GB797399A - Improvements in or relating to apparatus for making electrical measurements upon coated conductors - Google Patents

Abstract

797,399. Electric tests. ADDISON ELECTRIC CO., Ltd. June 14, 1956 [Nov. 10, 1955], No. 32200/55. Class 37. The thickness and eccentricity of the coating on a conductor is measured by means of auxiliary electrodes which form condensers with the central conductor, the sum of their capacities indicating the thickness of the coating and their relative capacities the eccentricity. Fig. 3 shows a combined arrangement for indicating both thickness and eccentricity in which the electrodes 3 and 4 in spring-loaded contact with the insulating coating each form a condenser with the central conductor. These two condensers form a pair of adjacent arms of a bridge circuit of which the other two are formed by differential condenser 20. An alternating supply from oscillator 7 is applied across one diagonal of a bridge by a transformer 22 and the total current applied is a measure of the capacities of both condensers and therefore of the insulation thickness and is measured by a second transformer 9 having its primary also connected in the same diagonal and feeding a meter 26 through amplifier 24 and rectifier 25. The eccentricity is determined by sensing the unbalance in the bridge by means of transformer 16, the primary of which is connected across the other diagonal. This transformer feeds variable gain amplifier 27, phase-sensitive rectifier 28 and meter 30. In use, ganged switches are used to transpose electrodes 3, 4 and the terminals of meter 30 until the reading of this meter is the same in both positions. The gain of amplifier 27 is then adjusted in accordance with the reading of meter 26 when meter 30 indicates eccentricity directly. Switch 31 allows the D.C. output of rectifier 28 to control the gain of amplifier 27 directly if desired. A single amplifier phase-sensitive rectifier and meter may be used and may be switched to measure either thickness or eccentricity. A further arrangement may be used comprising a pair of electrodes at right angles to those shown. The output voltages from these arrangements may be used to operate alarm devices or to control an extrusion process.