GB494886A - Improvements in and relating to electric thickness gauges - Google Patents

Abstract

494,886. Magnetic tests and measurements. BRITISH THOMSON-HOUSTON CO., Ltd. March 31, 1938, No. 9896. Convention date, April 1, 1937. [Class 37] A thickness gauge comprises a gauge head on which is mounted a permanent magnet arranged to pass a magnetic flux through the material and a flux-measuring element, and a second permanent magnet which tends to pass a magnetic flux through the flux-measuring element in the opposite direction past an adjustable air-gap. In the construction shown in Figs. 1 and 2, a sheet of paper, rubber, fabric, or other non-magnetic material 11 is passed over a roller 12, or a stationary platen, of steel or other magnetizable material. A gauge head 13 is provided with a pair of shoes 14 of soft iron or other magnetizable material which ride on the sheet 11 and may have wear-resisting inserts of cemented tungstencarbide. Within the gauge head 13 is a permanent magnet 15, the magnetic circuit of which passes through the shoes 14, roller 12, and a strip 23 of magnetic material that varies in permeability with variations in flux density, such as an alloy of 78¢ per cent of nickel and the remainder iron. A second permanent magnet 24 is so arranged that its magnetic circuit passes through the strip 23 and an air-gap 31 adjustable by a screw 28 and coarse and fine adjusting heads 32, 33. Surrounding the strip 23 is a coil 34 in series with a non-linear resistor 35, a D.C. instrument 36 shunted by a condenser 37, and an A.C. source 38. The resistor 35 may be composed of a mixture of silicon-carbide and carbon as described in Specification 292,110, [Class 37]. The permanent magnets 15, 24 tend to produce a flow of flux in opposite directions through the strip 23. The air-gap 31 may be adjusted to obtain a zero reading on the instrument 36 when the shoes 14 are resting directly on the roller 12, or the screw 28 mav be calibrated in terms of thickness when zero deflection is obtained. Any unidirectional magnetization of the strip 23 produces lack of symmetry in the wave form of the A.C. current in the coil 34, thereby causing a difference in the resistance of the resistor 35 to current in opposite directions. A unidirectional current is therefore measured by the instrument 36. A modified flux-measuring device is shown in Fig. 4, in which the variable permeability strip 23<1> is divided to form three legs 40, 41 42. An A.C. exciting winding 43, 44 produces an alternating flux in the circuit of the outer legs 40, 42. A unidirectional flux representing the difference between the magnets 15, 24 causes unbalanced permeabilities in the legs 40, 42 so that alternating flux is diverted to the middle leg 41, which is measured by means of a winding 45 in series with a rectifier 47 and D.C. instrument 46.