GB666581A - Measurement or control of physical quantities - Google Patents

Abstract

666,581. Electric control systems. THOMAS, H. A., and NEWMAN, E. A. Feb. 14, 1950. [Feb. 14, 1949] No. 3992/49. Class 40 (i). Apparatus for measuring or controlling a variable physical quantity comprises means for determining the change in capacity produced by the relative movement of two electrodes due to variations in the physical quantity. As shown for determining changes in temperature, a short annular electrode A (Fig. 1) surrounds the end of the mercury column M of a thermometer, the electrodes being screened by a tubular shield S, a guard ring SR below electrode A, and shields Sl covering the leads P, Q, to the two electrodes A., M. The equivalent circuit is shown in Fig. 3, where C is the variable capacity between the electrodes. In a modification where direct connection to the mercury column is avoided, a second annular electrode B (Fig. 5) is located round the mercury column, shielded from the first electrode A by an earthed guard ring E. Part of the shield S and electrode A may be cut away (Fig. 4) to assist in locating the assembly relative to the end of the mercury column, a cross wire X across the gap in A acting as a setting index. The equivalent circuit is shown to be still approximately that of Fig. 3. A further modification, Fig. 8, (not shown), enables the thermometer to be used in a conducting medium. Specification 666,582, [Group. XXXVI], is referred to.