The invention concerns a high frequency surgery apparatus for cutting and/or coagulating biological tissue and methods of operating same. The high frequency surgery apparatus includes at least one high frequency generator which in operation forms a high frequency circuit with the tissue to be treated, with the production of an arc, and at least one measuring and calculating device which is connected for signal transmission to the high frequency circuit and which is adapted in operation to ascertain both a DC voltage in the high frequency circuit and also the amplitudes of at least one even and at least one odd harmonic of a fundamental frequency of the high frequency generator and to form a speed parameter representative of a cutting speed from a quotient of the DC voltage and the amplitude of the harmonic and/or from a quotient of the DC voltage and the output current and to output a speed signal dependent on the speed parameter for subsequent processing
