used to transmit and receive power across the skin the skin at each coil location receives a fraction of the electromagnetic field strength required when using a single pair of coils as taught in the prior art alternatively a single load measurement is used and the coils l2 are controlled in accordance therewith in an embodiment for example each coil receives an equal fraction of the transmitted energy in another embodiment coils are driven as determined through calibration according to yet another embodiment each coil provides some feedback relating to coupling efficiency in order to maximise overall energy usage while maintaining electromagnetic radiation across the skin below the predetermined threshold of course phase correction circuitry may be implemented in association with the feedback information also other corrective action may be implemented in order to maintain a desired coupling between external coils and implanted coils during operation of the tet device different embodiments of control systems for maintaining sufficient power levels in the implanted circuit include the following features a feedback loop for responding to current and past measured values calibration of the circuitry to function with a specific implanted device and or calibration undertaken during an initial period of use and an ongoing estimation of tissue damage due to electromagnetic field strengths and overall exposure times a calibrated programmable circuit allows for corrective actions in anticipation of change the system for example includes storage means for storing past sequences of measured values that were controlled inadequately when these patterns occur the programmable circuit responds differently than in previous attempts within acceptable parameters to better address the measured values estimation of tissue damage permits control of multiple external coils in an attempt to minimise overall tissue damage for example.