Abstract: When developing a coating such as for instance a light-sensitive exposed layer on a length or plate of material through ultrasonic agitation, the material is carried at a short distance or closely past a bar or a pipe extending substantially perpendicular to the advancing direction of the material and in the entire width of the coating. The bar or the pipe is caused to vibrate by a plurality of ultrasonic transducers while a developer is applied onto the coating adjacent the bar or the pipe in such a manner that the vibrations are transferred from the vibrating bar or pipe to the developer and to a stripe-shaped transverse area of the plate/the length including the coating. In this manner only the part of the developer having touched the ultrasonically agitated pipe or bar or already having hit the stripe of material currently developed is caused to vibrate by the ultrasonic transducers.