added on the electrode side of the interface to shift the stimulus pulse duration above the chronaxie of the intended physiological target e g excitable tissue as shown in fig5 a for example a resistance of several hundred ohms may be added to slightly increase the impedance so that the stimulus pulse duration is shifted above the chronaxie conversely the series resistance r 2 can be removed or instead parallel resistance added r 3 as represented in fig5 d from the electrode side of the interface to shift the stimulus pulse duration below the chronaxie of the unintended excitable tissue in this way the strength of the stimulus signal may be compensated so that the target or intended tissue is stimulated even if the impedance of the skin changes it is also contemplated and within the intended scope of the present invention to control the resistance on a real time basis by detecting the impedance of the skin and adjusting a variable in line series resistor based on the detected impedance preferably the impedance of the skin in sampled or monitored in real time and from this measured value is derived the resistance and capacitance current decay the impedance of the skin may be measured using conventional schemes such as applying a current source constant or variable and measuring the skin impedance using a sensor or electrode place on the surface of the skin the variable resistance and thus stimulus pulse duration is preferably varied in direct proportion to that of the detected skin electrode impedance that is as the detected skin electrode impedance increases the variable resistance is increased to lengthen the duration of the stimulus pulse to allow a longer.