The Bioscope (Jul-Sep 1931)

September 9, 1931 MODERN CINEMA TECHNIQUE THE BIOSCOPE V HIGH INTENSITY POSITIVES The need for a high intensity of screen illumination became apparent with the introduction of (a) super cinemas, ( b ) Transvox screens and (c) denser films. The term High Intensity was used indifferently to denote the value of the screen illumination or that of the current in the carbons. Since these values are, broadly speaking, co-related, no serious harm is done by the confusion. As far as concerns carbons, the change in cinematography which was brought about by the introduction of the high intensity arc was anal ogous to that effected in transport by the introduction of steam or petrol locomotion. Low Intensity Limits Not necessarily that the high intensity arc completely superseded the low intensity — the horse still has its uses, as many a motorist knows — but that where the conditions of the modern " super talkie ” had to be met the low intensity arc was unequal to the occasion, just as a four-in-hand could not compete with the “ Royal Scot ” with much hope of success. The parallel can even be pushed a little farther, because just as the only point of similarity between the coach and the train is that they both form means of locomotion, so we can say that practically the only point of similarity between the high and the low intensity carbons is that they are both electrodes for the giving of light in an electric arc. In our study of the low intensity positive carbon we remarked that it consisted essentially of a rod of pure carbon, down the axis of which ran a squirted core. The luminous image which was presented to the mirror was the crater formed in the end of the positive carbon owing to the action of the arc. The area of this crater, as we saw, was directly proportional to the current flowing through the arc (excepting always extreme conditions), and it was this fact which set a limit to the application of the low intensity arc, because, apart from questions of intrinsic brilliancy, the optical design of the lamp demanded an optimum size of carbons, any increase in whose diameters was attended with losses, which increased up to the point where they offset any advantage to be gained. The core, too, served merely to centralise the crater in the positive carbon and did not in itself play any important part in the production of a luminous source. Core Composition Moreover, the low intensity positive was clamped at its nether end only, the body of the carbon carried the entire current throughout its length and it was fed into the arc by hand as it consumed away. It was submitted to no torsional or other mechanical stresses, and minute deviations from straightness or variations in diameter, while certainly undesirable, were at least not matters capable of spelling disaster to the arc as a whole. Let us contrast this with the design and the requirements of the high intensity positive carbon. In the first place the shell and the core change places in their relative importance as light-producing sources. From a light LIGHT FROM A DARK SUBJECT— VI. By R. WATKINS PITCH FORD FIG. I Lou) Intensity High Intensity giving point of view the shell of the high intensity positive, which is of pure carbon, achieves but little. Its function is, firstly, to form a protective container for the core, enabling the carbon as a whole to feed satisfactorily through the lamp, and, secondly, to maintain the shape of the gaseous light source which results from the consumption of the core. The core itself is the important factor in light production, and it is, relatively to its shell, very much thicker than in the case of the low intensity positive. (See Fig. I.) As in the case of the low intensity carbon, the H.I. core is intrinsically of a higher conductivity material, in order that the slower burning shell may form the necessary cup for the correct formation of the gaseous light source. But there is a vital difference between the compositions of the two types of core in that the high intensity variety is impregnated with cerium fluoride and other mineral salts. Impregnating With Chemical Salts The expedient of impregnating the cores of arc carbons with salts is not the novelty suggested by the comparatively recent introduction of high intensity operation. The alternating current arc, now rarely encountered in British cinema work, but still used extensively in Japan and other foreign countries, virtually demanded some sort of impregnation on account of the fact that current was being consumed in the formation of two craters simultaneously. Ordinary cored carbons could not, therefore, hope to give an illumination which was FIG. il C Toil Flame Features Good Carbons Should Show economically proportionate to the current consumed, and the impregnated A.C. or “ white flame ” carbon resulted. Carbon manufacturers, recognising the enormous gain in both optical and luminous efficiency which would result, immediately set about the application of this impregnated principle to the direct current arc. However, the task was not so simple as might be supposed, partly because of the problems in actual carbon manufacture which the innovation presented, and partly because of the reluctance of the lamp manufacturers to supply, and of the exhibitors to instal, the special apparatus necessary. Problem of Manufacturer Co-operation It may be remarked here that, of the difficulties which the carbon manufacturers has to face, those presented by the peculiarities of the trade (e.g., the number of different lamps in use and the diversity of their countries of origin) are far more formidable than the problems in carbon design encountered within their own walls. It may also be said that in this country there is a strong and highly commendable tendency for the manufacturers of carbons to cooperate with the manufacturers of lamps. Simultaneously there is a marked inclination for projectionists and exhibitors to favour the purchase of equipment which is manufactured in this country by firms who have studied at first hand, and provided for, the conditions peculiar to our British needs. This movement is only at its inception, but already the benefits to all concerned are beginning to be felt. These remarks may appear to be outside the scope of a technical article and to concern rather the political aspects of the case, but, such is the peculiar constitution of the cinematograph industry, they have a very real bearing upon practical issues. To substantiate this statement let us revert to the subject of the design of a high intensity positive carbon. We have seen that this consists of a comparatively thin shell of hard carbon containing a heavy core of softer carbon impregnated with mineral salts. (Although the core is referred to as being softer, we are here only concerned with what are known as “ hard inlaid cores,” these having been found greatly superior both for stationary and rotating positives.) The Real Light Source Now, when such a carbon is burned in an arc it is capable of giving an illumination of very high initial intensity. The incandescent shell plays a certain part in the light source, but by far the greater proportion of the light emanates from the luminous gases given off by the burning of the impregnated core. As in the case of the low intensity carbon, the light value varies (within limits) with the current used, but with the important difference that the size of the positive crater in H.I working is not materially affected by the current flowing and therefore questions of collection-angle do not obtrude. A typical high intensity arc is shown in Fig. Ii. Since, then, the light intensity increases with the current employed, it follows that