Magnetic flaw detector with exciting and sensing coils axially aligned on opposite sides of the material

Claim 1

APPARATUS FOR DETECTING A FLAW IN MATERIAL HAVING THICKNESS AS A MINOR DIMENSION WITH A MAJOR PLANE TRANSVERSE THERETO AND WHICH MATERIAL IS MAGNETIC OR ELECTRICALLY CONDUCTIVE, THE COMBINATION OF, FLUX PRODUCING COILS EACH HAVING A WINDING AXIS NORMAL TO SAID MAJOR PLANE, FLUX SENSING COILS EACH HAVING A WINDING AXIS NORMAL TO SAID MAJOR PLANE AND WHICH IS SUBSTANTIALLY AN EXTENSION OF THE AXIS OF ONE OF THE FLUX PRODUCING COILS WITH THE FLUX PRODUCING COILS ON THE ONE HAND AND THE FLUX SENSING COILS ON THE OTHER LOCATED ON OPPOSITE SIDES OF THE MATERIAL, MEANS INCLUDING CONNECTIONS FOR SUPPLYING ALTERNATING CURRENT TO THE FLUX PRODUCING COILS, DIFFERENCE DETECTION MEANS CONNECTED TO THE SENSING COILS AND PROVIDED FOR COMPARING SIGNALS FROM DIFFERENT SENSING COILS, THE ARRANGEMENT BEING FURTHER CHARACTERIZED BY THE FLUX PRODUCING COILS BEING SPACED FROM ONE ANOTHER IN A DIRECTION ALONG THE MATERIAL TRANSVERSE TO THE DIRECTION OF THE COIL AXES, THE FLUX SENSING COILS WHOSE AXES ARE EXTENSIONS OF THE FLUX PRODUCING COIL AXES THUS BEING CORRESPONDINGLY SPACED.