Which gravitomagnetic precession rate will be measured by Gravity Probe B?
General relativity predicts a "hyperfine" precession rate for a gyroscope moving in the gravitomagnetic field of a rotating massive body. The recently launched Gravity Probe B (GP-B) will test the predicted precession rate of 40.9 milliarc-seconds per year for a set of four gyroscopes in a Polar Earth Orbit (PEO). It may be possible, however, that the gravitomagnetic field from a rotating mass behaves in the same way as the magnetic field generated by a moving charge. In that case the predicted precession rate of a gyroscope will be zero, since the gyroscopes of GP-B have been shielded against external magnetic fields. Another possible manifestation of the equivalence of gravitomagnetic and magnetic field may already have been found. It is the so-called Wilson Blackett law, approximately describing the magnetic field of many rotating celestial bodies. In this work a review of the gravitomagnetic approach is given starting from the Einstein equations. Four gravitomagnetic equations, analogous to the Maxwell equations, are deduced. The Wilson Blackett relation follows from these equations, if the gravitomagnetic field is identified as a common magnetic field. In addition, the precession rate for a gyroscope in terms of the gravito-magnetic field has been derived, starting from the principle of general covariance. The gravitomagnetic field may again be identified as a common magnetic field, or can be evaluated in the standard way. The future observations from GP-B may discriminate between the alternative choices.