Calibration loop. A device used to calibrate [[induction log|induction logging]] tools. A test loop is an artificial ground loop which consists of a continuous loop (i.e., ring) of electrically conductive material containing a series connected precision resistor. The purpose of the loop is to provide a precise, calibrated, repeatable [[signal]] in the induction [[receiver]] coils when it is placed over the [[measure point]] of the activated [[tool]] in a zero signal medium (i.e., air). See [[induction log]].