GB578207A - Electrical devices for testing materials - Google Patents

Abstract

578,207. Magnetic tests ; determining physical qualities of materials ; inductance and capacitance measurements. STANDARD TELEPHONEB & CABLES, Ltd. April 28, 1944, No. 7989. Convention date, June 4, 1943. [Class 37] Materials that may vary in their magnetic or electrical properties are tested by placing a sample in a coil that is located in one arm of an A.C. bridge while the standard is placed in a similar coil in another arm of the bridge. The secondary 2 of a transformer is connected to voltage dividers 3, 4 in parallel, sliding contacts on which are connected to the grids of valves 6, 9. The cathodes of these valves are connected together and earthed through a resistance 10. The anodes of the valves 6, 9 are connected to the positive H.T. supply through resistances 11, 12 and are coupled by condensers 15, 16 to resonant circuits formed by inductance coils 20, 26 and condensers 21, 22, 27. These circuits form two arms of a bridge, of which the other two arms are formed by the valves 6, 9. A high frequency voltage from an oscillator is applied to the primary 1 of the transformer and a portion of the voltage from the secondary 2 is applied to the grids of the valves 6, 9. A standard piece of material is placed as a core in the coil 20 and the piece under test, of the same dimensions, is placed in the coil 26. If the test piece is the same as the standard the voltage impressed across the vertical and horizontal deflecting plates of an oscillograph 28 will be equal and in phase, giving a straight line on the screen. If the sample differs from the standard the trace becomes an ellipse. Amplifiers may be employed either at the input or output of the bridge, and distortion due to overloading the amplifiers may be avoided by employing buffer stages and limiter circuits such as diodes. Dielectric materials may be tested similarly by placing them in the condensers 21, 27.