Patent GB-681962-A (GB)
Title: Resistor element and method of fabricating same
IPC: H01C10/36
Filed: 1951-02-19
Abstract: 681,962. Resistance coatings and deposits; variable-contact resistances; electric analogue calculating systems. FAIRCHILD CAMERA & INSTRUMENT CORPORATION. Feb. 19, 1951 [March 23, 1950], No. 3988/51. Class 37. A variable resistor or potentiometer has a resistance element which consists of lines of low resistance film, such as silver, bridged by a deposit of thin high-resistance film, for example graphite, or a nickel-chrome alloy, and a moving contact which engages unbridged or exposed lines of the low-resistance film. The terms " low-resistance film " and " thin highresistance film " are defined in the Specification. In the sine-cosine resolver shown, Figs. 2A, 2B, a glass disc 35 forms a base for the composite resistance element which consists of commutator silver deposit lines 42 bridged by a diametrical 45 degrees angular disposed straight strip nickel-chrome deposit 43. The latter is a narrow straight band of uniform width so that if a contact were to traverse the commutator lines 42 between the terminal portions 42a, 42b along the straight line X-Y the resistance between such a contact and either terminal would vary linearly. With rotation of four quadrature-spaced sliding contacts 41a ... 41d to make a circular track over the commutator lines 42 there is produced between sliding contacts 4 Id, 41 b the required sine function change in resistance and between sliding contacts 41a, 41c the required cosine function resistance change. The two outputs are drawn through an appropriate series of brushes 44 bearing on collector rings 40a ... 40d rotatable with the operating shaft 37 and connected to the sliding contacts 41a ... 41d. In a further embodiment, Fig. 3B, a glass disc 65 is formed with inner and outer rings of silver deposit radial lines and on these rings are superimposed annular uniformly tapered nickel-chrome strip deposits 68, 69 which co-operate respectively with the exposed oppositely-tapered areas 66, 67 of silver deposit radial lines to provide two separate power law resistance elements. If three, Fig. 3B, resistance units are mounted at 56, 57, 58 in the ganged assembly, Fig. 3A, two power law outputs may be obtained from each unit via the appropriate output terminals and dual sliding contacts 62a, 62b, &c., with rotation of common operating shaft 59. The glass disc 65 may accommodate n resistance elements and the ganged assembly comprise m discs 65 or units 56 &c. onwards so as to obtain m.n separate outputs each of which may follow a different non-linear law. A further illustrated embodiment, Figs. 1A to Fig. 1D (not shown), and Fig. 1E, relates to a linear potentiometer. To manufacture the resistance elements, for example the linear form, Fig. 1E, the glass disc 11 is masked around the centre and the remaining annular part has applied to it a silver paint composition which is air dried and then fired at a temperature within the range 900‹ to 1200‹ F., preferably at 1100‹ F. The scribing to form the resolution or radial or commutator lines 25 at, for example, 500 per inch, and performed by a conventional indexing machine is done either before or after firing. An outer ring area of radial lines thus formed is then also masked and the annular nickel-chrome film 28 deposited utilizing a known thermal evaporation process of which details are given in the Specification. To complete the resistance element silver deposit terminals are formed at 26b, 26c.