT. — In projection, an opening in the front wall of the projection room.
POSITIVE. — As applied to photography, a "print" from a negative. The films used in projection are positive prints.
POSITIVE BRUSHES.— The commutator brushes of a dynamo or motor which connect with the positive wire of the circuit.
POSITIVE CARBON.— In a D. C. arc lamp the upper carbon ; the carbon to which the positive wire of the circuit is attached.
POSITIVE PRINT.— Film exposed to light passing through a negative. The film used in projection is a "positive" print.
POSITIVE POLE.— The positive (+) terminal of a generator from which the current is assumed to flow out to the external circuit.
POSITIVE WIRE.— The wire connected to the positive pole of an electric generator and charged with positive (+) E. M. F.
POWER. — The rate at which work is done, meaning work divided by the time in which it is done. The generally accepted unit is the horsepower, which is 33,000 foot pounds a minute. See "Foot Pound."
PRIMARY COIL. — In a transformer, a coil consisting of many turns of insulated copper wire wound around one "leg" of an iron core, or placed within a laminated iron core. In effect it is a powerful choke coil, its practical purpose being V.o create a magnetic field in order that a secondary current may be induced in a secondary coil placed within the magnetic field thus created, the voltage of which latter will be dependent upon the relative number of turns of wire in the two coils.
PRIMARY CURRENT.— The current in the primary coil of a transformer.
PROJECTION ANGLE.— See page 255.
PROJECTION DISTANCE.— Distance from projection lens to screen. Commonly referred to as "throw."
PROJECTION LENS.— The lens combination which forms the image upon the screen. The lens of a projector optical train corresponding to the objective in a camera. Also termed "projection objective."
PROJECTION SPEED.— The speed at which the film is projected, expressed in feet a minute.