  We calculate the minimum value of the power in kinetic bulk motion of the
galactic superluminal source GRS 1915+105. This value far exceeds the Eddington
luminosity for accretion onto a black hole of 10 solar masses. This large value
severely limits the possible carriers of the kinetic luminosity at the base of
the jet, and favours a jet production and acceleration controlled by a magnetic
field whose value, at the base of the jet, exceeds $10^8$ Gauss. The Blandford
and Znajek process can be responsible of the extraction of the rotational
energy of a Kerr black hole, if lasting long enough to provide the required
kinetic energy. This time, of the order of a day, implies that the process must
operate in a stationary, not impulsive, mode.
