A power transmitter for inductively transferring power to a power receiver, the power transmitter comprising: a resonance circuit comprising a capacitive impedance and an inductive impedance, the inductive impedance comprising a transmitter coil for generating a power transfer signal for wirelessly transferring power to the power receiver; a sampler for generating samples by sampling at sample times at least one of a current through the transmitter coil and a voltage over the transmitter coil; a message receiver for receiving messages load modulated onto the power transfer signal by the power receiver, the message receiver being arranged to detect the modulation in response to the samples; a driver for generating a drive signal for the resonance circuit; characterized in that the power transmitter further comprises: a resonance modification circuit for reducing the resonance frequency of the resonance circuit by slowing a state change for at least one of the capacitive impedance and the inductive impedance in a fractional time interval of each of at least a plurality of cycles of the drive signal, the state change being a change of at least one of a voltage of the capacitive impedance and a current of the inductive impedance and the resonance modification circuit being arranged to begin slowing the state change at a start-time of the fractional time interval and to stop slowing the state change at an end-time of the fractional time interval; and a sample time controller for controlling the sample times in response to at least one of start-times and end-times of the fractional time intervals. a resonance circuit comprising a capacitive impedance and an inductive impedance, the inductive impedance comprising a transmitter coil for generating a power transfer signal for wirelessly transferring power to the power receiver; a sampler for generating samples by sampling at sample times at least one of a current through the transmitter coil and a voltage over the transmitter coil; a message receiver for receiving messages load modulated onto the power transfer signal by the power receiver, the message receiver being arranged to detect the modulation in response to the samples; a driver for generating a drive signal for the resonance circuit; characterized in that the power transmitter further comprises: a resonance modification circuit for reducing the resonance frequency of the resonance circuit by slowing a state change for at least one of the capacitive impedance and the inductive impedance in a fractional time interval of each of at least a plurality of cycles of the drive signal, the state change being a change of at least one of a voltage of the capacitive impedance and a current of the inductive impedance and the resonance modification circuit being arranged to begin slowing the state change at a start-time of the fractional time interval and to stop slowing the state change at an end-time of the fractional time interval; and a sample time controller for controlling the sample times in response to at least one of start-times and end-times of the fractional time intervals. a resonance modification circuit for reducing the resonance frequency of the resonance circuit by slowing a state change for at least one of the capacitive impedance and the inductive impedance in a fractional time interval of each of at least a plurality of cycles of the drive signal, the state change being a change of at least one of a voltage of the capacitive impedance and a current of the inductive impedance and the resonance modification circuit being arranged to begin slowing the state change at a start-time of the fractional time interval and to stop slowing the state change at an end-time of the fractional time interval; and a sample time controller for controlling the sample times in response to at least one of start-times and end-times of the fractional time intervals.