Patent GB-592586-A (GB)
Title: Improvements in and relating to electric resistance units
IPC: H05B3/48
Filed: 1944-06-15
Abstract: 592,586. Electric resistances. BRITISH THOMSON-HOUSTON CO., Ltd. June 15, 1944, No. 11427. Convention date, June 15, 1943. [Class 37] An electric resistor comprises a resistance conductor wound upon a rigid conducting rod serving as one terminal thereof, the turns of the resistance being embedded in a casing of insulating material which is surrounded in turn by a layer of corrugated mica and an outer metal sheath. The terminal rod 10, preferably of nickel to withstand high temperatures without detrimental oxidisation, is preformed with an eye-shaped terminal 31 on a portion thereof bent at right angles to its main length. The rod is first inserted through an insulator head 17, for example, of porcelain, and secured thereto by applying vitreous insulating enamel 22c. A layer of insulation, preferably vitreous enamel 11, is then applied to the rod 10, the resistance conductor 12 wound thereon and secured by brazing to the rod 10 at one end and at its other end to copper strap connector 14, Fig. 3, which is brazed at its arcuate transversely extending part 33 to a terminal plate 15. The plate 15 has a square aperture 19 locating it on the reduced square end 18 of the insulator head 17 and has lugs 20, 21, 22 extending therefrom at right angles nesting against three of the sides of the end 18. The resistor 12 is made of a heat resistant or refractory alloy of nickel and chromium and as shown has an insulation covering of woven glass. This covering may, however, be dispensed with when the resistor wire or strip is large enough to be self-supporting when wound into a helix. In this case, the resistor turns are spaced from the insulated rod 10. The metal sheath 25, for example, of nickel, may be preheated to 1000‹ C. for 15 minutes to oxidize and therefore darken its outer surface to facilitate radiation of heat therefrom. A porcelain insulator 26 is secured in the sheath 25 by spinning and contracting the sheath at 27, 28. A layer of mica 24 corrugated longitudinally is then inserted in the sheath and forced outwards by a suitable tool and the sheath is contracted at 29. A predetermined quantity of insulating material 23 in the form of a thick liquid or paste is then inserted in the sheath, and the terminal rod and resistor assembly, after a preliminary dipping into a container of the prepared insulating material to coat the rod between the resistor turns, is then pressed into the sheath, surplus insulating material being forced out of the rod-locating hole in the insulator 26 and at the enlarged end of the sheath. A mica or other insulating ring 30 insulates the sheath from the contact plate 15. The insulating material 23 is preferably formed by adding water to a powdered mixture of sodium silicate, feldspar, flint and waterglass. After assembly, the unit is allowed to dry for 12 hours while held over water so that moisture may escape from the interior before the outer portions harden to form a seal. The unit is then heated in an oven to about 200‹ C. for three hours when the insulating material is hardened into a compact mass rigidly securing the resistor and rod assembly in the metal case 25. Apertures may be formed in the mica layer 24 so that the insulating material 23 forced therethrough intimately engages and further secures the outer casing.