GB988262A - Improved variable resistance device and method of making such device - Google Patents

Abstract

988,262. Variable resistors. BECKMAN INSTRUMENTS Inc. Jan. 25, 1962 [Feb. 23, 1961], No. 2941/62. Heading H1S. A variable resistance device of the continuous rotation type includes a toroidal support, a coil having a plurality of turns wound on the support and consisting of a live portion and a dead portion, and a contact member in sliding contact with successive turns of the coil along a contact path, the turn at each end of the dead portion being severed at a point not on the contact path, and the number of turns in the dead portion being greater than the number of turns simultaneously engageable by the contact member. A resistance element 19 (Figs. 2, 4) comprises a helical winding of resistance wire 30 extending over substantially the whole length of a toroidal support 28 (Fig. 1). A terminal lead 70 is near but spaced from one end of the wire 30 and a terminal lead 71 is near but spaced from the other end of the wire 30. The wire 30 is severed between each lead and the adjacent wire end. Preferably a plurality of severed points 75 are provided at each end. Before severing the wire 30 a coating 58 (Fig. 3) of enamel is applied over 270 degrees of the surface of support 20 to prevent displacement of the turns. Alternatively, the entire surface of the support 20 may first be coated with enamel and a portion of enamel extending over 90 degrees removed. The toroid is preferably of enamelled copper wire, the enamel being polyvinyl formal polyester or phenolic epoxy resin. Severing of the wire 30 is performed under the enamel coating 58 by a sharp-edged tool 76, Fig. 5, or by contacting electrical leads 77, 78 to adjacent regions of a single turn of wire and " blowing " the section between the leads with current from a source 80. The resistance element is situated in a two-part housing 10 (Fig. 1) together with a wiper 51 (Fig. 2). The wiper 51 is continuously rotatable by a shaft 31 and the wiper moves equally smoothly over the live and dead portions. Electrical connection from a terminal 55 to the wiper 51 is provided by a brush 56 riding on a bush 50 which also serves to attach wiper contact arm 39 to a block 33 carried by shaft 31. The resistance element 19 is made by providing an elongate deformable core, helically winding resistance wire on the core and bending the core to form a substantially closed ring. Specifications 782,938, 935,732, U.S.A. Specifications 2,334,880, 2,620,990, 2,783,361 and 2,860,455 are referred to.