An in-hole current-measurement system for measuring an electrical current in an electrical conductor, the in-hole current-measurement system comprising: four or more magnetic-field sensors configured to be inserted into a hole in an electrical conductor , each sensor operative to generate a signal at an output in response to a magnetic-field strength at the sensor's location, wherein the four or more magnetic-field sensors are aligned on an axis configured to be mounted with at least a component of the aligned axis perpendicular to a direction of an electrical current in an electrical conductor when inserted into the hole; a stray-rejecting current-measurement module coupled to each of the four or more magnetic-field sensors' outputs to receive each of the sensors' respective signals representative of the magnetic-field strength at each of the sensors' respective locations, and to calculate, using the received signals and the sensors' locations, an odd-order spatial derivative of the magnetic-field strength at a measurement location along the aligned axis, wherein the calculated odd-order spatial derivative of the magnetic-field strength at the measurement location represents a measure of the electrical current; and a selection module coupled to each of the sensors' outputs to receive each of the sensors' respective signals representative of the magnetic-field strength at the sensor's location; characterised in that the in-hole current measurement system further comprises: a control module operative to control the selection module to select a subset of the magnetic-field sensors at a scheduled time of a predetermined disturbance event, and to cause a set of the non-selected magnetic-field sensors, which are scheduled to receive a disturbance associated with the predetermined disturbance event, to be deselected at the scheduled time of the predetermined disturbance event. four or more magnetic-field sensors configured to be inserted into a hole in an electrical conductor , each sensor operative to generate a signal at an output in response to a magnetic-field strength at the sensor's location, wherein the four or more magnetic-field sensors are aligned on an axis configured to be mounted with at least a component of the aligned axis perpendicular to a direction of an electrical current in an electrical conductor when inserted into the hole; a stray-rejecting current-measurement module coupled to each of the four or more magnetic-field sensors' outputs to receive each of the sensors' respective signals representative of the magnetic-field strength at each of the sensors' respective locations, and to calculate, using the received signals and the sensors' locations, an odd-order spatial derivative of the magnetic-field strength at a measurement location along the aligned axis, wherein the calculated odd-order spatial derivative of the magnetic-field strength at the measurement location represents a measure of the electrical current; and a selection module coupled to each of the sensors' outputs to receive each of the sensors' respective signals representative of the magnetic-field strength at the sensor's location; characterised in that the in-hole current measurement system further comprises: a control module operative to control the selection module to select a subset of the magnetic-field sensors at a scheduled time of a predetermined disturbance event, and to cause a set of the non-selected magnetic-field sensors, which are scheduled to receive a disturbance associated with the predetermined disturbance event, to be deselected at the scheduled time of the predetermined disturbance event. <EOS>