Projection engineering (Jan-Dec 1931)

Page 16 PROJECTION ENGINEERING Answers to examination questions IN the October issue of Projection Engineering were presented the first ten of a series of questions intended as a month to month examination for projectionists. The answers to the first ten questions were published in the November issue, and in that issue appeared also the second set of ten questions. Herewith are presented the answers to questions Nos. 11 to 20: 11. When white light is passed through a spherical lens, both refraction and dispersion (the decomposition of white light into several kinds of light) occur. This causes a separation of the white light into the various colors, and causes images to have colored edges. This effect, which is most observable in condenser lenses, is due to the unequal refrangibility of the simple colors. 12. The distance from a point half way between the front and back combination of lenses to the film in the gate of projector while the film is in focus on the screen, is the equivalent focus. This can be obtained by measuring the distance between the front and back combination, then dividing by two and adding the result to the back focal length. The equivalent focus of a plurality of lenses in combination is the focal length of a simple lens, which will under all conditions form an image having the same magnification as will the given lens combination. 13. Lens loose in lens holder, dirty lens, buckling of film, incorrect film tension in gate of projector. 14. Sound is a wave motion. Wavelength is equal to velocity divided by the number of vibrations per second. 15. Sound waves, unlike light waves, do not travel in straight lines. 16. Sound does not travel at the same speed in different media, as a matter of fact an increase of speed will be shown with an increase of temperature. Sound waves travel at the rate of 1087 feet per second at 0° C, or 1126 feet per second at 20° C. Sound cannot be transferred through a vacuum, and in this respect differs from light and heat. 17. Ohm's Law is the fundamental principle on which most of the electrical mathematics are worked. Given two known quantities it is an easy matter to find the third unknown quantity. To find the amount of current, divide the voltage by the resistance, or to find the resistance, divide the voltage by the current, or to find the voltage multiply the resistance in ohms by the current in amperes. 18. A watt is the electrical unit of power. It is equal to one joule per second ; that is, when one joule of work is expended in one second, the power is one watt. The power is obtained by multiplying the current by the voltage. The watt is sometimes referred to as the volt-ampere. A kilowatt is 1,000 watts, the common abbreviation for kilowatt is kw. The kilowatt-hour is a unit of energy, and is the energy expended in one hour when the power is one kilowatt. The volt is the practical unit of electromotive force. The volt is equal to the electromotive force which, when applied to a conductor having a resistance of one ohm, will produce in it a current of one ampere. The coming together of the two opposite polarities in an electrical system without any controlling device. 19. Reverberation is the persistence of sound in an enclosed space. It is measured by the time required for the sound to decrease to one millionth of the initial intensity. Sound waves travel by air 1,100 feet per second. Sound generated in a room proceeds outward in spherical waves from the source to the walls, where it is reflected, absorbed and transmitted. The reflected sound proceeds to a second wall, where it is again reflected, absorbed, transmitted and so on, experiencing from 200 to 300 reflections to an average sound. The greater the dimensions of the room or hall, the more prolonged will be the reverberations, other things being equal. 20. The control of apparatus from a point some distance removed therefrom, as, for instance, a motor generator located in a basement may be started, stopped and controlled from the projection room. AAA EDISON'S LAST HONOR CONFERRED BY MOTION PICTURE ENGINEERS The last honor bestowed upon the late Thomas A. Edison was that of honorary membership in the Society of Motion Picture Engineers ; and this final tribute to the great electrical genius given during his lifetime was part of an important ceremony which took place recently at the New Ocean House at Swampscott, Mass. In this connection Mrs. Edison requested a full story of the event, and Mr. Clement E. Kennedy, president of the New Ocean House, was able to furnish one of the menus used at the banquet of the S. M. P. E. A NEW YORK AND CHICAGO SECTIONS OF S.M.P.E. ELECT OFFICERS Officers for both the New York and Chicago Sections of the Society of Motion Picture Engineers have been elected for the ensuing year. In the New York Section, P. H. Evans of Warner Brothers has been elected chairman; Donald E. Hyndman, Eastman Kodak Company, secretarytreasurer ; and the managers elected are M. C. Batsel, RCA-Photophone, and J. L. Spence of the Akeley Camera Company. For the Chicago Section, the officers are: R. Fawn Mitchell, chairman; Burton W. Depue, secretary; and Robert P. Burns and Oscar B. Depue, governors. A PHOTOPHONE RECORDING FAR AHEAD IN ENGLAND AMERICAN-MADE sound apparatus may be credited with having maintained constantly increasing activities in the studios of England, according to a report received by Van Ness Philip, manager of the foreign department of RCA-Photophone, Inc. With fourteen pictures as the high peak in production for the summer period, twelve of which were features and two of which were comedies, ten of the features and both of the comedies were re corded by the RCA-Photophone system. English studios which employ the Photophone system of recording are British International, British Lion, Gainsborough, Gaumont, First National Pathe, Ltd., Mansfield Markham, ARP (Dean) Studios, Teddington Film Studios, which are operated as a Warner Bros, subsidiary, Twickenham, Ltd., and Nettlefold Productions. A METRIC PREFIXES COMMONLY USED WITH ELECTRICAL QUANTITIES Meg-or Mega = 1,000,000 = 10" Myria = 10,000 = 104 Kilo = 1,000 =103 Hecto = 100 =10a Deka = 10 = 101 Deci =.1 = 10"' Centi =.01 = 10a Milli =.001 = 10s Micro =.000,001 = 106 Milli-micro =.000,000,001 = 10B Micro-micro =.000,000,000,001 = 1012 The prefix "pico-" is sometimes used in place of "micro-micro-"