Patent GB-520328-A (GB)
Title: Magnetic analysis
IPC: G01N27/82
Filed: 1938-05-16
Abstract: 520,328. Magnetic tests. FAIRWEATHER, H. G. C. (Magnetic Analysis Corporation). May 16, 1938, No. 14550. [Class 37] , In a method of magnetic analysis, an alternating signal voltage in a coil inductively influenced by the body to be analysed, is applied to the input side of a thermionic valve biassed so as to prevent the flow of anode current except for a brief interval at corresponding instants in successive cycles of the signal voltage, the anode current being an index of the magnetic characteristics of the test body which may be moved relatively to the coil to test various portions thereof and also compared with a standard. In the apparatus described three circuits are employed (a) a comparison circuit whereby a test specimen is compared with a standard specimen, (b) a flaw detector circuit for detecting flaws such as blow holes, &c., and (c) an indicator control circuit arranged to prevent end effects," i.e., straining of indicating instruments in circuits (a) and (b) as a test specimen is being passed into or from the test coil assembly. Test coil assembly. Comprises a primary coil 4, Fig. 1, mounted on an insulating tube 3 secured to insulating end plates 1, 2, mounted on a non-conducting baseplate 7 and a removable secondary coil assembly, consisting of non-conducting end rings 8, 9 connected by tubes 10, 11 as secondary coils 12, 13, 14, 15, 16, 19, 20, metallic rings 17, 18, being mounted on the tube 10 through which the test specimens are passed. Further auxiliary coils 24, 25 are arranged on the base 7 at right angles to the other coils. Flaw detecting circuit. The primary coil 4, Fig. 2, is connected in series with the auxiliary coil 24 and potentiometer 28, 29, across an A.C. supply 26. The two coils of each pair 13, 14 and 15, 16 of flaw detecting coils are connected in series opposition, the pairs also being in opposition in a bridge circuit including potentiometers 30, 33 provided for initial adjustment purposes. A flaw in a test specimen upsets the balance of the bridge, the resultant e.m.f. being amplified by transformer 31, amplifier 34, and transformer 35. The amplified voltage appearing across the secondary of transformer 35 is rectified at 38, the rectified voltage across the load resistance 40 being applied to trigger off a gas discharge tube 43, whereupon a coil 48 in the anode circuit of the discharge tube rocks a switch 51 to connect a neon lamp 50 across a battery 44. The neon lamp flashes at each discharge of a condenser 49 to indicate the presence of the flaw which may alternatively or additionally be indicated, if the test specimen is only moving slowly through the apparatus, by a rectifier type microammeter 37 connected across the output circuit of the amplifier 34. The arrangement of the detector coils 13-16 enables the apparatus to distinguish flaws which set up transverse field distortions from strains caused by heat treatment which generally cause longitudinal distortions of the field, the latter distortions not affecting the apparatus as they are apparently balanced out in the bridge circuit : " End effect," i.e., the violent fluctuations which are imposed upon the apparatus when a test specimen is passed into or from the apparatus is prevented by closing a switch 32 as hereinafter described to short circuit the input circuit of the amplifier 34. Standard, comparison circuit. The grid 70 of one triode portion of a double triode rectifier 69 is biassed by the voltage obtained directly from the A.C. supply 26 through a phase changing transformer 59 or condenser 62, transformer 61 and full wave rectifier 66, the load resistance 67 of which is connected across the grid-cathode circuit 68-70 of rectifier 69. In the right-hand position of a switch 71 the grid 72 is similarly biassed. These biassing potentials permit the two triode portions of valve 69 to become conducting only during a small period of each half cycle of the supply 26, the phase of the conducting period being variable throughout the cycle by the phase changer 59 or maintained at a fixed point in the cycle by condenser 62. The current pulses produced in the plate circuits of rectifier 69 are amplified in corresponding portions of a double triode amplifier valve 86, but as the impulses thereby produced in the circuits of the plates 91 92 are equal no deflection is produced in a galvanometer 93 which is connected through a potentiometer 94 across a resistance 89 in the two plate circuits. If there is any deflection it is balanced out by movement of the slider 90. Voltages-induced in the detector coils 12 of the test coil assembly are rectified in a full-wave rectifier 57, and, in the left-hand position of switch 71, the rectified voltage across the load resistance 58 is applied to the grid 72 in series opposition with the biassing voltage across the load resistance 67. This creates an unbalance and, the galvanometer 93 is deflected. The balance is restored by introducing a D.C. potential in the circuit of grid 72 by adjustment of a potentiometer 73. The potentiometer potential indicated by a voltmeter 96 is thus a measure of the potential at a particular instant in the cycle of the signal wave induced in detector coil 12 and its value at, all points in a half-cycle may be obtained in the above manner by operating the phase-changing transformer 59 whereby the wave form may be plotted. It is stated that the potential wave varies with the metallurgical characteristics of a specimen being tested. If a comparison test is required, then a standard specimen is'first inserted in the test coil assembly, and with switch 71 in the left-hand position, balance is effected at some predetermined point in the wave cycle determined by condenser 62 or transformer 59, by adjustment of potentiometer 73. On replacing the standard by the test specimen, any deflection of the galvanometer 93 then indicates dissimilarity. between the two specimens. " End effect" is eliminated by means of a switch contact 108 in the galvanometer circuit which is opened automatically while the ends of a specimen are passing into or from the test coil assembly as hereinafter described. The massive short circuit rings 1.7, 18, are also provided to reduce by eddy current effect, the distortion of the magnetic field of the coil assembly as the end of a specimen is passed therethrough. Indicator control circuit. This is arranged to close the switch 32 and open switch 108 as either end of a test specimen approaches or passes an end of the primary coil 4. For this purpose the secondary coils 19, 20 are disposed at opposite ends of the coil 4 and connected in opposition in opposite arms of a bridge circuit, each arm including a transformer primary 99 or 100 and a potentiometer 97 or 98. The auxiliary coil 25 is also connected in the bridge circuit in parallel with the other arms so that normally with no test specimen present in the test coil assembly, the three e.m.fs. in the three arms are balanced. No e.m.fs. are therefore impressed through the transformers 99, 100; the grids of the two diode-triode valves 101, 102 are not biassed and the resultant current from the parallel triode anodes actuates the coil 105 to close switch 32 and open 108. If a test specimen is now inserted into the test coil assembly so that it first approaches coil 19; the balance of the bridge is upset ; the increased' induced e.m.f. in the coil 19 is impressed through the transformer 100, rectified in the diode portion of valve 102 and applied to the grid thereof. The coil 105 is not yet released however, as sufficient current therefor is still provided by the valve 101, being released to allow switches 32, 108 to move to the positions shown only when the test specimen has similarly influenced the coil 20 to reduce the anode current in valve 101 also.