Patent GB-253165-A (GB)
Title: Improvements in electrical resistances
IPC: H01C7/22
Filed: 1925-02-10
Abstract: 253,165. Ledward, T. A. Feb. 10, 1925. Resistance coatings; resistances with specially related inductance. - Resistances, particularly applicable to thermionic valve amplifiers, are made by coating, for example by dipping, a support of insulating and heat-resisting material with a liquid containing finely divided carbon in suspension, drying the coating preferably by heating, subjecting the coated former to heat treatment at a higher temperature than that at which the unit is to be used, for example, at 500 to 1000‹ F. in order to increase the stability of the coating, and subsequently removing the coating locally to form a conducting path of greater length than the nearest distance between the end connections. A potential of 200 to 300 volts may he applied until the resistance value is steady. After the final heat treatment and before the element has cooled below 100‹ C., it may be dipped in molten wax above 100‹ C. The coating is removed locally by scraping, a resistancemeasuring instrument being connected to the element, or by removal of a pattern laid on the former before the coating was applied. The final resistance coating on a former such as a glass tube may be in the form of a helix c<2>, Fig. 1, connecting and covering connections B at the ends of the tube, or of a helix reversed in direction at the centre of the tube, or of a transverse or longitudinal zig-zag. The end connections B may have portions b<2> embedded in the support to take any strain applied to the free ends of the connections and prevent such strain from being communicated to other portions b<1> of the connections bent over the outside to contact with the coating. The portions b<1>, b<2> may be bent as shown in Fig. 1, and held by heating and pressing the glass tubes to embed the connections, the ends of the tube being at the same time sealed to close access to its interior. The element so prepared may be enclosed in ebonite or other insulating tube filled with wax or other insulating material, conical brass caps being screwed on to the ends of the insulating tube and soldered to the connections B. In modifications, the portions b<1>, b<2> are shaped as in Fig. 1<B> or 1<D> prior to embedding in the ends of the glass tube A. In further modifications, the end connections may be formed by binding wire around the end of the tube and the free end b<3>, Fig. 2<A>, of the binding B<3> may be bent into the end of the tube to form a shock-absorbing loop b<4> absorbing slight strains on the ends of the connections. In another modification, Fig. 4, an insulated strip cut with slots a<4> is covered with the resistance coating which is then removed from opposite faces of the strip along the edge a<6> and the centres of the projections a<5> to form a zig-zag conductor. The Provisional Specifications describe forms in which the end connections are of copper wire softened and oxidized on the surface or of metal foil or gauze or of electrodeposited coatings of copper or other metal or of wire embedded in a spiral groove, in which two or more coats of Indian ink are applied and each dried before the application of the next, in which the tubular support is heated to 150‹ F. before coating with the liquid, in which a single straight conductor between the end connections is formed by locally removing the coating, in which a screw-threaded former is coated and surplus coating removed, and in which two or more elements are mounted in a single container and connected in series or parallel permanently or by means of external switches. Wire resistances, cored.-One of the Provisional Specifications describes wound resistances with end connections described above, for example, with parts embedded in the former or with a shock-absorbing loop or spiral,