A magnetic search coil was constructed that is equivalent to two differentially connected pick-up coils equally spaced from a centrally wound exciting coil. The coils are wound on a bakelite tube and are con-structed so that the tube and coils can be opened along a direction parallel to the axes of the tube, and placed around a pipe. The voltage Induced in. the pick-up coils by the exciting coil is rectified by a mechanical recti¬fier and indicated on a d.c. galvanometer both in magnitude and direction. This gives a measure of the space change of magnetic permeability along the length of the pipe. Hard spots in piping give indications corresponding to an area of low permeability. Sections along the pipe that are found to have low permeability contain hard spots that might be in the pipe. A ball-punch hardness tester, constructed from an automatic center punch, is used to test the areas of low permeability for hardness. An abstract of operational instructions is appended to this report.
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