Apparatus and associated methods relate to a measurement system that calculates a current in a conductor based on an odd-order spatial derivative function of signals representing magnetic-field strengths within a hole in the conductor. In an illustrative embodiment, the odd-order spatial derivative function may generate an output signal representing a spatial derivative of the in-hole magnetic field greater than the first-order. The three or more magnetic-field sensors may be configured to align on the hole's axis when inserted into the hole. When inserted into the hole, the sensors may be aligned on an axis perpendicular to a direction of current flow and be responsive to a magnetic-field directed perpendicular to both the direction of current flow and the aligned axis. Some embodiments may advantageously provide a precise measurement indicative an electrical current in the electrical conductor while substantially rejecting a stray magnetic field originating from an adjacent electrical conductor.