The Bioscope (Jul-Sep 1931)

4 i > ■ August 26, 1931 Light from a Dark Subject — IV. The main considerations governing the design of a negative carbon for use in a low intensity arc are those connected with conductivity. To avoid obscuration of the crater the negative must be of a diameter three or four millimetres less than the positive with which it is paired. At the same time it must carry the same current. The negative temperature coefficient of carbon is so marked that it is not a simple matter to give a formula for cross sectional area as a function of current-carrying capacity but it will be apparent that for each millimetre by which we reduce the diameter of the negative in the interests of avoiding crater obscuration, we must increase the conductivity factor in something like a square law proportion. Otherwise the current-carrying capacity of the carbons considered as a pair is going to be cut down by the introduction of undue resistance in the negative, and as a direct result we should seriously limit the available intensity of the arc. Conductivity Problems The conductivity of the negative carbon, therefore, is a matter of great importance and receives particular attention in the design. At the same time, there are considerations such as arc steadiness which must be given their due weight. It is a case, in fact, where compromise is desirable. A negative carbon of solid pure carbon can, no doubt, be expected to have a particularly high conductivity, and certain Continental carbons are, in fact, so made. But, on reflection, it must appear that if two electrodes, each of pure carbon, are to have the same current-carrying capacity they must be of the same diameter. If they are of the same diameter it is difficult to see how the crater (which must itself be of smaller diameter than the positive carbon in which it is formed) can fail to be more or less obscured by the negative. Moreover, with a negative of solid carbon there is lacking a most important feature in that there is no provision made to hold the arc steadily on the tip. British practice, therefore, favours a negative carbon made up of three distinct parts — the core, its copper covering and the shell. Copper Coating Must Not Be Thick The core is of pure hard retort carbon, it is covered by an electrolyticallv deposited coating of copper to a thickness of some two thousandths of an inch, and the coated core fits with the greatest possible accuracy into the core hole in the shell of pure carbon. We may note here that the thickness to which the copper is deposited on the core is limited. A greater thickness than that employed might certainly aid us in our quest MODERN CINEMA TECHNIQUE By R. Watkins Pitchford for conductivity, but disadvantages woul^ be introduced. In the first place we should risk damage to our mirror from spots of molten copper, and, in the second place, we should impart a green colour to the arc stream, thereby reducing the general visibility and definition of the picture. It will now be apparent how essential is the accuracy with which the core hole is formed in the shell. Not only must this hole be of perfectly constant internal diameter (3 millimetres in most cases), but it must be dead straight and perfectly coaxial with the shell. How Internal Arcing Starts Consider for a moment what would happen were these precautions neglected. Fig. 1 shows a core hole with a " bulge ” (greatly exaggerated) at A. As soon as current is passed through an arc containing such a carbon, a local arc sets in at A, with the result that before long the shell develops a waist as in Fig. 2, and, finally, a complete fracture occurs, as in Fig. 3, with disastrous results to the arc and to the picture alike, because, of course, such a thing is bound to happen at the crucial point or climax of the whole film. In a well-made negative, therefore, extreme accuracy of the core and the core hole is rigidly insisted on, and the greatest possible intimacy is provided between core and copper, and copper and shell. We have, in fact, so intimate a union between the three parts that they can be considered as forming one solid rod — from a mechanical point of view, that is. Electrically, of course, we have once more our old friend of three resistances, or conductors, in parallel, and, as we know, the current is going to divide itself between the three, the greatest current flowing through the best conductor, and vice versa. That, of course, is just what we require. The main function of the negative is to lead the current THE BIOSCOPE Fig. 4 away from the arc back to the generator terminal, hence the importance of its conductivity. But it matters a lot how it leads this current away. As we shall see when discussing the subject of the low intensity arc as a whole, we aim at getting a steady source of light which can be focussed by optical equipment. Obviously then it will not do to have this source of light waltzing round the top of the carbon. It must behave in a predetermined manner so that we can rely on it. Examine Fig. 4, which shows a view of the end of a burnt negative taken from an angular low-intensity arc. You will notice the hard inlaid core protruding from the shell. Actually, of course, this core is copper covered, but the copper volatilises in the arc and burns back slightly along the core, consequently it is not visible in the view, but the small space which previously it occupied may be observed between the core and the shell. This volatilisation of the copper is a highly necessary feature in a negative carbon, since any excess of copper would have undesirable effects as mentioned above. Time Element in Carbon Consumption But the point to remark is that the negative shell, provided the carbon is being run at the appropriate current, should burn back just fast enough to expose constantly a small tip of core. In this way there is enough " body " to the tip to hold the arc steady, while at the same time the negative presents constantly its low resistance path (the coppercovered core) to conduct the current efficiently back to the generator. The coppercovered core in fact behaves somewhat in the nature of a lightning conductor in this respect. You will notice the provision above as to the carbon being run " at its appropriate current.” Like any other piece of engineering workmanship, each carbon is designed by its makers to perform a certain duty under certain carefully stated conditions. In the case of the negative carbon under discussion, the manufacturer chooses for his shell a certain combination of graded millings, so much 80 mesh, so much 120 mesh, etc. Then he chooses the particular grade of retort carbon for the core, and he decides how much copper he will deposit on it. Co-ordinating Specifications and Conditions He observes certain precautions in the duration of baking, in the temperature employed, in the rapidity or otherwise with which the oven is brought to maximum temperature, and so on. All the way through he is working to a certain specification which he knows will produce a carbon guaranteed to behave in a certain stated manner, provided that the projectionist observes (with a generous Negative Carbon Design \ \ r\ WK rtnra « «* n m < vi i m ■ mn w nxran rrm wmm mm rr warm mmw w sm rr #r # wm jtw i 1 . a :i • * >3 1. 8: m • i‘ a ^ E • A llNCMSH & Cdo 1/rDo SHIP “ HILO ” CARBONS SHIP axe recommended by us, and can be supplied • g n either from our Head Office at U — I || g f J j Vy 94, Wardour Street, London, W.l, ^ ;rrL*» Gerrard 5137, 0T f \ pD fD /TA EN Q\ or from any of our Branches below. ^ B\ MJ In! Birmingham, Cardiff, Glasgow, Leeds, Manchester. Newcastle. J—r jmm m a/ MLjm mm am a mm a a a. mm m a a m mm m mm m am l mm mma m mm ■ am m i mmmmm j