Patent GB-577712-A (GB)
Title: Improvements in electromagnetic testing devices
IPC: G01N27/82
Filed: 1944-03-28
Abstract: 577,712. Magnetic tests and measurements. MAGNETIC PRODUCTS CORPORATION, BINDSCHEDLER, T. S., and DAVIS, C. K. March 28, 1944, No. 5655. [Class 37] [Also in Group XXXV] An electromagnetic device for testing the physical, chemical, or heat treatment properties of the surface of magnetic materials comprises a core of high permeability and having a rounded base portion and a pair of legs extending therefrom which are curved and tapered towards each other whereby their free ends are in proximity and adapted to contact material to be tested, an exciting coil surrounding the base so as to expose a minimum of the surface of the base to the air, and a source of high frequency current connected to the coil. The test piece 1 is placed so as to bridge the air gap in the core of an electromagnet having a winding 2, Fig, 3, fed with alternating current. The reluctance of the magnetic circuit is dependent on the physical properties of the test piece 1 and varies the impedance of the coil 2 and hence the current, which is measured. The core is composed of several groups of laminations, of which the group 20 are C-shaped. The laminations 21 are shaped as shown and are pressed downwards by springs 52 so as to make good contact with the test piece 1. Additional small laminations may be placed above the spring-pressed laminations 21 and within the C-shaped laminations to complete the core. If the coil 2 is excited by a high-frequency current the flux only penetrates the surface of the test piece and enables the depth of case-hardening to be measured. With a low-frequency current the penetration is deeper and a conglomerate test from the surface to the interior is obtained which may be employed for determining the effect of heat treatment, and other physical and chemical properties of the material. Fig. 6 shows the circuit arrangement. A voltage stabilizer 25 consists of two transformers with their primaries in series and their secondaries in series opposition. One of the transformers is operated at a high flux density and is tuned by a condenser. The output from the stabilizer feeds the primary winding 26 of a power transformer, the secondary winding 27 of which is connected to a full wave rectifier 28 which supplies the anodes of oscillator valve 37, 41. The valves 37 form a high-frequency oscillator the circuit of which contains an aircored transformer 32 having three windings 33, 34, 35. The winding 35 is tuned by a condenser 36 to the required frequency. The winding 33 is connected to the grids of the valves 37, and the winding 34 supplies power to the test unit assembly. The valve 41 forms a low frequency oscillator supplying an iron cored transformer 38 having windings 39, 40, 45 which are connected similarly to the windings 33, 34, 35. A two-way switch 24 enables the coil 2 to be connected to either of the ocsillators 37, 41 through a rectifier 47 and D.C. ammeter 48. This is shunted by a group of resistances 52, 53, 54 in series with a resistance 50, enabling'the calibration of the ammeter 48 to be adjusted on both frequencies independently.