Motion Picture News (Jul-Aug 1923)

August 2 5, i 9 2 3 Simplified Electricity for Projectionists (Cntinued from page 928) automatic apparatus, is often overlooked by the projectionists who "gives her the juice" and " freezes " the carbons, is the proper voltage for the arc in order to maintain it. There is a formula, by means of which one, providing he has the proper laboratory equipment and time at his disposal, can determine the correct voltage. It is given only to show the projectionist, in a very general manner, what causes his arc to " nutter " and so forth. This formula is r = m + n 1 where r is the "apparent resistance" ; m and n constants which hold only while the arc is steady (constant current) and I is the arc length. From this formula it can be seen that as long as the arc is steady the " resistance " of the arc varies directly as the distance between electrodes, meaning that as the electrodes are pulled farther apart the resistance will increase and as the electrodes are moved closer together the " resistance " will decrease. Of course, any change in its resistance will also be reflected in a corresponding change in the amperage of the arc so that if the resistance is increased the current will decrease and vice versa. Such a change, of course, will mean a 55 1 1 1 1 1 1 1 1 1 io zo so 4-o So 6c 7o Go Figure 92 change in the constants m and n which as pointed out before, only hold for any particular case while the current is constant. The exact value of these constants, by the way, was and still is, the subject of much discussion among scientists, so no effort will be made to give them. The work is much complicated since very rarely will two carbons give exactly the same results. It is interesting to note in this connection, how the voltage of an arc performs when it is once started and left to burn. According to Mrs. Ayrton's " Electric Arc," the voltage of an arc, between carbon electrodes, which had been mechanically formed before the arc was struck, first was about 30, then rising gradually to 50 in 10 minutes' time, remaining constant for about 20 minutes and gradually simmering down to 45 volts (steady state) in 80 minutes or over one hour. This is shown in detail in Fig. 92 which shows variation of voltage with time. Even when the arc is steadv (constant current) a slight change in current will require several minutes for the arcto again become steady. So it can be safely said that the arc as used for projection purposes is never steady. Stability of Arc When an arc is supplied with power from a constant source of current such as a constant (Continued on next page) 931 Immediate Profit Successful theaters everywhere have the same experience: immediate profit ' comes from installing the right lighting control system. Patrons more thoroughly enjoy the performance that is attended by sumptuous color effects and therefore they come back oftener! Perfection in controlled theater lighting that sums up the Major System. It is proved by the experience of more than 100 of the country's largest and most successful theaters. The Major System is extremely flexible in installation, as well as operation, and can be adapted to any theater, being composed of standardized units, it is capable of extended remote control for any desired number of points and with any desired grouping of lights. It can not injure the electrician, or anyone else, neither can it suffer damage through careless handling. The Major System is the only one of its kind in existence. When you are ready to install your lighting control system, there is only one to consider. Write for the complete and practical book, " The Control of Lighting in Theaters." It is a publication full of useful information for every theater manager. 3*rank oidam ELECTRIC COMPANY ST LOUIS Oe roit. New York, Dallas. Minneapolis. Kansas City. District Offices: Cincinnati, Cleveland, New Orleans. Chicago, San Francisco, Los Angeles, Seattle, Boston. Pittsburgh, Philadelphia. McVicker's Theater Chicago Thomas W Lamb Architect Henry L. Ncwhousc Resident Architect MAJOR SYSTEM ADVANTAGES: Remote Control Extended Remote Control Remote Dimmer Control Cumulative Control Pre-selection Flashless, noiseless switch operation Minimum stage space Unit construction Experienced design Perfect workmanship Highest grade materials Consult the Equipment Dept. of the "News" when buying THEATRE CONSTRUCTION Consultations and advice on all classes of theatre projects, from inception to completion. Financial assistance. Send us your proposition for analysis H. ROBINS BURROUGHS 70 East 45th St. Consulting Engineer New York Economical and Efficient Equipment for Theatre COOLING and VENTILATING Write for Particulars James M. Seymour Lawrence St. Newark, N. J. Est. 1886