Motion Picture News (Apr - Jun 1927)

1862 Motion Picture N eiv s iO\ Project! or) ics^Electrlcityfiddkallde^ Inquiries and Comments Some Practical Considerations of the High Intensity Arc Part II HERE appear to be no small number of projectionists who entertain the idea that a high intensity arc possesses the same operating characteristics as the low intensity type and can, therefore, be subjected to the same flexible regulation, as regards current, which is ordinarily resorted to in the case of the old style arc. In spite of the fact that there are available only four sizes of high intensity positive carbons, on each of which the proper operating current is plainly marked, instances have been noted wherein the particular electrode being used was operated (■ither under its rated current value or above it. Appreciable differences existed between the rated and operating current values in all the cases observed, such as, for example, the operation of a 120 ampere electrode on 100 amperes; the use of a 75 ampere electrode on 100 amperes; and the deliberate use of 170 amperes in connection with a 16 mm. 150 ampere carbon. Strange to say, most of these errors were deliberate ones and resulted, in some cases, from ignorance on the part of the projectionist, and in others from a mistaken idea that additional illumination could be obtained by working a given size of electrode much in excess of its proper current value. ''Boosting'' the High Intensity Effect In one prominent Broadway house, the chief projectionist labored under the impression that the use of 170 amperes in <^onneetion with a 16 mm. 150 ampere carbon would give him greatly increased screen illumination — believing that by so doing he could "step things up" in a fashion similar to what he had been accustomed to with the old style of arc. The result of these efforts to force the carbons (already loaded to a high current density to obtain the high intensity effect) nearly 20 per cent over their rated current capacity was a "dark" screen as often as twice during a single performance since the overloaded crater naturally tried to adjust itself to this abnormal condition without success. It was pointed out to this projectionist that he stood no chance of obtaining appreciably greater screen illumination by so over-working the electrodes for the reason that the tcmiierature at which the rare earth fluorides contained in the case vaporized, and was fixed so that the brightness of the incandescent flame (gas ball) could not be raised above its normal value. Nor could a larger crater be established by the excess current since the normal size of the crater established by the proper rated current was of such a size as to constitute nearly the entire end section of the carbon. The net result, therefore, was unstable arc operation with no material increase in screen light. Carbons Designed for One Current Value It is necessary to remember that high intensity positive electrodes are designed for a particular current and will deliver their best results only when operated on hte current for which they were made. Thus, the proper size for 50 amperes is 9 mm. ; for 75 amperes, 11 mm. ; 120 amperes, 13.6 mm. ; 150 amperes, 16 mm. While no additional illumination is obtained by operating them over-amperage, the effect of running them under their rated current value, as pointed out last week, is to entail an appreciable loss in screen light, which increases rapidly as the under-loading of the carbon is increased. Thus, in those theatres using 120 ampere electrodes on 100 amperes, as mentioned earlier in this article, the loss in illumination would be of the order of 40 per cent. Even a reduction of 10 amperes involves a light loss of 20 per cent and the editor has been in any number of theatres where under-loading of the electrode to the extent of 10 amperes was considered hitting close enough to the 120 ampere rating; which goes to illustrate the attitude taken by many I)ro,jectionists that the manufacturers' rating on the carbon is placed there more as a matter of policy than something to be seriously considered. It is true that in some eases voltage conditions are such that the full rated current is imjiossible to obtain but the easy tolerance of such grossly inefficient conditions should not be so lightly overlooked by any chief projectionist worthy of the name. Use Next Smaller Size It would be better, in cases where the difference between rated current and that actually obtained was great, to change to the next lower size of carbon, thereby obtaining probably the same screen illumination with a great reduction in current. As an illustration, suppose the 120 am pere size of electrode was being used and it was found that 100 amperes was the most which could be obtained. Where the conditions permitted, it would be more economical to switch to the 75 ampere electrode, thus taking an additional 5 per cent loss in light and saving approximately 2.5 kilowatts per hour. Or, if it was necessary to obtain greater illumination, then the low voltage condition should be corrected and not alloAved to stand. It is interesting to note, in this respect, that a 75 ampere electrode operating at its rated current will deliver practically the same amount of screen light as will a 120 ampere carbon operated at 100 amperes. Use Care in Trimming Carbons In the high intensity arc, the real source of light is the incandescent gas ball contained in the crater proper so that anything which tends to disturb this gas has an immediate and direct effect upon the screen illumination. It is essential, therefore, to maintain this gas ball steadily at all times and it is also quite necessary to keep the crater well filled with gas from top to bottom. Since it is naural for the intensely hot gases to rise and spill out over the top of the crater, the various forces present in this type of lamp are used to ])revent this spilling of the gas beforr the crater is completely filled. This is accomplished by so directing the force of the gas stream issuing from the negative carbon as to cause it to strike the crater just above the axis of the positive carbon with the result that the crater gas is packed into the crater, completely filling it and thus maintaining a good source of light from crater edge to crater edge. Some projectionists, in their haste to trim the lamp, frequently advance the positive too far forward so that the gas stream from the negative strikes the crater hclow its center. The result of this misadjustment is immediately evidenced by a light cloud of black soot which is pure carbon formed by incomplete combustion of the core ingredients. This is caused by the negative gas stream violently "scooping" out the crater gases before they are completely consumed. This action is immediately evidenced in the screen intensity, which drops quite sharj)ly. The remedy for this is to pull back the positive electrode slowly until the screen illumination again reaches its maximum value — at the same time making the arc more steady in operation. The intersection of the negative axii with the crater should be carefully watched as automatic feeding progresses, in order to determine whether or not the feeding is proper at all times.