Tow bar

The coupling has a retaining unit engaged at an end (42) of a ball neck (40) and fixed at a motor vehicle, and a coupling ball (46) arranged at another end of the ball neck. A magnet (80) e.g. button magnet, is arranged in a recess of the coupling ball. The magnet is aligned such that magnetic flux magnetomotive force of the ball neck is carried out. A magnetic field sensor (130) is arranged at the ball neck for detecting the magnetic flux magnetomotive force of the ball neck, where the magnetic field sensor is connected to a trailer detection circuit (154).