The Bioscope (Jul-Sep 1931)

September 30, 1931 MODERN CINEMA TECHNIQUE THE BIOSCOPE vii LIGHT FROM A DARK SUBJECT— IX THE HILO ARC By R. WATKINS PITCHFORD No discourse on carbons would be complete without a reference to the recently introduced miniature high intensity carbons intended for use in low intensity lamps. These carbons were introduced as the result of pioneer work on the part of British manufacturers which has as its object the overcoming of certain specific difficulties more or less peculiar to conditions in this this crater was directly proportionate to the current flow. Fig. 1 shows a 12 mm. positive carbon and gives the crater diameters for various current intensities. It will be noted that the largest crater occurs when the carbon is run at its maximum safe current. If current greatly in excess of this value is used the flame lick will tend to run the positive to a point, thereby seriously re high intensity carbons must be devised such that they would operate at the comparatively low current output of the generators found in low intensity installations. Now, as we have seen, when dealing with the chemical arc the illumination intensity is proportionate (within the physical limits of endurance of the carbon) to the current flowing. There is, in fact, a relation which is almost constant between intrinsic brilliance on the one hand and current per square millimetre of positive carbon core on the other. Now our available current is determined by our generator output, and in low intensity equipment about 55 amperes is the most we can hope to use, 45 amperes or so being nearer the mark. It is obvious, therefore, that to get the necessary intrinsic brilliance we must employ carbons of small cross sectional area. This is, in fact, what is done. Hilo Positive Carbons range in diameter from 6 to 9 millimetres, with corresponding negatives of from 5 to 7.5 millimetres. As will be observed, there is only a millimetre difference between the negative and the positive. Coring a 5 mm. Carbon ! Moreover, since low intensity lamps do not provide for forward contacts on the positive immediately behind the crater, it is necessary for the Hilo Positive Carbon to carry the full volume of current throughout its length. It is therefore copper-coated in order to assist in getting the maximum current intensity at the crater. The negative which has to carry the same current value must also be copper-coated. In designing a carbon pair suitable to the Over running Mirror Carbons results in a smaller effective crater and hence less light on the screen Co) 1 5 amps, under s=> (=5 (b) 2 5 amps, normal crater diameter Jm/m (d) 35 amps, abuse crater diameter 6 -5 m/m •fM\ (c) 30 amps. max. crater diameter 7 5 m/m s=> cm FIG. I country. Further reference will be made to this matter later ; meanwhile it is to be remarked that these carbons have met with an immediate acclamation over here — that their counterpart has already been introduced upon the market of America, and will no doubt be produced before long by carbon manufacturers on the Continent. On account of the fact that these carbons have high intensity characteristics, but are designed for use in low intensity lamps, they are, in this country, termed hilo carbons. It can be said that the widespread adoption of “ sound ” films was, more than any other single factor, responsible for the introduction of the hilo carbon. These films necessitated the use of screens designed to permit the passage from behind of sound waves from loud speaker reproducers, and such diaphonous screens were, naturally, less efficient as reflectors of light waves than their more substantial predecessors. Getting More Out of Low Intensity Arcs This change, therefore, like almost every innovation in the motion picture world of recent years, demanded of the lamps a higher initial light intensity. It was necessary to make up for the lack of reflective powers of the new screens in order that patrons should enjoy at least the same standard of brightness to which, in the days of silent films, they had become accustomed. A state of affairs thereupon arose in regard to the existing low intensity lamps which it may be worth our while to study. We noticed when dealing with low intensity arcs that the light emanated from the crater formed in the incandescent tip of the positive carbon. We saw, further, that the area of For much information contained in this article the writer is indebted to Messrs. Charles H. Champion &■ Co., Ltd., the " office of origin ” of the Hilo Carbon. ducing the crater area and consequently the light intensity obtainable. Projectionists therefore wisely refrained from overloading their carbons and sought to obtain the required increase of intensity by employing a pair of carbons of larger diameter. The normal crater diameter of the larger positive was naturally larger, and a gain in illumination intensity was observed. Limits to Carbon Increase It was soon found, however, that there was a limit to this increase in carbon size, beyond which the gain from a larger crater was offset by an inefficient angle of collection. In other words, it was found impossible to focus an economic spot on the gate aperture, and so most of the light gained by using larger carbons was wasted. Fig. 2 will explain this diagrammatically. The outcome therefore was the realisation that there is, for any given lamp, a critical size of carbon, and for that carbon there is a critical current value. Beyond this it is inefficient either to increase the current or to increase the carbon size. What happened in practice was that in spite of using the correct carbon size at its normal current, and in spite of keeping mirrors, port holes and screens scrupulously clean, projectionists still found that they were unable to satisfy their patrons’ requirements in this matter of screen intensity, and the only alternative appeared to be to instal high intensity lamps, together with their generators — a formidable expansion. It was to meet this situation that there was introduced the hilo carbon. At the outset it was realised that since the limit of illumination in ordinary plain carbons had been reached it would be necessary to design carbons which would give the " chemical arc ” associated with impregnated positive cores. In other words, miniature ENTERTAINMENT TAX BURDEN REDUCED As an expression of the goodwill that has always existed between Willsons and Cinema Managers throughout the country, we are arranging to take upon ourselves a portion of the extra burden which the new Entertainment Duties will impose upon the Exhibitor. We therefore propose to make special reductions in price on all roll and booking tickets, either specially printed or taken from stock, ordered in connection with the increased taxes, prior to November 9th. NfiTr This is not an advertisins llU 1C “stunt,” but a genuine endeavour to lighten the burden imposed upon theatres and cinemas by the new taxes. This announcement will, therefore, not he repeated, and any interested managers are cordially invited to write to us for further details. WILLSONS (Leicester), ’Phone : Gloucester Mansions, TEMple Bar Cambridge Circus, 5326 LONDON, W.C.2