Abstract: The components for operation of the invention are three: (1) a downhole tool consisting basically of a sensitive coil suitably protected by an encasing sheath as it is run downhole inside a string of drill pipe, tubing or the like, (2) surface apparatus to pulse the coil by discharging a capacitor through it, and later to detect small voltages induced in the coil as a result of running it past magnetic spots in the string produced by the capacitor discharges, and (3) connecting elements for both running and pulling the downhole tool, e.g., a wire line, and for electrically connecting the coil to the surface apparatus.Such apparatus is used in conjunction with standard equipment at the wellhead to stress the string by either pulling up on it or trying to rotate it, or both.