  We consider the magnetoelectric effect produced by a capacitor formed by two
semispherical perfectly conducting plates subjected to a potential difference
and surrounded by a spherical shell of a topologically insulating material. The
modified Maxwell equations are solved in terms of coupled electric and magnetic
scalar potentials using spherical coordinates and in the approximation where
the effective magnetoelectric coupling is of the order of the fine structure
constant. The emphasis is placed in the calculation of the magnetic field for
several relevant configurations designed to enhance the possibility of
measuring this field. The magnitudes we obtain fall within the sensitivities of
magnetometers based upon nitrogen-vacancy centers in diamond as well as of
devices using scanning SQUID magnetometry.
