To interrupt the current to the drive motor (3) of a motor-driven arm (1) which carries a pipette (2) when something or someone unintentionally contacts the pipette (2) during its movement, the arm 1) has a lower, rigid part (5), and an upper, flexible part (6) which, at its one end, carries the pipette (2) and, at its other end, is rigidly connected to the lower arm part (5). The upper, flexible part (6) is in releasable contact with the lower, rigid part (5) at the end that carries the pipette (2), and is adapted to, at least partially, the released from that contact when something or someone unintentionally contacts the pipette (2) during its movement. A detecting means (9) is adapted to detect when the upper, flexible part (6), at least partially, is released from the contact with the lower, rigid part (5) and in response hereto cause a switch to brake the current to the drive motor (3) for the arm (1).