A resistance welder, which includes a clamp for supporting a detachable arm, the arm being constituted by a fixed part supporting a fixed electrode and a rotationally mobile part supporting a mobile electrode. The electrodes are intended to be crossed by an electrical current. The mobile arm of the clamp is pushed in its rotational motion by a thruster so that the extremities of the electrodes approach each other and grip the elements to be welded. The clamp further includes a shift measurement sensor, the value measured by this sensor being proportional to the distance between the extremities of the electrodes. It is thus easy to measure the distance between the electrodes and the measurement made on the piston can be used for other types of arms.