A stationary part has a conduit for supplying fluid to a passage located in a rotary part on the rotation axis thereof, the supply conduit having a valve seat. A body comprising a magnetizable material has a fluid-transferring channel coaxial with the rotation axis and includes a valve element; and an electromagnet is operable on this body to hold the valve element against the seat to close the supply conduit against the fluid pressure therein. Upon deenergizing of the electromagnet, the fluid pressure in the supply conduit displaces said body axially so as to unseat the valve element and engage the body with the rotating part, whereby the passage therein is supplied with fluid through said channel in the body.