second voltage measuring device 36 with a series connected filter 33 the conducting state frequency of which is matched to the frequency 350 khz of treatment current i hf the output signal of the second voltage measuring device 36 which is supplied to evaluation device 30 reflects the voltage drop which arises due to the treatment current i hf flowing between measuring electrode 14 and main electrode 10 evaluation device 30 is connected to a display or signal generating device 31 in operation after affixing the electrode device to a skin section 5 of a patient a measuring current i m is produced continuously or intermittently by generator 32 the voltage measured by voltage measuring device 35 thus expresses with constant measuring current i m a measure for the impedance which is present between measuring electrode 14 and main electrode 10 if the impedance rises in relation to a predefinable u201c standard value u201d or a standard value ascertained from earlier investigations then it may be assumed from this that the resistance between measuring electrode 14 and tissue 6 and or the resistance between main electrode 10 and tissue 6 is very high or has risen in turn it can be identified from this that there is a contact failure thus the electrode has become detached or too high a.