Exhibitor's Trade Review (Aug-Nov 1925)

38 Exhibitors Trade Review THEATRE EQUIPMENT AND ACCESSORIES News and Facts Bearing on the Design and Improvement of the House and Its Facilities NEW TRANSVERTER MODELS Increasing Popularity of Mirror or Reflector Arcs Necessitates Specially Designed Transverter Sets Conserving Space and Supplying Proper Current Control THE use of the electric arc for motion picture projection has made another great advance with the adoption in this country of the mirror arc. The mirror or reflector arc, or as sometimes called the low intensity arc, is not new but has been used considerably in Europe for quite some time, being nothing but the adaptation of the search light construction to the lamphouse and projector. There have been a number of makers, particularly in Germany and France, whose lamps were available and could have been bought and used but nothing happened until about three years ago when interest became aroused. Today there are a number of such lamps made in the United States and rumors of others. The horizontal position of the arc, while it permits the use of a mirror to gather the light at the best position facing the crater, also leads to a more sensitive condition of the arc stream and unless the proper sizes of carbons are used, so that they are neither underloaded nor overloaded, the arc is more likely to be unsteady than if in the vertical position. The arc cannot be drawn out as long as the vertical arc, hence the voltage must be kept more nearly normal. For use with these lamps, it is necessary to have a source of current that will, at least, not tend to increase the sensitivity of the arc, and every effort must be made in the design of the generator to produce such results. In the Transverter it has been the aim of the designers to produce a machine that will deliver the current with the greatest degree of stability and no pains are spared to achieve this end, either in workmanship o r material. The two units intended for operation of the mirror arc are the double F,S 1 30 and double 20, The "Double 30," ver the former operatt i c a 1 construction ing at from 30 to Transverter Unit, oper 20 amperes, and ating at from 30 to 20 the latter from 20 amperes. to 10 amperes. By John H. Hertner President Hertner Electric Company This covers the usual range of such arcs. These sets are designed along the lines of the larger units except that on account of their shorter length a close coupled two bearing mounting has been adopted, the bearings as in the case of the larger generators being Hess Brights, and the standard the vertical type. The machines are built for horizontal operation only when so specified. The electrical design follows the general lines of that of the larger units, being a bipolar interpoled generator driven by an rHE Hertner Electric Company of Cleveland, O., is known throughout the country for the excellence of its Projection Room Equipment. They are now among the first to recognize the growing importance of the horizontal carbon arc in motion picture projection and have specially designed "The Double 30" and "Double 20" Transverter Units for operation of the Mirror Arc. induction motor. The latter is designed to give as near as possible constant speed characteristics so as to eliminate the effect of line voltage fluctuations and to maintain its speed under heavy overload such as can occur during the change-over period if the arcs are permitted to draw out unusually long. '"PHE generator is not differential compound wound, but develops its constant current performance by proper rocking of the brushes. Regulation, produced in this manner, is apparently closer and more prompt when the force that maintains the constancy of the current is located at the point where the disturbances originate. In machines designed for the usual constant potential operation, it is sought to make the effect of armature reaction as small as possible so as to disturb the field as little as can be and to promote satisfactory commutating conditions. A multipolar construction is generally used, as in it, the effect of the armature on the field is reduced since it is split between a larger number of poles while the magnetic strength of each field coil is practically the same as though the generator were bipolar, hence a minimum of field distortion with increasing load. IN the bipolar generator the armature resistance being a maximum, advantage can be taken of this fact by using a bipolar design where this reaction can be put to use and in a machine of constant current characteristics the armature reaction can be employed in maintaining constancy of current by limiting and decreasing the increase of field strength a . the voltage increases. In any generator of this kind, the field strength at ordinary operation is very low, the generator having an open circuit voltage of from three to four times its single arc working voltage and the magnetic strength is in proportion so that good commutation could hardly be expected without the use of interpoles. In many houses the projection room is very badly crowded and all efforts must be made to conserve space. With this in mind, the earlier machines were all built vertically but with the more modern theatre the room allotted to the projection machinery has become more generous in size. The cut, Fig. 1, is of the vertical construction, being a double 30. The unit is built to suit the A. C. supply for any commercial frequency voltage or phase. Fig. 2 The "Double 30," built horizontally. The close coupled construction requires no sub-base for the maintenance of alignment. Fig. 2 shows the same unit built horizontally. It will be noted that this close coupled construction requires no subbase for the maintenance of alignment. From all appearances the future of the mirror lamp is secure. The American manufacturers are turning out a product that assures correct working from a mechanical standpoint and all indications point to a continual decrease of the use of the standard vertical arc and of the incandescent lamp in the projector, and a rapid increase of the use of the mirror arc.