Patent GB-444056-A (GB)
Title: Improvements in or relating to apparatus for measuring the thickness, moisture content or other characteristics of materials
IPC: G01N27/22
Filed: 1935-10-28
Abstract: 444,056. Capacitance measurements. JACKSON, L. MELLERSH-, 28, Southampton Buildings, Chancery Lane, London.-(Foxboro Co.; Foxborough, Massachusetts, U.S.A.) Oct. 28, 1935, No. 29729. [Class 37] [See also Group XIX] In apparatus for measuring the thickness, moisture-content, or other characteristics of a material, in which the material is arranged to be included in a highfrequency oscillatory test circuit and to influence the reactance thereof, the test circuit is energized from a separate high-frequency oscillatory circuit through an aperiodic coupling circuit which is loosely coupled to both the oscillatory circuits. In this way, the energizing circuit and the measuring instruments may be placed at a distance from the test circuit and material to be examined. In the arrangement shown, the material 4, the thickness of which is to be measured, is earthed and forms one side of a condenser 1, of which the other side is constituted by a pair of plates 2, 3 connected together. Variations in the thickness of the material 4 vary the capacity of the condenser 1, and affect the tuning of an oscillatory circuit provided with condensers 9, 10 which are connected in parallel with the condenser 1 and in series with an inductance 5 and a thermocouple 11. The latter generates a direct current which is proportional to the alternating current excited in the circuit including the condenser 1, the direct current being measured by an ammeter 20 which may be placed at any desired distance from the testing circuit and connected thereto by leads 21, 22. The exciting oscillatory circuit comprises a thermionic valve 25 excited from a power unit 32, the grid circuit of the valve including a condenser 46, a resistance 45 and an inductance 47. The plate circuit of the valve includes a condenser 37 and coil 38, between which the circuit is earthed through a condenser 39 and through a condenser 40 and radio frequency ammeter 41. One plate of the condenser 39 is formed of thermostatic material, so that variations in frequency of the exciting circuit due to temperature changes of the oscillator are compensated by corresponding variations in the capacity of the condenser 39. The two oscillatory circuits are so adjusted as to be slightly detuned from each other, so that variations in the frequency of the excited circuit due to changes in the characteristic to be measured vary the current response in the excited circuit. The two circuits are coupled together by a low-voltage coupling circuit comprising a coil 60 loosely coupled with the coils 38, 47 and a coil 63 loosely coupled with the coil 5, the coils 60, 63 being connected together by leads 61, 62, which may be enclosed in the same cable as the direct current leads 21, 22. The exciting circuit and the parts associated therewith may be assembled together in a casing 100 divided into three compartments by partitions 101 ... 103. The upper compartment 106 contains the D.C. ammeter 20 and one of the lower compartments houses the valve 25 and a radio frequency choke 50. The remaining elements of the high-frequency exciting circuit are mounted in the third compartment. Specifications 276,468, 276,469, [both in Class 37], and 281,693, [Class 97 (iii)], are referred to.