Patent GB-616381-A (GB)
Title: Improvements in and relating to the manufacture of electric resistors
IPC: H01C17/245
Filed: 1946-08-06
Abstract: 616,381. Resistance coatings and deposits; non-metal resistances; wire resistances; variablecontact resistances. PHILIPS LAMPS, Ltd. Aug. 6, 1946, No. 23357. Convention date, Aug. 4, 1945. [Class 37] The manufacture of a resistance begins at one extremity and continues in the longitudinal direction, the value of the resistance of the completed part being continuously compared to that part of a standard resistance which is located between a fixed point and a contact which moves along this resistance at a speed proportional to that at which the resistance is manufactured. The process may be applied to wirewound resistances, resistances having a metallic layer applied by vaporisation or a layer of conductive powder, such as graphite, and an insulating binder, such as synthetic resin, on an insulating carrier, or to resistances formed wholly of insulating material. As shown in Fig. 1, the resistance being manufactured consists of a carrier 1 coated with a layer of conductive material 2. This is rotated against a rotating grinding wheel 3, which is movable parallel to the axes of the resistance and also normal thereto so that the thickness of material removed may be varied, A sliding contact 7 is moved axially so as to be at the same distance from an end cap 6 as the grinding wheel 3. The completed portion 4 of the resistance between sliding contacts 5, 7 forms one arm of a wheatstone bridge, of which the other arms are formed of a portion 8 of a standard resistance 9 and resistances 12, 13. A slider 11 on the resistance 9 is mechanically coupled with the slider 7 so that the bridge is balanced so long as the ratio between the resistances 4, 8 is constant. If this is not so the deflection of the galvanometer 14 indicates whether the wheel 3 has to be moved closer to or further from the axis of the resistance. A coated disc resistance for a potentiometer may be similarly manufactured. Fig. 3 shows apparatus for winding a wire resistance in which a flat insulating strip is mounted in a rotary holder 19. The resistance wire 18 is fed from a reel 18a, the end being held by a clamp 20 at the free end of the strip. This engages a slide 21 which is movable by means of a piston 22 in a cylinder 24. Oil is supplied to the cylinder by a pump 25 the output of which may be regulated by a lever 26. The pump 25 is driven by a shaft 27, from which the holder 19 is driven. The slide 21 drives, through gearwheels 28, 29 a rack 30, so that its displacement is proportional to that of the slide. The rack 30 carries a contact 11 engaging the resistance 9, the part 8 of which forms one arm of a Wheatstone bridge. The rack 30 also drives a cam 31 through gear-wheels 32, 33, 34 and a chain 35. The assembly 32, 33, 35 may be displaced so that the gear-wheels 33, 34, engage or not. The cam 31 operates the lever 26 through a rod 36. The lever is additionally controlled by a hand lever 37 and eccentric 38. The direction of flow of the oil may be reversed by a cock 39. The completed portion 4 of the resistance, between the clamp 20 and a roller contact 5 is compared with the portion 8 of the resistance 9 by the Wheatstone bridge. The cam disc 31 regulates the oil supply so that the resistance 4 acquires as much as possible the desired value. Small differences cause a deflection of the galvanometer and are corrected by the hand lever 37. As shown, the winding speed is constant whereas the displacement of the carrier is adjustable, but alternatively the displacement may be constant and the speed adjustable. The complete resistance may be bent into a circle and used in a potentiometer with a rotary sliding contact. The strip on which the resistance wire is wound may be of variable width.