in u s pat no 3 791 043 or u s pat no 3 862 499 with the probe being electrically connected by a flexible line 29 to a readout unit 30 at the surface of the earth as in those patents probe 28 may be magnetically responsive to the earth 39 s magnetic field and if so is contained within a non magnetic section 10a of the drill pipe 10 the lower end of the instrument probe 28 may be anchored within an attaching sub 31 connected into the string between section 10a and the drill unit 16 instrument 28 may be located centrally within section 10a in a position of extension along the main axis 18 of the drill string with a space 32 being left about probe 28 to pass pressurized circulating fluid downwardly past the instrument and to the drilling unit for attaching the lower end of probe 28 rigidly to sub 31 in centered relation the lower or inner end of the probe may carry an externally threaded row 33 carrying a nut 34 which is tightenable upwardly against the central portion of a fluid passing spider 35 in sub 31 tightening of nut 34 on rod 33 pulls a series of circularly spaced radially projecting fins 36 on the lower end of probe 28 downwardly against.