Abstract: An apparatus for measuring the resistance and reactance of a subject, or a segment of a subject, includes a constant current source supplying a periodic high frequency input current to the subject. The constant current source controls the input current to a targeted value by measuring the actual current through the subject and comparing that actual current to the target current. The resulting error signal controls the input current. The input current is supplied to the segment through a coupling transformer, and another coupling transformer coupled to a resistor in the subject path supplies the value of the actual current. The apparatus also includes an impedance measuring circuit that detects an output voltage across the segment, preferably using a detection coupling transformer, and produces at least one of a reactance output signal and a resistance output signal using the output voltage. In one aspect, an automatic shut down circuit is included.