Abstract: A temporarily inaccessible object such as a locator in a buried pipe has a first transmitter (1) which produces an electromagnetic field rotating about a horizontal axis (A--A) and a second transmitter (7) which produces a solenoidal field centered on that axis. The rotating field is remotely detected at a receiver using a vertical axis coil (6) which locates the object and a reference signal is obtained by a horizontal coil (9) at the receiver. The roll .alpha. of the object may be determined by detecting the phase difference between the detected signal and the reference signal. The yaw or angle .beta. of departure from a desired horizontal direction is obtained from a maximum output of the horizontal receiver coil (9). With a third transmitter (15) producing another phase related field rotating about a horizontal axis transverse to the first one (A--A) a similar technique at the receiver gives the pitch angle .gamma..

Abstract: Two members 21,22 are relatively displaceable along a linear or circular path. One member 21 provides a plurality of coil portions along the path. Their individual inductances are dependent on the configuration of the second member 22. For example, the first member 21 may be an annular stator with inward radial projections k,l,m . . . on which the coil portions are wound, alternately in different senses so that the flux path tends to loop in and out. The second member 22 may be an annular rotor, one section 23 being ferromagnetic and the other section 24 being non-ferromagnetic and/or having a conductive screening layer. Alternatively the second member may be mercury 42 half-filling an annular conduit 40 which surrounds a static ferromagnetic core 44 (FIG. 9). A particular characteristic is that for any relative position of 21 and 22, half the coil portions of 21 have a different inductance from the other half.

Abstract: A high frequency and two low frequency electromagnetic signals are generated (1), the low ones being coherent with the second being a sub-harmonic of the first. The high frequency is modulated (4) by the second low frequency and applied (7,9) as a composite signal to an underground conductor, while the first low frequency is applied (9,10) additively. A receiver traversed over the line has aerials (12,21) or ground probes (24,25) for picking up these signals in separate channels. The high frequency is demodulated (18) to provide a reference signal for a synchronous detector (20) to which the first low frequency is fed. That suffers a phase reversal if there is a fault, and the output of the synchronous detector is arranged to indicate that.