The Bioscope (Jul-Sep 1931)

August 12, 1931 MODERN CINEMA TECHNIQUE THE BIOSCOPE v Light on a Dark Subject -II. By R. Watkins Pitchford Carbon Construction Outlined Meticulous Care Through Multifarious Operations The carbon arc plays so important a part in the projection of good pictures that it will repay us to consider briefly at the outset how the carbons themselves are made. There will be no necessity to go deeply into the various factory processes, as these primarily concern the manufacturer, but we ought to note those points in design and manufacture which have a particular bearing upon the behaviour of the carbon when it comes later to be used in the lamphouse. One of the important points, for instance, is purity. If you have been over a factory where carbons are made, it may strike you as ludicrous to talk about there being purity in what must assuredly be the filthiest process ever. But this is only a paradox, like that of the carbons themselves which give from their utter blackness the whitest light we know. The fact is that dirt is matter out of place, and so when we consider the scrupulous care that is taken to preclude even a single speck of grit from the carbons right through their fifty odd processes we can justly claim the cleanliness of a dairy where all is, to outward appearances, filth. Soot from Naptha Oil The first ingredient in carbon manufacture is a blended oil rich in naphtha. A sample of this having passed the scrutiny of the laboratory, the main tank is fed to the burner, and a constant stream of dense sooty smoke is passed through tubular cooling chambers where the soot precipitates. This soot, collected with meticulous care in order that no incombustible matter such as grit or metal particles may gain admission, is then mixed to a stiff paste with tar, pitch or other carboniferous matter and undergoes its preliminary grinding and mixing in heavy mills. From here it is compressed by hydraulic pressure into thick cylinders about 1 ft. long and 6 in. in diameter. Rods Formed by Hydraulic Pressure It is now necessary to drive off from these cylinders the oils present in the tar, and this is done by baking in ovens — the first calcining process — a process which also bums up any combustible impurities which may have crept in. The residue is now pure carbon, but in its present state it is hard and brittle and in no way suitable for forming into rods or electrodes. It is, therefore, put once more through grinding mills, and the millings are carefully graded according to the sieve mesh through which they pass. Bricked-up ovens in which baking — the most delicate operation in carbon manufacture — is carried out under expert supervision Photo by courtesy of Charles B. Champion & Co., Ltd. When any particular type of carbon or shell is to be made, an appropriate selection is taken from the graded millings and this is again mixed with tar into a fine paste, the mixture being kept plastic through warming. Again the mixture undergoes hydraulic pressure, being put into the press in the form of a thick cartridge, and being extruded by a hydraulic ram through a hardened nozzle which determines the diameter of the carbon rod and of the core hole, if any. As the rods are extruded they are cut roughly into lengths and allowed to roll down chilled steel plates. This operation performs in effect a case hardening and prevents the still plastic rods from receiving any impressions which might affect the mathematically accurate circular cross section imparted by the extruding nozzle. The carbons are now in their rough state and require cutting to length, grinding, pointing, labelling and packing. Before this can be done, however, there takes place the operation which is probably the most important in the whole cycle. It will be remembered that tar has been added to the calcined carbon in order to render the mass plastic for conversion into rods. It is obvious that the oils, etc., contained in this tar must be eliminated in order that the carbons may be chemically pure. Why Baking is a Delicate Process Accordingly the rods of roughly 4 ft. in length are carefully parcelled together, tied to keep them straight and placed into fireclay seggars, in which they are surrounded by packing and carbon grindings. The seggars are then conveyed to one of a series of ovens which is raised gradually in temperature to a predetermined figure. The speed at which the temperature is caused to rise and the time for which an oven dwells at any particular temperature are matters which have been determined as the result of years of experiment. It will be apparent that the more volatile oils in the tar will vaporise at the lower temperatures, and it is essential that these gases be dispelled even from the rods in the middles of the bundles before the temperature is raised to higher degrees. It is, in fact. in this matter of accurate control over oven conditions that the skill and experience of the manufacturer shows itself. Throughout the three weeks during which the carbons are baking, the scientists of the oven house work in shifts, noting continuously the temperature of each oven as it rises to the fierce white heat of 1,400° Cent., noting each shift of the wind, the humidity of the atmosphere and a score of other apparently irrelevant factors which experience has shown to make all the difference. They are “ chefs ” if you will, who know as if by instinct when their carbons are " done to a turn ” — theirs is the human touch of craftsmanship which no instruments devised by man can ever hope to supplant. Electrical and Mechanical Considerations Such, in brief, is the process through which the carbon passes between the soot stage and the branded article which the projectionist fits into his lamp preparatory to striking up. There are, of course, wide differences between the various types of carbon intended for high intensity and low intensity lamps, between the positives and the negatives of any type, etc., etc., but it will be best possibly to defer a discussion of these differences until we come to deal with each type in turn. In the meanwhile we can profitably consider some of the qualities common to most if not to all carbons intended for arc lamp work. These fall generally under two headings : (a) electrical, and (b) mechanical. Every one of the precautions observed in the manufacture of carbons is based on ensuring efficiency in one or other of these directions. Getting Current through Carbons We must bear continuously in mind that the carbon is designed to form part of an electrical circuit. Moreover, since we are concerned almost exclusively with direct current circuits we are constantly called upon to apply the relationship which Herr Ohm laid down as existing between current, E.M.F. and resistance. In some of our carbon problems this law is capable of application in its simplest form, We Stock and Recommend the New SHIP “HILO” CARBONS NO ADDITIONAL CONTROL IS NECESSARY WITH “ ARTISOL ” LAMPS WALTURDAW CINEMA SUPPLY CO., LTD. LONDON DUBLIN BIRMINGHAM LEEDS LIVERPOOL MANCHESTER