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907,575. Photographic apparatus; electrolytic development. MINNESOTA MINING AND MANUFACTURING CO. Sept. 24, 1958 [Sept. 25, 1957], No. 30519/58. Classes 98 (1) and 98 (2). In apparatus for exposing a photo-conductive sheet material, the exposed image area is developed by electrolysis of a developer solution at the photo-conductive surface as the sheet is advanced to bring a fresh section into the exposure position. The sheet material preferably consists of a thin aluminium foil backed by paper and coated on the exposed side with a photo-conductive composition of zinc oxide in a styrene butadiene binder. As shown, in a micro-film reader and copier, the foil is electrically connected to spindle 31 of supply roll 29 and the circuit to batteries 38 is completed through sponge-like developer applicator pad 35, developer tank 37, switch 20, variable resistor 23 and ammeter 22. Thus, when switch 20 is closed the developer between pad 35 and the photo-conductive surface is electrolysed. Micro-film 16 is wound between spools 17 and 18 and its image is formed on ground-glass viewing screen by lens 15 and mirror 25. Exposure is effected by momentarily moving the mirror to the full-line position and returning it to the viewing position. Immediately thereafter, switch 20 is closed and the material 30 is withdrawn at a constant speed through slot 33. Contact with pad 35 is assured by spring strip 36 which light-traps slot 33. In another form (Fig. 4, not shown) the material is fed out of the slot from between a pair of developing rollers. Rollers surfaced with formaldehyde hardened gelatine plasticized with glycerol or other humectant may hold sufficient electrolyte or moisture for tank 37 to be omitted. However, when the rollers are made of such materials as compressible rubber, metal or ceramic, the lower roller dips into a tank 37. Alternatively, the developer may be brought into contact with sheet 30 by a brush arrangement (Fig. 5, not shown) dipping into the tank and the sheet advanced by sprockets engaging marginal perforations in the material. Unperforated material may be advanced by a spiked sprocket which pierces the material and, since the spikes make electrical contact with the aluminium foil, the electrical connection may be made to the spiked sprocket instead of to spindle 31. Preferably, development is carried out by electrolysis of an aqueous solution of cadmium nitrate, tartar emetic and silver nitrate. Development may be effected by electrodeposition from solutions copper or nickel ions or by chemical action which follows the deposition of a non-metallic ion. Nickel may be deposited to produce a lithographic printing plate directly. In another arrangement (Fig. 6, not shown) material which recovers its electrical resistance rapidly after exposure is immersed in a tank of developer which is electrolysed during exposure through a ring-shaped electrode.
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