the sensors 22 to detect the set of keys which is actuated in a predefined time window following triggering the central analysis system 23 is also connected to an automatic control system not shown for controlling the height of the tong 1 11 in use as explained at the beginning of this document a string of tubulars is restrained from falling into the well by applying the slips of a spider not shown located in the floor of the drilling platform in order to add a new stand of tubulars the new stand is moved from a rack nearby until it is correctly aligned above the top of the stand held in the spider the new stand is now spun by a drill pipe spinner not shown located above the spider so that the threaded pin screws down into the threaded box at the top of the drill pipe string in order to complete the joint the power tong 1 11 is now moved into position the recesses 3 13 are aligned and the tong is pushed forward so as to encircle the tubulars the detection apparatus 15 is pushed aside by the tubulars so that it pivots on the shaft 16 as the tubulars enter the tong and once the tubulars are past the detection apparatus 15 it swings back into its original position the arrangement of the detection apparatus 15 at the moment of contact with the tubulars 24 25 is shown in fig5 the thickness of the tubulars varies near the joint the higher tubular 24 has a thicker portion 26 designed to be gripped by the wrenching tong just above the pin 27 similarly the box 28 of.