The motion picture projectionist (Nov 1931-Jan 1933)

42 Motion Picture Projectionist December, 1931 Good Projection Requires Good Rectification M. P. 25-25 Good Rectification Means Forest Rectifiers THIS Forest Rectifier meets the demand for a single unit to supply direct current for two projectors, and will furnish 15 to 25 amperes to either projector continuously. It supplies a steady direct current, free from pulsations, and will produce a better light than other current supply devices. The only wearing parts are the bulbs which will last at least one thousand hours and usually much longer since only two bulbs are being used at a time (except during change over) and the load is alternately carried first by one set of two tubes, then the other two, as the projectors are alternately used. This Forest Rectifier embodies the use of four rectifier tubes which are connected to supply current to two direct current circuits independent of each other, thus preventing loss of current at the first arc when the second arc is struck. Both arcs can be operated at the same time during the change over period and there will be no diminishing of the light i from one projector while lighting up the second. Two Ammeters are mounted on the unit which will show at a glance the amperage being used at either arc. Links are provided for operating from 110-220 or 240 volts. Rectifiers for all purposes made in 15 amp., 25-25 amps., 30, and 65 amps, sizes. We are specialists in our field. Send us your technical problems. Literature on request. Forest Electric Corp. New and Wilsey Sts. Newark New Jersey tirely the fault of the set but may be due to excessive line drop. Taking up this last item first. Suppose the generator is located several hundred feet away from the projectors. The bus lines are led into a panel from which the several circuits lead to the respective projectors and their ballasts. Allowable Line Drop It is obvious that any drop in the line between generator and panel will affect both lamps and if this circuit is long, even though the Underwriter's rules as to wire sizes are followed, there is a noticeable amount of line drop. In such cases a very marked improvement will be made if separate lines are run to each projector from the generator. In such case the resistance of these lines becomes part of the ballast and the smallest wire size permitted by the Underwriter's will suffice. If the fault is in the generator it may be due to any one or a combination of several contributing causes. The motor may have such a high squirrel cage resistance that it drops its speed on overload. The generator may be under compounded. The whole set may have been chosen too small for the load it is to carry. The line voltage may be too low to suit the winding of the motor. The line drop in the supply circuit may be too great so that when the overload comes on and the motor demands full voltage, it is actually getting a voltage too greatly reduced. This condition is mentioned in particular because there have been cases where standard 220 volt machines have been put on 208 volt lines and while this comes within the 10 per cent latitude permitted under the rules it makes it rather uncomfortable for the motor when called on to deliver a heavy overload and maintain its speed so as to deliver full D.C. voltage at the arc. The matter of voltage ripple is one of eternal vigilance in electrical design. It is only one of the additional items of refinement demanded brought about largely by the addition of sound to the picture. Index of Advertisers INSURE YOUR JOB! Study F. H. Richardson's Handbook of Projection Three volumes containing over 1,400 pages of vital knowledge with question and answer index. Every projectionist needs this authoritative guide. See your local theatre dealer or send for full details. CHALMERS PUB. CO. 516 Fifth Ave., N. Y. Best Devices Co 41 Brenkert Lt. Proj. Co 6 Chalmers Publishing Co 42 Eastman Kodak Co 5 Electrical Research Products, Inc. 3 Fish-Schurman Corp 5 Forest Electric Corp 42 General Machine Co 35 G-M Laboratories, Inc 41 GoldE Mfg. Co 33 Griswold Machine Works 38 Hall & Connolly, Inc 39 Hertner Electric Co 4 Hoffmann-Soons, Inc 40 International Projector Corp Back Cover Kaplan, Sam, Mfg. & Supply Co.. 2 Keasbey & Mattison Co 40 McAuley, J. E. Mfg. Co 8 Movie Supply Co 42 National Carbon Co 9 National Theatre Supply 43 Projection Optics Co 39 RCA Photophone, Inc 7 Roth Bros. & Co 10 Service-On-Sound Corp 36 Strong Electric Corp 35 Telephoto & Television Corp. ... 33 Universal Electric Welding Co. . . 38 Vallen Electrical Co 37 Walker Sound Screen 41 Weber Machine Co 37 Whiting & Davis Co 41 THEATRE EQUIPMENT Q>. s~\ NEW AND USED \ '■ i| Sound Equipment, Moving Picture Machines, Screens, Booths, Opera . Chairs, Spotlights. SterV "'^* eopticons. Film Cabinets. -A J Portable Projectors, M. ("T P. Cameras, Generator II Sets, Reflecting Arc Ca i=3 Lamps, Carbons, Tickets. k/P Mazda Lamps and Sup plies. Projection Machines repaired and overhauled. Repair parts for all makes opera chairs. Attractive 3 x 10 ft. muslin banners, $1.50 ; on paper, 50 cents. We Pay Highest Prices for Used Projection Machines, Opera Chairs, etc. •Evtrythinf for the Theitre— Write for Catalog K. OVIE SUPPLY CO.. 844 So. W«ba«h. ChicMoJg