arm 1206 is pivotally deflected from its normally outwardly biased position against the surface of the pipe by a distended surface feature on the pipe with reference to fig2 it is there shown that dependent upon the pipe utilized any one of a predetermined number of distended portions on the pipe may be used to actuate the sensor arrangement embodying the teachings of this invention the position of the box end taper fig2 having a predetermined diameter of approximately 5 70 inches when using 5 00 inch drill pipe is such a convenient location on the pipe when the predetermined location on the pipe is encountered by the sensor roller 1208 and the arm 1206 is pivotally deflected from the normal position the switch emits an electrical signal on the line 1201 that the joint has been sensed when the predetermined location on the pipe is encountered the distance 1230 from that location to the joint is a known value the tongs lift is then stopped the sensor is retracted and the backup tong is locked due to the standardization of drill pipes in the oil drilling industry detection of a predetermined location such as a predetermined diameter on the box end taper insures that any other surface feature on the pipe such as the joint itself is then a predetermined known distance 1230 from the location which generated the deflection signal thus it.