GB661313A - Improvements relating to thermostats - Google Patents

Abstract

661,313. Thermal switches. BRITISH THOMSON-HOUSTON CO., Ltd. May 6, 1949. [May 7, 1948] No. 12165/49. Class 38 (v). A switch suitable for controlling an electric oven, comprises a member 17 for transmitting the movement of a bimetal strip 14 to one, 9, of a pair of resilient contact-carrying arms 9, 10, biased to engage one another by being anchored on sloping shoulders on a block 4. The low (room) temperature setting of the switch is obtained by adjustment of arm 10, and the high-temperature setting is obtained by adjustment of the position of member 17. For low-temperature setting, a shaft 20 is screwed down until an insulating extension 21 thereon engages the contact arm 10 and just opens the circuit, whereupon an extended portion 24 is secured to the shaft 20 so that a pointer 27 on a, knob 25 indicates the room temperature on a plate 26. The knob 25 is then rotated until the pointer indicates the highest temperature required, e.g. 400 degrees F, and the switch is immersed in a bath or placed in a box maintained at this temperature. Whilst in the bath, an arm 15 carrying the member 17 is slid along a support 13 until the contacts just open, and is then welded in this position. Owing to the parallelism of the arm 9 and bimetal strip, the room-temperature setting is not upset by this alteration of the position of member 17. In a modification, Fig. 8 (not shown), the member 15, 17 is replaced by a member having a screw-and-slot connection with the arm 9, and in a further modification, Fig. 10 (not shown), the arm 15 has a screw-and-slot connection with the support 13. In these embodiments, the bimetal strip is placed remote from the arms 9, 10 so as to be unaffected by the heat produced by the current therein, but in a further modification, Fig. 12, the bimetal strip 52 is placed between the contact arms 48, 50, a further slidable arm 53 having an insulating transmitting member 54 being placed between arm 50 and strip 52. The shaft for varying the low temperature setting comprises a hollow portion 58 screwed into a fixed boss 60 and a portion 62 threaded into the upper part of portion 58 and adjustable by a screwdriver inserted through the bore 61, the two portions being then cemented together. The strip 53 has a screw-and-slot connection with a support 55. In a modification, Fig. 14 (not shown), the upper setting is adjustable in use by making the member 54 spherical and replacing the strip 53 by a rod, an extension of which screw-threads into a support. In all embodiments, the rotation of the shaft is limited by stops.