Patent GB-672984-A (GB)
Title: Improvements in or relating to electrical resistors
Filed: 1949-06-16
Abstract: 672,984. Wire resistances; resistance casings; sets of resistances. GENERAL ELECTRIC CO., Ltd., MAWSON, R., and NOAKES, W. F. June 16, 1950 [June 16, 1949], No. 16104/49. Class 37. A resistor for use at high frequencies comprises a resistance element having a low inductive reactance and supported on a planar assembly comprising a rigid sheet of insulating material and two conducting terminal members, which are attached to and spaced apart by the insulating material and connected to the ends of the resistance element. Such a resistor may form a component of a variable attenuator operating in the frequency range of from 0 to 30 megacycles per second. Fig. 1 shows a resistor having a low value, at which it is more important that the self-inductance of the resistor shall be kept to a minimum rather than the self-capacitance. The resistance element 1 is formed of wire covered with insulating material and formed into a single bifilar loop which is maintained in this condition by coating it with a thermoplastic varnish. Two metal members 2, 3 are bent up from sheet-metal with upstanding lugs 10, 12, 13, 14, 18 and a flap 15. The members 2, 3 are joined by a rectangular mica sheet 4, the members being separated from one another and insulated by a narrow strip of the mica sheet. The members 2, 3 have portions which are folded under the mica sheet 4, the joints being soldered down and four punched indentations 20 being made in the metal and mica sheet to secure the assembly. The resistance element 1 is secured by thermoplastic varnish to the metal member 2, the bifilar loop being bent into narrow U-shape. The ends of the resistance wire are secured under and soldered to the lugs 14, which are bent down over the wire. A mica insulating sheet similar to the sheet 4 is placed over the element 1 and the flap 15 is bent down on to it. A further mica sheet is then placed over the flap 15 and clamped in position by bending over the lugs 10, 12, 13, 18. Fig. 3 shows a construction for a resistance of intermediate value where it is' necessary to consider its inductive and capacitative reactances equally. The large flap 15, Fig. 1, is omitted owing to the high self-capacitance it introduces and the two metal members 2, 3 are made identical. As the length of the bifilar loop 1. is much greater than that shown in Fig. 1 it is formed into two U-shaped portions 21 bridged by a straight portion 22. For resistance values where the U-shaped portions would,be too large the elements are wound in two equal flat coils bridged by a straight portion and fixed with a coating of thermoplastic varnish before transference to the resistor assembly. Fig. 6 shows a modification for a resistor of high resistance value, where it is important that the selfcapacitance should be kept to a minimum rather than the self-inductance. The resistance element 23 is wound on a former 24, Fig. 7b, in the form of a rectangular mica sheet out of which a slot 25 is cut, so that the former 24 has two parallel limbs 26 joined by a yoke 27. The resistance wire, which is a single insulated conductor and not a bifilar loop, is wound outside the sheet and passed through the slot 25 after passing around one of the two limbs, the winding being fixed by thermoplastic varnish. The winding does not extend to the tips of the limbs 26. The yoke 27 of the former 24 is clamped between bent-under portions of a metal member 28 and fixed by punching indentations 36. The tips of the limbs 26 are similarly clamped between bent-under portions of a similar metal member 37. Transparent plastic strip is then wound around the exposed part of the former for protection. The ends of the winding are soldered to lugs 38, which are then bent down. In each construction the two metal members 2, 3 or 28, 37 serve as terminals. Attenuators are made by assembling appropriate values of resistor in pads having shunt and series elements, the pads being connected and disconnected by switches to vary the attenuation.