Motion Picture News (Apr - Jun 1927)

1978 Motion Picture N ezvs Project! or) !iesJkctrieity,Prddicd Ideas ^ advj Inquiries and Comments Some Practical Considerations of the High Intensity Arc Part III X view of the general activity among manufacturers, it would seem logical to believe that a high intensity arc, constructed on the mirror arc principle, is a tiling of the near future. At Ipast one lamp of this type has already been tried out in theatres, and although it represented but little more than the first crude stages of evolution, it is quite likely that the difficulties which now suiTound the problem will be cleared up to the point where the device will become a practical reality. There is some question, however, as to whether such a lamp is at all required. From a purely economical standpoint it is, since it is hoped, by means of it, to obtain, with approximately 50 amperes, screen results which are comparable to those now produced by the present type of lamp operating at 150 amperes. A reduction to 50 amperes of the current required to operate the high intensity lamp at its fullest advantage would certainly be desirable if for no other reason than that it would place all light projectors now used for motion picture projection within the same current field instead of having this field range from 25 amperes to 150 amperes as is the case at present. Effect on Film The item of operating economy is not, however, the only thing to be considered in this connection, for it is possible, and, indeed, even i)rol)able, that this economy may be obtained at the expense of the film so that the resultant savings in cost of current would be counteracted by an increased rate of destruction of film. The same thing was true of the reflector are using low intensity carbons and there was, for a time, considerable agitation in certain quarters to curb the use of these lamps. Lately, however, little has been said of this. Just what effect a reflector arc higii intensity lanij) will have upon film is largely a matter of conjecture and must remain no for some time, even should this principle be found practical, since the results of even the present mirror arc in this respect are not yet clearly understood in spite of the fact that this lamp has now been in use for some four or five years. Danger of ''Over-Shooting" In the meantime, theatres must continue the use of the present type of lamp, which is satisfactory in all respects but that of current required for operation. There is another angle to this problem of the reHector are high intensity lamp and past experience leaves no doubt but that a recurrence of the over-lighting of screen will take place, following its availability, such as haj)pened when the old reflector arc was brought out. The conditions in the case of the widespread adoption of the high intensity form, however, will be so much worse since this source then will be made available for even the small theatres, which would have no more use for such an outfit than they would for a triple console organ. The Logical Field It was pointed out in another article that some theatres are reaching for still higher screen intensities for the reason that their long projection distances made it difTficult to obtain the same level of screen illumination as is enjoyed by many smaller ones. There is good reason to believe that a reflector arc high intensity lamp Avill assist these theatres by enabling them to obtain the maximum illumination possible with the high intensity source and here would seem to be the logical field for the new lamp. This field, although now somewhat restricted, is rapidly becoming larger, due to the trend toward de luxe houses in all large cities. Attention also was called to the fact that the projectionists in some of these large houses were overloading the carbons in a mistaken effort to force them and so obtain more illumination and that the ordinary result of such efforts was a dark screen of frequent intervals. It is necessary to bear in mind that high intensity electrodes are designed for one specific current and should be operated neither above nor below the rated current capacity. When efforts are made to force them much above the proper current, the arc becomes increasingly unsteady as the current is raised. This unsteadiness is caused, principally, by the rapid boiling of the gas in the crater, which causes it to swirl about. Since the gas i-eally constitutes the source of light, any marked unsteadiness of the gas will naturally be evident upon the screen. Magnetic Action in Arc The most serious effect, hoAvever, comes from the magnetic action of the arc and it is the joint action of two magnetic field, present in the arc when it is in operation, that is resjxjnsible for the snuffing of the One of these magnetic field surrounds the carbons when cuiTent is flowing through them, in the same manner that a magnetic field is established around a wire eaiTving current. The second field is the result of the arc stream and the combined action of the two fields is to tend to force the arc stream upward so as to break it. This action is the same as that in a magnetic arc snuffer as used on large circuit breakers to prevent the establishment of a continuous arc. Now the intensity of this ujjward thrust will depend upon the strength of the magnetic field and the amount of current flowing through the arc. The intensity of the field is also proportional to the current so that the intensity of the force which tends to })reak the arc will increase as the square of the current. Thus, if the current is doubled the arc-breaking force will be increased four times. A relatively small increase in cuiTent, therefore, will cause a large force to be exerted on the arc stream. In the ease of a 150 ampere arc, using a 16 mm. electrode, an increase of 20 amperes (to 170 amperes) is sufficient to snuff out the arc as was experienced in one large Broadway house. Incidentally we have since learned that the manufacturer of the lamps used in this theatre was blamed for this failure of the arcs, as it was believed by parties who, presumably, should have known better, that the fault lay in the lamps rather than with the pi'ojectionist. The principal thing to remember in the case of all high intensity lamps is that the electrodes are constructed for one, and only one, specific current and if the particular electrode is operated below its rated current the arc Avill decrease in steadiness and the screen illumination will drop. The loss of light will increase rapidly as the current is lowered. If, on the other hand, the electrode is operated above its rated current, the arc will become unsteady, no increase in screen illumination will be obtained and if the current is raised too much the arc will be frequently wiped out. National Theatre Supply Reduces Prices That the National Theatre Supply Company is living up to its avowed policy of making every possible reduction of prices on theatre equipment and supplies is evidenced in a statement issued last week by H. A. R. Dutton, presid(>nt of the organization. In pointing to a numl)er of reductions recently made in general supplies, Mr. Dutton attributes this move to economically correct principles enabling attractive prices through quantity buying power. It is further said that the treiul of prices of the smaller items of equipment and sui)uli<'= is also downward.