The Bioscope (Jul-Sep 1931)

August 19, 1931 MODERN CINEMA TECHNIQUE THE BIOSCOPE iii Light from a Dark Subject III Having discussed the constituents of carbons as applied to cinematograph work, and having followed the various processes through which the constituents pass before they emerge as the familiar polished and ground rods, we can now turn our attention to specific types. Taking first the Low Intensity Positive Carbon, we will consider its mechanical and electrical features before examining its negative counterpart on similar lines. In subsequent talks we shall treat High Intensity and various other carbons in the same manner, examining their properties and seeing in each case how the ingredients, construction and operating data are modified in accordance with the purpose to which each type of carbon is to be put and seeing what bearing these factors have upon modern projection practice. The L.I. Positive In the Low Intensity Direct Current Arc the Positive Carbon forms the incandescent crater whose magnified image is focussed upon the projection gate aperture. The underlying principle in the manufacture of the L.I. Positive is therefore that of assuring the formation of a perfect crater and all the qualities of this carbon as they are enumerated below will be found to keep this consideration primarily in view. In the first place the positive must be considered as an electrical conductor, and, since we are dealing with direct current, it follows that we can apply our formula of ohm’s law. If then for any given E.M.F. or line volts our current will be inversely proportional to the carbon resistance (ignoring for the moment the resistance of the arc) it follows that our aim should be to produce positive carbons which for any given diameter show the lowest possible voltage drop per unit of length. In other words, the carbon should be of the highest possible conductivity, commensurate, of course, with mechanical considerations. Conductivity, Cost and Core The exact connection between this conductivity factor and crater formation will be dealt with in a subsequent article, but for the moment we must concentrate upon the qualities of the carbon itself. In any carbon intended for general commercial use there are, of course, considerations of expediency and production cost, which limit the extent to which this quest for low resistance can be carried. There are also other factors connected with crater formation which obtrude. For example, a positive made solid throughout of the purest grade carbon would have a higher conductivity than one of the same grade and diameter containing the customary soft squirted core. But such a carbon would POSITIVE CARBONS For Mirror Arcs be quite unsuitable to the purpose in hand, since there would be some difficulty in keeping the crater central. At the same time, it can be said that in a good mirror positive every effort should be made by the manufacturer to ensure that the main body of shell of the carbon consists of the purest possible carbon of absolute homogenity subjected to great hydraulic pressure, in order to ensure the high density upon which conductivity largely depends. When the core of the Positive is strictly co-axial with the carbon a perfect cup-shaped crater can be obtained with consequent economy of focus on the gate Such is, in fact, common practice, at any rate amongst the manufacturers in this country. The carbon destined for mirror positive shells is selected with meticulous care, numerous tests and cross checkings are imposed in order to ensure the elimination of any impurities, and the result is that it is possible to guarantee without reservation the quality of the carbon forming the sticks of varying diameters which emerge from the extruding presses. This shows the bad crater formation which would result if the Positive core-hole were not central. Note the wasteful reflection It is probably a matter of common knowledge that the sticks come through the extruding presses in a plastic state with the core holes already formed. But it may be of interest to note that in the manufacture of good Mirror Positives skilled hands are employed whose sole duty it is to ensure that the mandril which forms the core hole is accurately centred. By R. WATKINS PITCHFORD The plastic core material is passed through the core hole and stopped off by hand in order to ensure uniformity of core packing Without anticipating the subject of criter formation we can see that the soft core which locates the crater must be perfectly co-axial with its shell. The slightest deviation will cause the carbon to fall short of the high standard at which the manufacturers aim, and so it is not surprising to learn that the tolerance here is never permitted to exceed thousandths of an inch and that the maintenance of such a standard implies a continuous observation in order that instant adjustments can be made to counteract any wear on the mandril due to the passage of plastic carbon under high pressure. Centred Crater Saves Light Loss Before passing to the subject of the core itself it may be of interest to observe the two sketches given here. Figure I shows a truly centred core hole, while Figure II shows (in exaggerated form) the evil which would result if the centring precaution were neglected. It will be apparent, that in the case of Figure I the crater will be maintained dead central on the positive throughout its length, whereas in Figure II there is only one point, namely, half way along the carbon at which the core is truly central. Having now obtained our mirror positive shell of pure, high conductivity carbon, complete with its accurately centred core hole, we are ready to insert what is known as a soft squirted core. The function of this core is to assist in locating the positive crater and to hold it in a central position as the carbon is burnt throughout its length. Composition of the Core The core itself consists generally of a liquid paste composed of a mixture of carbon and silicate of sodium. While ideal for its purpose, this paste hardens to solidity rapidly on exposure either to the atmosphere or to the porous walls of the shell. Precaution must therefore be taken to ensure that the core hole is free from any obstruction which might cause the paste to cease flowing through and to congeal. For this reason the core hole is first blown through with compressed air, and then, as a final precaution, sodium silicate is passed FOR DENSE FILMS THE ANSWER IS— CARBONS