GB556464A - Improvements in or relating to variable electric resistances - Google Patents

Abstract

556,464. Variable resistances. MORGAN CRUCIBLE CO., Ltd., and MILLER, L. W. March 2, 1942, No. 2762. [Class 37] A variable resistance suitable for volume control in radio reception on aircraft is made as small as possible and capable of withstanding extreme conditions of humidity and temperature. The case consists of a metal body 1 having a bearing bush 2 for the operating spindle 3 sealed in the bottom and a lid 5 of moulded insulating material having on the inside a segmental resistance strip 6 encircling an inner contact 7 and on the outside terminal tags 8, 9 connected to the ends of the strip 6 and a third terminal tag (not shown) connected to the contact 7. The body 1 and lid 5 are sealed by a ribbon of thermoplastic or other jointing material in a groove 12 which is melted, after assembly, to form a hermetic seal between the lid and the body. The lid is further secured in position by bending tags 13 on the body over the top of the lid. The terminals are stamped out of sheet metal and formed into bucket-like pieces 14 with upstanding flexible tags. They are mounted at the base in sockets in a box 16 on the lid 5 and secured by rivets passing through the lid. The bearing bush 2 is sealed into the bottom of the body 1 by moulded synthetic plastic material 21 which is formed with an annular channel 22 to accommodate the rim 23 of the lid with the interposition of a film of jointing material. A recess is formed in the layer 21 to engage a projection 25 in the lid to ensure correct angular relationship between the body and the lid. This projection may also serve as a stop to limit the travel in both directions of a rotor head 4 carried by the spindle 3. The sliding contact comprises a spring strip 27 pivoted in the rotor head so that one end bears on the resistance strip 6 and the other bears on the contact disc 7, The strip 27 is shaped as shown with projections 28 engaging recesses in walls 41 projecting from the rotor head 4. The head 4 carries a boss 29 which passes through the central aperture of the contact 27 and abuts against the centre of the contact 7 to serve as a spacer. The bottom of the rotor head 4 has a short boss 30 surrounding the top of the spindle to bear normally on the top of the bush 2 when the rotor contacts are bearing with the desired spring pressure on the contacts in the lid. An insert 31 may be moulded into the head 4 and formed with a central hole into which the reduced and splined end 32 of the spindle 3 may be forced and held frictionally so as to enable a standard rotor head to be employed with different spindles.