A double-acting cylinder (1.0) guides cylinder pins towards a measuring device. The pins are separated by means of a continuous to and fro movement. The oscillating motion can be started by means of a valve with selector switch. The duration of the forward stroke of the cylinder is to be t1 = 0.6 seconds, the return stroke t3 = 0.4 seconds. The cylinder is to remain in the forward end position for t2 = 1.0 seconds, resulting in a cycle time of t4 = 2.0 seconds.