Patent GB-606102-A (GB)
Title: Improvements in and relating to the electrical measurement and control of the moisture content of photographic films and papers
IPC: G01N27/04
Filed: 1943-06-18
Abstract: 606,102. Determining physical qualities of materials; resistance measurements. WILLIAMSON, S. June 18, 1943, No. 9854. [Class 37] Apparatus for measuring the surface moisture content of the photographic emulsion on sensitised paper or film comprises two electrodes adapted to make relatively stationary contact with the sensitised surface and thereby bring into circuit between them a strip or strips of the surface, the path through which is of short length but considerable width, and means for measuring the resistance between the electrodes. The strip of material under treatment, for example a continuous web of photographic film or paper on leaving the drying apparatus after development &c. is passed over a roller as shown in Fig. 1. A central rod 1 is secured at one end to the frame 2 of the drying apparatus. Secured upon this rod is an insulating sleeve 3 on which are mounted two slip rings 4. Next to these slip rings there is threaded on the rod an insulating end plate and brush carrier 5, and at the other end of the rod is a second end plate 6 and an insulating cap 7. Two metal rods 8, 9 pass through the end plates 5, 6 and carry brushholders 11, 12 provided with spring-pressed brushes 13, each of which bears on one of the slip rings 4. On the rods 8, 9 are threaded alternate metal and insulating washers 14, 17. The insulating washers 14 are a sliding fit on both rods. Each of the metal washers 17 has, besides its central opening, a large hole 15, Fig. 4, and a small hole 16. Alternate metal washers have the rod 8 passing through their small holes 16, which are a close fit so that the rod makes good contact with the washers, while the rod 9 passes through the larger hole 9 without touching the washer. The remaining metal washers have their smaller holes 16 threaded on the rod 9 so as to make contact with it, and do not touch the rod 8. The surface of the roller is turned after assembly so that conductors and insulators are flush. The rods 8, 9 may be included in a circuit containing the operating coil of a relay which controls the operation of a heater for drying the web. Alternatively, the resistance between the terminals of the contact roller may form one arm of a Wheatstone bridge. In another construction of contact roller an insulating roller 23, Fig. 5, has a double helical groove of semi-circular cross-section cut in its periphery. In these grooves are laid wires 24, which are turned down level with the surface of the roller. Alternatively, the conductors may lie parallel to the axis of the roller, alternate conductors being connected to a ring at one end of the drum, and the rest to a ring at the other end. Instead of the electrodes forming part of the surface of a roller which is rotated by the web travelling over it, they may form a convex grid consisting of two interleaved combs, over which the web slides. Alternating current may be employed to obviate electrolytic action, or the apparatus may be operated with minute currents which are amplified by a thermionic amplifier.