Relation between changes in the dipole field and those in the earth's rate of rotation has been examined for three different periods, 8000, 400 and 65 years, based on such various kind of data as those of archeomagnetism, observation of moon's longitudes and recent instrumental observations. It has been confirmed that fluctuation exists in the rate of rotation corresponding to the dipole change for these periods. The magnitude of the variation in the rotation to be related to the dipole change increases as the period decreases.
Time variation in the drift velocity has also been examined for several features of the geomagnetic secular variation. It is very probable that a change in the drift velocity accompanied the 400 year period fluctuation in the earth's rotation.
The results thus obtained are compared with a theoretical result. Except for the phase of the angular velocity of the mantle for 65 year period and the magnitude of the geomagnetic westward drift for 400 year period, the observed results are well accounted for by an electromagnetic coupling caused by a dipole change at the core-mantle boundary, provided that the electrical conductivity of the lower mantle is as large as 10^-8emu.