Handbook of projection for theatre managers and motion picture projectionists ([1922])

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26 HANDBOOK OF PROJECTION FOR CRITICAL ANGLE.— The angle of incidence beyond which rays of light are no longer refracted into a transparent medium, but are reflected from its surface. CYCLE. — A series of operations. As applied to A. C, the cycle is two complete alternations. DIMMER. — An adjustable resistance inserted in an incandescent circuit by the manipulation of which the lights of the circuit may be gradually dimmed or brightened. DIOPTER. — The unit for expressing the refractive power of a lens. It is the power of a lens whose focal length is one meter. DIRECT CURRENT (D. C.).— A current constant in direction, though not necessarily in value. A direct current constant both in direction and value is called a continuous current. Direct Current, which, while continuous in direction, pulsates as to pressure, is often wrongly called continuous current. DIRECT CURRENT CONVERTER.— A machine for converting D. C. of one voltage to D. C. of a different voltage. DISSOLVE. — The gradual transition or fading of one projected image into another. DIVERGING BEAM.— A light beam which diverges away from its immediate source. DOUBLE THROW SWITCH.— A knife switch which may be thrown over into either of two sets of contacts. D. P. SWITCH.— Abbreviation for double pole switch. DEGREE. — A unit of measurement of temperature. DEGREE. — The circumference of every circle is divided into 360 equal parts called degrees, hence a degree is l/360th part of the distance around the circumference of any circle. It is, therefore, evident that with every increase or decrease in circle diameter the linear measurement of a degree changes, insofar as applies to that particular circle. Each degree is divided into sixty equal parts, called minutes, and each minute is divided into sixty seconds. DENSITY OF FIELD.— The quantity of electromagnetic lines of force existing in a unit of cross section area of an electro-magnetic field. DETERIORATION OF INCANDESCENT LAMP.— The decrease in candle power of an incandescent lamp which takes place after prolonged use. DIFFERENTIAL WINDING OF FIELD.— A method of winding a field magnet with double coils in such a way that each exerts a pull against the other.