The instrument has a first arm (19) including an electrically isolating base body (23) with a groove-like recess and a guide slot. Retaining units are formed on the base body to mount an electrode having a curved shape. A second arm (20) is movably mounted on the base body and moved toward and away from the electrode, where the second arm includes a longitudinal slot. A knife (21) is arranged in a guide slot of the base body such that the knife is longitudinally movable and projecting-arranged or moved into the longitudinal slot. The second arm is formed by a solid metal body.
