Patent GB-476613-A (GB)
Title: Method of and device for measuring the thickness of foils
IPC: G01B7/06
Filed: 1936-06-27
Abstract: 476,613. Measuring resistances ; compensating for temperature errors. URMENYI, L. June 27, 1936, No. 17888. Convention date, June 28, 1935. [Class 37] A method of measuring the thickness of foil or the like consists in arranging the foil between a primary and a secondary coil, the former of which carries alternating current, whereby eddy currents are produced in the foil which vary the amplitude and phase of the current induced in the secondary coil to an extent dependent on the foil thickness and the secondary voltage is completely or partially neutralized by a compensating voltage which is approximately opposite in phase thereto, the difference between these two voltages being maintained constant by adjusting the phase and amplitude of the compensating voltage, the degree of adjustment required serving to indicate the thickness of the foil. The apparatus may be used for detecting variations in the thickness of foil during manufacture by passing the foil in a length between the two coils, any variation in thickness causing a variation in the voltage differences, which is indicated by a suitable measuring instrument. The foil to be measured is passed between the primary and secondary coils P1, S1, and the eddy currents induced in the foil change the voltage induced in the coil S1 to an extent which is dependent on the foil thickness. The voltage change is measured by means of a second pair of coils P2, S2, of which the primary coil P2 may be in series or in parallel with the coil P1 while the secondary coil S2 is in series with and acts in opposition to the coil S1. The coils S1, S2 are connected as shown to the grid of a pentode valve V1, the output of which is applied to the grid of a valve V2. A constant comparison voltage is applied to the grid of a valve V3 from a potentiometer P and the output circuits of the valves V2, V3, are connected to two resistances R1, R2 and a current meter I, so as to form a bridge circuit. The reading of the meter I is zero before the foil is introduced between the coils S1 P1 and after the foil is brought between the coils the reading is again brought back to zero by inserting between the coils S2 P2 a metal plate which may be slidably or pivotally mounted. The thickness of the foil is found from the position of the metal plate relatively to a scale. In addition the relative positions of the coils S2 P2 may be simultaneously varied to alter the voltage induced in the coil S2. If desired, the voltages may be amplified by connecting the condenser C and resistances W into the circuits, so as to form a tuned circuit. The valves V2, V3, may be replaced by rectifiers. In a modification only one valve is used, the voltages of the coils S1, S2 neutralizing one another either partially or completely when no foil is present. On introducing the foil between one pair of coils the change of voltage alters the grid bias of the valve and the plate current of the valve, amplified if necessary, is reduced to its original value by introducing a metal plate between the other pair of coils. In a further modification, a further pair of coils is employed ; the secondary coil S1 of one pair is connected to the grid of one valve, the secondary coil S2 of a second pair is connected to the grid of a second valve while the secondary coil of the third pair is earthed in the middle and the ends are connected one to each of the cathodes of the valves. In the plate circuits of the valves there are connected resistances and an ammeter, as shown in the Figure. Under these circumstances, the meter reading is zero when no foil is inserted and is also zero when the foil and the metal plate of known thickness are in position, provided these are of equal thickness. Should the foil be thinner the reading of the meter becomes increased, while if the foil be thicker the reading falls. Specification 429,227 is referred to. The Specification as open to inspection under Sect. 91 comprises also a circuit in which the point B is connected to the grid of one valve while the point A is connected to the grid of another, the plate circuits of the valve being connected to resistances R1, R2, and a meter I as shown. The meter reading is zero before the foil is introduced and is reduced again to zero with the foil in position by adjusting the metal plate between the coils S2, P2 and also by adjusting the relative position of these coils. In order to correct for temperature variations of the foil, a copperconstantin thermo-couple may have one junction arranged close to the foil, the other junction being maintained at a constant temperature. The thermo-couple is connected across a potentiometer, a tapping on which is connected to the grid of one of the valves V1, V2. Owing to the varying sensitivity of the apparatus with the foil thickness, the tapping must be varied with the foil thickness and the contact is therefore connected mechanically with the indicator which indicates the foil thickness. Alternatively, the change in resistance with the temperature of the foil of a copper wire R1, Fig. 6 (Cancelled), arranged in the vicinity of the foil may be used, two copper wires R1, R2 and two constantin wires R3, R4 being connected in series, the two points A1 and B1 being connected by a potentiometer P1 the tapping from which is connected to the valve grid. The wires R2, R3, R4 remain at a constant temperature. This subject-matter does not appear in the Specification as accepted.