A scoop controlled fluid coupling has a toroidal working circuit W defined by vaned impeller and runner elements 11,12. Working liquid is supplied to the working circuit W from a rotating reservoir 9 by an adjustable scoop tube 15 having a scooping orifice 16, the radial position of which is to determine the degree of filling of the circuit W. To counteract unintentional changes in the filling due to thermal expansion of the liquid, the runner diameter is about 10% smaller than that of the impeller, and the runner has two sets of holes 54,55 which permit liquid to escape from the circuit.