A power transmitter for wirelessly providing power to a power receiver via an inductive power transfer signal; the power transmitter comprising: a transmitter coil; a driver for generating a drive signal for the transmitter coil; a plurality of sets of balanced detection coils (207, 209) coupled in series, each set of balanced detection coils (207, 209) comprising two symmetric detection coils arranged such that signals induced in the two detection coils by an electromagnetic field generated by the transmitter coil compensate each other; a foreign object detector coupled to the plurality of sets of balanced detection coils (207, 209) and arranged to perform foreign object detection during the foreign object detection time interval, the foreign object detector being arranged to detect a foreign object in response to signals from the plurality of sets of balanced detection coils (207, 209) meeting a foreign object detection criterion, the foreign object detection criterion including: a first requirement that a signal from at least one of the plurality of sets of balanced detection coils (207, 209) indicates an imbalance between signals induced in the two detection coils of the at least one of the plurality of sets of balanced detection coils (207, 209) exceeding a first threshold; wherein the power transmitter is characterized by: the driver being arranged to generate the drive signal for the transmitter coil to generate the power transfer signal during at least one power transfer time interval of a repeating time frame and to generate the drive signal for the transmitter coil to generate an electromagnetic test signal during at least one foreign object detection time interval of the repeating time frame and by the foreign object detection criterion further including a second requirement that a number of signals from the plurality of sets of balanced detection coils (207, 209) indicating an imbalance between signals induced in the two detection coils of the set of balanced detection coils (207, 209) exceeding a second threshold is less than a threshold number, the threshold number being at least two. a transmitter coil; a driver for generating a drive signal for the transmitter coil; a plurality of sets of balanced detection coils (207, 209) coupled in series, each set of balanced detection coils (207, 209) comprising two symmetric detection coils arranged such that signals induced in the two detection coils by an electromagnetic field generated by the transmitter coil compensate each other; a foreign object detector coupled to the plurality of sets of balanced detection coils (207, 209) and arranged to perform foreign object detection during the foreign object detection time interval, the foreign object detector being arranged to detect a foreign object in response to signals from the plurality of sets of balanced detection coils (207, 209) meeting a foreign object detection criterion, the foreign object detection criterion including: a first requirement that a signal from at least one of the plurality of sets of balanced detection coils (207, 209) indicates an imbalance between signals induced in the two detection coils of the at least one of the plurality of sets of balanced detection coils (207, 209) exceeding a first threshold; wherein the power transmitter is characterized by: the driver being arranged to generate the drive signal for the transmitter coil to generate the power transfer signal during at least one power transfer time interval of a repeating time frame and to generate the drive signal for the transmitter coil to generate an electromagnetic test signal during at least one foreign object detection time interval of the repeating time frame and by the foreign object detection criterion further including a second requirement that a number of signals from the plurality of sets of balanced detection coils (207, 209) indicating an imbalance between signals induced in the two detection coils of the set of balanced detection coils (207, 209) exceeding a second threshold is less than a threshold number, the threshold number being at least two. the driver being arranged to generate the drive signal for the transmitter coil to generate the power transfer signal during at least one power transfer time interval of a repeating time frame and to generate the drive signal for the transmitter coil to generate an electromagnetic test signal during at least one foreign object detection time interval of the repeating time frame and by the foreign object detection criterion further including a second requirement that a number of signals from the plurality of sets of balanced detection coils (207, 209) indicating an imbalance between signals induced in the two detection coils of the set of balanced detection coils (207, 209) exceeding a second threshold is less than a threshold number, the threshold number being at least two.