Patent GB-581195-A (GB)
Title: Improvements in or relating to rheostats or potentiometers
IPC: H01C10/20
Filed: 1944-06-01
Abstract: 581,195. Variable contact resistances ; combinations of resistances. MORRISON, P. H., TAYLOR, S. W., and PLESSEY CO., Ltd. June 1, 1944, No. 10530. [Class 37] A rheostat comprises two or more variable resistance elements built into a cradle made from a single piece of material, the whole assembly being housed within an airtight casing. As shown, three carbon or like resistor elements 4, 4a, 4b are mounted in annular recesses in insulating discs 1, 1a, 1b fixed in a cradle 8, Figs. 4, 5 by means of tags 10, 10a, 10b on the cradle which are bent over upstanding lugs 6, 6a, 6b on the discs. Radial extensions on the discs serve to hold them in position by engaging transverse groove 11, 11 a, 11b in the cradle base. The resistor elements 4 have track terminals 5, 5a, 5b and are engaged by roller contacts 23, 23a, 23b carried by leaf-spring wiper arms 17, 17a, 17b clamped between insulating bushes 18 and washers 19 on hubs 20 secured to squared portions of a shaft 14. The wiper arms are extended at their opposite ends and bifurcated to engage diametrally opposite points on collector rings 2, 2a, 2b fitted in the insulating discs 1. The hubs 20a, 20b are flanged at 21 to provide electrical screening and are fitted with insulated rings 22, 22a to take the contact pressure reactions of the wiper contacts. The shaft 14 is rotatably mounted at one end in a bush 115 secured in the collector ring 2b and at its other end fits into a transverse slot 13 in an operating spindle 12 rotatably mounted in the end walls 9 of the cradle 8. The casing 129 has a bush 130 soldered thereto at its closed end and threaded externally so that the complete rheostat may be mounted on a panel 33 by means of a nut 34. The cradle with the three potentiometers mounted therein is slid into the casing 129. An airtight bearing between the spindle 12 and the bush 130 is effected by rubber sealing rings 136. Rotation of the casing relative to the cradle is prevented by the inwardly extending casing tab 35 projecting through a hole in the cradle, this tab 35 also serving to limit rotation of the operating spindle 12 in either direction by forming an abutment for a radial extension 16a on a ring 15, Fig. 7 (not shown) rotatable with the spindle 12. The open end of the casing is then closed by an insulated disc 38, ring 39 and terminal head 42. A rubber sealing ring 49 effects an airtight joint between the terminal head and casing and tags 50 integral with the casing are bent over the outer face of the terminal head to secure it in position. Leads 24, 25 from the terminals 5, and collector rings 2 pass through holes in the disc 38 and through V-shaped notches 41 between the ring 39 and a rubber pad 40. The ends of the lead wires are bared and turned radially inwards where they are resiliently pressed against the rubber pad 40 by the inner hollow riveted ends 47 of the terminal contact pins 43. The rivet ends 47 project slightly from the inner'face of the head 42 to effect good contact. The head 42 is rotatably positioned relative to the casing 129 by a lug 51 engaging a corresponding slot in the casing and is preferably moulded with a central key shaped extension to provide a fool-proof location for connecting the potentiometer to an external circuit.