In a tapping unit according to the invention, the tap is held in a tap-carrier which is screwed into a pattern retained and fixed against rotation in a casing. The latter has a flange part with a flange with which the pressure cap is fastened to an end face and annular sealing surface of a housing by being screwed into place. A user selection for a warning wait period is received and stored. An AC signal not containing a DC component is input from the AC-signal source to the electrode units through the source impedance, and a signal representing the status of electric connection of the electrode units is output. The multi-pulse train has a pulse period longer than T which represents a reference period of the recording modulation code.