Patent GB-584325-A (GB)
Title: Apparatus for electromagnetic testing of metals
IPC: G01R33/12
Filed: 1944-08-01
Abstract: 584,325. Magnetic tests and measurements ; current and voltage measurements. STANDARD TELEPHONES & CABLES, Ltd. Aug. 1, 1944, No. 14673. Convention date, Jan. 22. [Class 37] ' In order to test or analyse a metal without destroying it or changing its properties, a specimen is inserted in a coil to which A.C. is applied, and the results that occur are observed. A source of A.C. 1 is connected through a transformer 2, 3 and a switch 4 to the primary 5 of a testing transformer. The secondary 6 of this transformer is earthed at one end and connected at the other end to a lead 9 through a switch 7 ganged with the switch 4. Tuning circuits are employed to feed the first, third, and fifth harmonics to amplifiers, while filtering out the other harmonics. These circuits comprise resistor-capacitor filters 10, 11, 12, each containing two tuned, rejector circuits, and feed the first, third, and fifth harmonics to the grids of amplifiers 13, 14, 15. Other resistor-capacitor tuned networks 16, 17, 18 connect the ends of the filters 10, 11, 12 to the anodes of the amplifiers 13, 14, 15 through capacitors 20, 21, 22. The anodes of the valves 13, 14, 15 are connected to a D.C. source S through load resistors 35, 36, 37. Each of the one-stage amplifiers 13, 14, 15 has a maximum gain at the frequency to which it is tuned and sharp attenuation at other frequencies. The anode of the valve 13, to the grid of which the first harmonic only is fed, is coupled by a capacitor 20 and lead G to a gain control 40, the sliding contact of which is connected to the grid of a cathode loaded amplifier 41. A resistor 42 shunted by a condenser 43 provides bias for the cathode of the valve 41. The cathode load of the valve 41 comprises the primary winding 44 of an iron core transformer, the secondary 45 of which is earthed at its centre and is connected through a capacitor 46 and variable resistor 47 to provide a controllable phase shifter. This is coupled through a resistor 48 to the control grid of a pentode 50 which, together with a second pentode 51, forms a phase-inverter deflection amplifier. The anodes of the pentodes 50, 51 are coupled through capacitors 56, 57 to the horizontal deflection plates 54 of a cathode ray tube 55. The anode of the valve 14, the grid of which is fed with the third harmonic, is similarly coupled through a gain control 59 to the grid of an amplifier 60, which is coupled through a phase-inverter deflection amplifier 65 to the vertical deflection plates 70 of the cathode-ray tube 55. Positioning controls 72, 73 are provided for the vertical and horizontal deflection plates. The anode of the valve 15, to the grid of which the fifth harmonic is fed, is coupled by a capacitor 22 and lead 80 to a gain control 81 and thence to the grid of an amplifier 82. The anode circuit is provided with a load resistor 83 and is coupled by a capacitor 84 to a rectifier 85, the anode of which is connected through a resistor 87 to a resistor-capacitor filter 88 and thence to the control grid of a pentode 89 serving as a D.C. amplifier. The output from the pentode 89 is fed to the grid of a D.C. amplifier 92, the anode of which is connected to the operating coil 93 of a relay 94. This relay is connected by a lead 95 to a point 96 directly connected to the grid of the valve 14, so that the third harmonic is short circuited when the fifth harmonic reaches a predetermined value. When a standard specimen is placed in the primary and secondary windings 5, 6 of the testing transformer the first and third harmonics produce on the screen of the tube 55 a Lissajous pattern having a 3 to 1 ratio which can be rotated by means of the phase shifter 45, 46, 47 for convenience of observation. The standard is then removed and a test specimen of the same size and shape inserted in its place. If the same figure is produced on the screen the first and third harmonics are the same as in the test specimen. The standard is then replaced and the gain control 81 adjusted until the relay 94 closes and short-circuits the third harmonic thus causing the Lissajous pattern to disappear and leave only a straight horizontal line due to the first. harmonic. The test specimen is again substituted for the standard and if the fifth harmonic is the same the third harmonic is again cut out with the same setting of the controls. When the first, third, and fifth harmonics of the standard and test piece are the same, the pieces are sufficiently alike.