An earth boring apparatus includes a drill rod, a source for a drill fluid, and a voltage source. The drill rod has a hollow rod casing provided for throughflow of the drill fluid, and a conductor located within the rod casing, with the voltage source being connected with the conductor and the rod casing. The voltage source is so connected as to prevent the electric conductor to act as anode in the event the drill fluid establishes an electrically conductive connection between the rod casing and the conductor.