Motion Picture News (Jan-Feb 1916)

Construction, Equipment and Operation Section of Motion Picture News Everything for Theatre, Exchange and Studio Motion Picture Projection Simplified By J. H. Hallberg 50 — Realizing the importance of exposing both carbon craters at the same time and understanding that this effect can be produced by certain means, will you please explain why the special transformer referred to accomplishes this result. Ans. — Y o u have been told that an ordinary rheostat composed of a resistance unit made from nickelsteel or similar wire, is of such nature that its resistance increases if the current flow through the same is materially increased. In other words, the rheostat easily absorbs and dissipates the power in heat, instead of forcing the power over the shorter arc. P/tiM/iftr *r — when the carbons are brought closer together, so as to lower the voltage across the arc. A rheostat of large enough capacity to reduce the voltage from 110 to 40 volts, which is the usual voltage maintained for an A. C. arc, will easily absorb and dissipate the few extra volts which will be forced upon the resistance when the carbons are put closer together when you try to focus both i if the craters at one and the same time. The effect on the arc is, under this condition, such that the loss of the extra voltage without a consequent and proportionate increase in amperes prevents proper combustion of the carbons. Another effect tending to also reduce the brilliancy of the illumination is caused by the watts at the arc dropping with the least change of resistance in the arc, as would happen in case the core should gas more freely than normally, which will disturb the illumination, causing the arc to lose its intensity with consequent loss of light on the screen. (Copyright, 1916, CO/V£ FIG. 35. With the transformer, however, which is an electromagnetic device, the effect upon the arc when the carbons are brought closer together is entirely different. There is no such cushioning or absorbing effect in the properly designed transformer when the carbons are put closer together, as required when it is desired to reduce the arc voltage from 40 to 30 or 33 volts. As a matter of fact, an arc controlled by a transformer which may pass 50 amperes with 33 volts across the arc, will force the amperes to maybe 55, if the carbons are put closer together, which reduces the arc voltage to 30. This change, you will understand, does not lower the watts across the arc, because the transformer under that condition forces more amperes through it, so that the loss in volts is compensated for by a proportionate increase in amperes, and as a result the effect is to maintain a nearly constant watt consumption in the arc, which in turn keeps the "magnetic blow-out" effect in perfect working condition, allowing the carbon points to be kept very close together without any tendency of the arc to what you might call "die down" or burn the carbon points with a yellow glow. 51— Explain the action or Ans.— Fig. 35 illuseffect of the "magnetic blow t rates a regulation out" effect on the arc. lamp house with the arc lamp, carbon-holders, carbons and asbestos-covered leads, connected to a transformer such as referred t<> in questions No. 48 and No. 49. An analysis of this system, as illustrated in Fig. 35. shows the A. C. line, which in this instance is 110 volts, connected to a set of line fuses which should be of about 30 ampere capacity, with leads to the motion picture machine switch. The other side of the switch is connected to the proper connections on the primary or line side of the transformer. The secondary or lamp winding on the transformer is made of considerably larger wire with a lower number of turns connected to the flexible asbestos-covered cables feeding the current to the carbon-holders and to the upper and lower carbons placed within the lamp house itself. by J. H. Hallberg.)