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366 RICHARDSON'S BLUEBOOK OF PROJECTION
other kinds as regards the material composition of their body or shell. The materials in the core of a carbon differ widely from those composing its shell, the exact composition being largely a trade secret. Rare earth metals, including cerium, lanthanum, neodymium and praseodymium may be used in varying proportions. Very naturally carbon manufacturers do not disclose their formulas, but such knowledge is of no practical value to projectionists. The high intensity arc is treated more fully later in this chapter.
Effect of Carbon Diameters
(28) To secure best results in projection the capacity of the carbons must be very carefully matched with the amperage. (29) If the positive carbon is too large in diameter the arc will be inclined to unsteadiness. If the negative is too large there will be undue interference by the unnecessarily large tip, which means waste of light.
While it is quite true that carbons of excessive current capacity make it unnecessary to trim the lamp so often, and may even save a bit in carbon cost, such saving nevertheless represents very poor practice. It results in an unsteady light of relatively poor quality, and at such a price any small saving in carbon cost is an absurdity. Any projectionist worthy of his hire will select and use only carbons of a kind and diameter in combinations recommended by the carbon manufacturer for the amperage and type of light source he is using.
(30) Keeping within range of proper current capacity, the smaller the negative carbon the smaller will be its tip and consequently the less light it will obstruct and waste ; also, if the negative tip be unnecessarily large it encourages a tendency in the arc to be unsteady and to "travel," producing light fluctuations which are revealed on the screen.
Use the smallest negative carbon possible without inducing spindling but remember that the carbon makers have set up limitations which should be strictly adhered to. Copper coating aids materially in diameter reduction.