Patent GB-694364-A (GB)
Title: Adjustable potentiometer
IPC: H03G3/06
Filed: 1950-04-06
Abstract: 694,364. Variable resistances. SYLVANIA ELECTRIC PRODUCTS, Inc. April 6, 1950 [April 16, 1949], No. 8773/50. Class 37. [Also in Group XL (c)] An adjustable potentiometer comprises an electrically continuous resistive element, a conductive member supported in insulated relation to at least the major length of the element and including a portion movable in a path along the resistive element and electrically coupled to the part thereof adjacent to this portion, and a conductive tab fixed at a selected point along the length of the element and separated therefrom by dielectric material to provide capacitive coupling between the tab and the selected point of the element. As shown in Figs. 1 and 1A, the resistance element 10 comprises an open ring of graphite &c., coated on one surface of a member 11 of material of high dielectric constant, such as titanium dioxide ceramic, titanium dioxide magnesium titanate, or titanium-zirconium dioxide ceramic. The ends of the element 10 are connected to terminals 12, 13. A conductive member or slider 20 is mounted on a shaft 24 of insulating material and is insulated from at least the major part of the length of the element 10. The member 20 includes an arm 27 which is movable along the element 10 but spaced therefrom by the dieleotric member 11, which provides capacitive coupling between the arm 27 and the portion of the element 10 opposite to it. The member 20 is connected to a terminal 30. Conductive tabs 32, 33, 34 are fixed at spaced points along the length of the element 10 but separated therefrom by dielectric material so as to be capacitively coupled with the corresponding portions of the element. The tabs 32, 33 are cemented to the dielectric member 11 on the side opposite to the element 10, while the tab 34 is secured to a dielectric layer 35 which is secured to the surface of the element 10. The potentiometer is adapted to be used as a volume control in a radio receiver having an intermediate frequency transformer 38, Fig. 2, a diode rectifier 41 and an audio amplifier valve 43 with an input circuit resistor 44. The tabs 32 33 and the portion 10a of the element between them form a low-pass filter. The tab 34 and a resistor 42 form a compensating network for low volume settings. The capacitive coupling between the movable arm 27 and the element 10 is indicated as a condenser 21 adjustably coupled to portions 10b, 10c of the resistance element by a slidable tap. The resistor 42 may be painted on the lower surface of the dielectric member 11 between the tab 34 and the terminal 12, and the resistor 44 may be similarly formed on the upper surface of the member 11. In a modification, Fig. 3, the resistive element 10 is extended beyond the terminal 12 and the terminal 30 is bent down and connected to one end of the element by a rivet 46. The portion of the element 10 between the terminals 12, 30 may form the fixed resistor 44, Fig. 2. In an alternative construction, Fig. 4, for providing a larger capacitive coupling between the arm 27 and the element 10, the arm 27 carries at its end a number of condenser plates 51 sandwiched between layers 52 of dielectric material. Fig. 5 shows a dual potentiometer wherein one potentiometer comprises a resistance element 10, slider arm 20, and shaft 24. A ring 54 of dielectric material is fixed between the element 10 and the arm 20. The second potentiometer comprises a resistance element 55 on the opposite side of the member 11, and an arm 56 carried by a hollow shaft 57 surrounding the shaft 24. A ring 58 of dielectric material separates the arm 56 from the element 55. Specifications 694,418 and 694,419 are referred to.