The observed correlation between the decade variations in the earth's rotation and geomagnetic secular variation probably can be accounted for by electromagnetic core-mantle coupling as suggested by Bullard long ago. The new observations, combined with a simplified earth model, provide some indications on the thickness of the outer core layers which are responsible for geomagnetic secular variation. This thickness appears to be much smaller than previously thought (of the order of 100 to 200 km). The radial velocity of the fluid at the base of the layer is found to be of the order of 10-6 m s-1, about 2 orders of magnitude smaller than the horizontal velocity in the layer.