The model of Anufriyev and Braginskii (1975) and Moffatt and Dillon (1976) is further studied, with a view to calculating (i) the mean tangential stress on the core-mantle interface, and (ii) the level of the α-effect due to flow over the surface bumps. The stress calculated agrees in order of magnitude with the stress of 0.04 Nm−2that is inferred from the decade variations in the length of the day.