comprises a control electrode squaring circuit integrating circuit comparator and a relay whereby high frequency current is cut off when the body tissue heat build up becomes excessive application_number US 70157685 A description referring to fig1 by means of a control electrode 1 attached to the body surface of the patient and isolated from the neutral electrode 2 and any other grounded part the body voltage u k is measured relative to the ground potential the body voltage u k is essentially independent of the point of application of the active electrode 3 and the position of the neutral electrode 2 so that the location of the control electrode on the body surface is not critical the body voltage u k which is picked up by the control electrode 1 is connected to a squaring circuit 5 by means of conductor 4 the squaring circuit 5 may comprise electronic circuits known in the art such as integrated analog squaring elements alternatively the squaring circuit 5 may be a thermal convertor a thermal convertor has the advantage that an lf voltage is available at its output which simplifies the following circuits the output u k 2 of the squaring circuit is connected to the integrator 7 via input conductor 6 electronic integrators are readily available in the computing art the.