Patent GB-964820-A (GB)
Title: A method and means for the production of resistors and the like
IPC: H01C17/04
Filed: 1962-03-19
Abstract: 964,820. Electrical resistances. INSTALLATIONSMATERIEL A.B. March 19, 1962 [March 29, 1961], No. 10380/62. Heading H1S. The spaced groups of serpentine loops of resistance wire, each of which groups comprises a wire resistance element, are supported in position and are carried in a continuous band by catching the transverse arms of each resistance wire loop in the loops of one or more chain-looped threads running the length of the band. The serpentine loops may be made by bending straight resistance wire but as shown in Fig. 4, the loops are preferably bent from resistance wire 2 coiled about a core 1 of nylon or silk. The chain-looped threads 4 and 5, of which there are preferably three, are of nylon. Individual resistance elements may be severed from the band and be cast in a ceramic paste as shown in Fig. 5, where one half of the mould 10 is to be seen. The element is supported in the mould by tubes 13, 14, which engage the free ends of the resistance wire and by entering into the interior of the mould and being withdrawn from the ceramic when this is set, the tubes provide cavities about the ends of resistance wire so that terminal wires tipped with silver solder may be attached within the cavities. A number of such moulds may be employed in line to receive the resistance elements without cutting the supporting threads. A continuous process of embedding the resistance elements in ceramic is shown in Fig. 9 in which the band of elements is fed between two rollers with a layer of ceramic paste extruded from nozzles 24, 25, on each roller face. The band of elements is preferably wetted with a thin paste to facilitate adhesion between the extruded layers of paste. To provide cavities within the ceramic for the attachment of terminal wires, wax or plastic rods are attached to the band by compressing semi-cylindrical halves across the resistance wire and supporting threads between the elements. The rods are subsequently melted out of the ceramic, as may be done in a firing process, and suitable cavities exposing the resistance wire remain. Knife edges are provided on the rollers to cut waists into the ceramic paste and facilitate separation of the final resistors. Such separation may be made automatically, as shown in Fig. 10, where alternate wax or plastic rods are cropped short so as to allow the longer ones to operate a micro-switch 43 which triggers the drive to cutters 38 and 39. The cutters thereby remove the individual resistance wire elements embedded in ceramic. In this process two additional threads or wires 36, 37, are fed into the rollers between the outer edges of the extruded paste layers so that the waste part of the ceramic band left after cutting can be carried clear of the separated resistor elements as they are transferred to a conveyer. Additional support may be necessary for the bridging portions of the waste material and this may be provided, as shown in Fig. 15, by the formation of transverse bights 8<SP>11</SP> in the resistance wire between the elemental serpentine groups. Terminals may be attached to cavities formed in the ends of the embedded resistor elements by the use of wax or plastic rods 21, 21<SP>1</SP>, as shown in Fig. 11. The rod 21<SP>1</SP> will provide cavities in both adjacent embedded elements which, if they are not cut from the band, require the rod 21 to provide an escape for the molten material. In all embodiments the wax or plastic rods may be enlarged at the inner ends of the cavities they produce so that the soldered terminal wire will be keyed into the cavity. To increase the length of resistance wire within the space of a given element the serpentine loops may be corrugated along their length. Corresponding corrugations are in this event formed in the opposed faces of the extruded layers of ceramic paste. The embedded elements are dried and fired and are given a coat of glaze to seal off imperfections. During firing the core 1 on which the resistance wire is wound, and the supporting threads to which it is looped, may be combusted and the resistance wire sticks to the surrounding ceramic. The core 1 may, however, survive firing as it would if it were of glass fibre. The elements may be embedded in plastics material in place of ceramic material. Instead of being soldered within their cavities the terminal wires may be clamped by the use of expanded or barbed ends.