Abstract: Two vertical, parallel, horizontally spaced signal coils are connected in series but in opposite phase across an a.c. source. Thus a buried conductor located centrally beneath the coils experiences an alternating magnetic field whose strength is due to the addition of the fields induced by the two coils. The field strength may be further enhanced by one or more of: providing a ferromagnetic flux diverter between the upper ends of the coils; placing a conductive nonmagnetic screen between them; connecting a tuning capacitor in parallel with the coils; or providing a tuning circuit having series-connected secondary coils which are flux-linked to respective signal coils and are in parallel with a capacitor. There may be a switch arrangement for switching the signal coils to in-phase connection, for use when they are oriented horizontally.

Abstract: The invention relates to a motor driven actuating mechanism which incorporates means of sensing torque and deriving a mechanical movement therefrom. The invention provides a mechanical brake which utilizes the mechanical movement of the torque sensing means to brake the mechanism if a preset torque is exceeded.In the preferred embodiment of the invention the mechanism comprises a worm and wormwheel in which the worm shaft is movable axially against a spring in response to torque reaction. The shaft movement is utilized to de-energize the drive motor and the mechanical brake comprises a brake disc attached to the worm shaft.