To reduce the rotational power, an apparatus with a rotational body that is rotationally driven in a fluid-filled housing a rotational directing body is provided between the rotational body and the housing, which is rotatably supported coaxially with respect to the rotational body. The rotational directing body is configured such that in operation it rotates at an intermediate rotational frequency in comparison to the housing and the rotational body. The apparatus is particularly an X-ray radiator having a cathode and anode that are mounted in a vacuum tube in a spatially fixed manner in relation to the tube, the vacuum tube being rotationally driven as a rotational body in a coolant housing.