GB392549A - Improvements in or relating to electrical resistances, heaters and the like - Google Patents

Abstract

392,549. Grid resistances. EXPANDED METAL CO., Ltd., Burwood House, Caxton Street, WILLIAMS, E. T., 5, Meadway Close, Golders Green, both in London, and MELSOM, S. W., 28, Lansdowne Road, West Hartlepool. Nov. 10, 1931, No. 31179. [Class 37.] A resistance element comprises a sheet of metal slitted so that the current path is a zig-zag passing to and fro across the sheet and stiffened at its edges. The metal sheet is slit in the manner shown in Fig. 3 by passing under a punching tool having an angle of movement making about 45‹ with the sheet so that the sheet is formed with a succession of V's both in plan and end elevation. The sheet is then passed through rollers or put under a press so as to press it into one plane. The end portions 1, 2 are left unslitted and may be stamped as hooks &c. 4, 5 to take connecting members. The edges are stiffened by U -section channelling 6, 7, insulating material 8, 9 being inserted between the channelling and the resistance sheet. Holes 10 .. 13 may be formed in the resistance sheet and the channelling is deformed so as to lock into these holes. A number of units may be assembled together by prolonging the U-section carrier clamps and cutting away half of the U so as to leave flat projections, which are then bolted together. A number of elements in series may be constructed from a single piece of metal so as to avoid joints. The connecting pieces are bent so that the elements are in parallel planes, in which position they are held by insulated bolts and spacing collars. Alternatively, as shown in Fig. 10, the elements are formed separately, and the tops and bottoms of the unslitted end portions Y are bent over and welded. Instead of edge channelling, central plates 19, 20 are employed for stiffening. Rods 16 in insulating tubes 17 are passed through holes in the unslitted end portions, the elements being spaced by bent-up members 21, 22 and clamped together by nuts 23. To protect against deformation due to expansion by heat, the units may be bent so that the zigzag path lies in a curved surface. A number of elements as shown in Fig. 3 may be mounted in a box having grooved or corrugated sides, the grooves or corrugations receiving the stiffening members 6, 7. The resistance may be made from stainless steel, silicon iron, &c., or from metals of higher specific resistance such as nickel-chromium alloys. Hot spots at the corners of the zig-zag path may be avoided by making the distance Z, Fig. 3, relatively large. The invention is particularly applicable to motor-starting resistances, but is also applicable to heating resistances. According to the Provisional Specification. the adjacent strips forming the zig-zag path may be bent in opposite directions out of the general plane of the path.