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374 XII. PEOCESSING AND RELEASE PRINTING
of parts make for flexibility of machine design as well as lower cost. Thus, if a machine were to be altered to provide a development time of 6 minutes {e.g., for sound track originals), it would be necessary to add but one developing tank to a machine that is operating satisfactorily at a development time of 3 minutes when used for release prints. Thus, the output of the machine in feet of film per hour would be the same when developing 6-min. sound film as it is in developing 3-min. release prints. Without the extra tank, it would be necessary to reduce the machine speed to one-half, thereby reducing its film output per unit time to one-half.
In general, machine drive is of two kinds: friction and sprocket. Different makes of film "stretch" different amounts when wet with solution; the increase in length is quite large (it may be as large as 0.5%). "Overdrive" is provided in the case of friction-driven machines; it is important that the machine compensate automatically for the increase in the desired amount. In the case of sprocket drive, the sprockets used must be designed for the proper "stretch" and for the proper shrinkage ranges. Fortunately, there has been great improvement in the dimensional stability of films and smaller dimensional change during development and there is promise for still better stability in the films of the future. At present, most catalog developing machines offered for sale are of the friction-drive type.
Developing
It is impracticable in a single chapter to cover the many important factors in developing machine design or operation. Suffice it to say that among positive developer baths the Eastman D-16 is widely used although in a variety of concentrations and with minor variations that depend upon specific conditions of use and, in too many cases, upon the whim of the man in charge. A developer bath usually consists of:
(1) Reducing agents — Elon, Metol, Hydroquinone, Paraphenylenediamine, Glycin, Pyrogallate, and others. Metol (monomethylparaaminophenol sulfate) is sold under a variety of trade names; these include Rhodol and Pictol. The reducing agent reduces the exposed emulsion to metallic silver without affecting the unexposed emulsion.
(2) An alkali — sodium carbonate. This is needed because most developing agents are relatively inactive except in alkaline solution. In addition, the alkali aids in swelling the gelatin to permit action by the reducing agent upon the emulsion.
(3) A preservative — sodium sulfite or potassium metabisulfite. This is used to prevent oxidation of the reducing agents.