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174 MOTION PICTURE HANDBOOK roo feet than it does fifty. As I have previously explained, it is the size of the picture that determines the necessary light strength to produce a given brilliancy and not the dis- tance the picture is proiected. (2) There is no use "assum- ing" that a 60-ampere D. C. arc would be satisfactory, for it would not. I have known of sixty amperes being used, but I have seen no appreciable advantage in increased cur- rent strength after passing the fifty ampere mark. You can- not handle the arc to advantage with current exceeding fifty amperes—or at least that has been my experience. There would be no danger so long as the film is kept mov- ing. I would not care to risk less than forty feet per minute on that kind of light, however. (3) One-half second would probably be sufficient. It would depend to some extent on how small a spot you had. The smaller the greater amount of heat, of course. * * * Carbons Needle.—Sapulpa, Okla.. writes setting forth his troubles as follows: "My carbons burn back from the point, sometimes as much as an inch or two. There are two other houses here and we all use current from the same gener- ator. One of these houses and myself are using compens- arcs and the other fellow has a rheostat for resistance. They both seem to be using more current than I do vet their carbons burn fine. The same trouble has existed in this house for a long time—long before I became its ooerator. Can you suggest a remedy or tell me what is wrong?" T have not quoted all this letter, but sufficient to show what Sapulpa is up against. By "burning his carbons back," Sapulpa undoubtedly means what is called "needling." That is to say, his carbons burn to a long, slim point. This^ is caused by undue heating in the carbon, the effect of which is to burn out the binder and allow the carbon to drop away (disintegrate) without being consumed (volatilized) in the arc. Sapulpa will find a heap of fine, black carbon in the bottom of his lamphouse just under the carbons. Needling may indicate several things, or any one of several, rather. You may be using carbons which are too soft. You may be using carbons of poor quality. It may be that your trans- former (Compensarc) is giving an excess of current, some error having been made in its constniction (windings). I would suggest that you have the electric light company test your current flow with an ammeter, if possible. But first proceed as follows: With a file clean out the inside of your carbon arms thoroughly so that the carbon will make good electrical contact with the arms. Get a few of the other fellow's carbons and put them in your lamp for a trial. Let the other fellow try some of your carbons. Thus you will determine whether or no the fault is in the carbons themselves. If not, then it is pretty near a cinch that you