  We investigate the properties of electromagnetic fields in isobaric
$_{44}^{96}\textrm{Ru}+\,_{44}^{96}\textrm{Ru}$ and
$_{40}^{96}\textrm{Zr}+\,_{40}^{96}\textrm{Zr}$ collisions at $\sqrt{s}$ = 200
GeV by using a multiphase transport model, with special emphasis on the
correlation between magnetic field direction and participant plane angle
$\Psi_{2}$ (or spectator plane angle $\Psi_{2}^{\rm SP}$), i.e. $\langle{\rm
cos}\ 2(\Psi_B - \Psi_{2})\rangle$ [or $\langle{\rm cos}\ 2(\Psi_B -
\Psi_{2}^{\rm SP})\rangle$]. We confirm that the magnetic fields of
$_{44}^{96}\textrm{Ru}+\,_{44}^{96}\textrm{Ru}$ collisions are stronger than
those of $_{40}^{96}\textrm{Zr}+\,_{40}^{96}\textrm{Zr}$ collisions due to
their larger proton fraction. We find that the deformation of nuclei has a
non-negligible effect on $\langle{\rm cos}\ 2(\Psi_B - \Psi_{2})\rangle$
especially in peripheral events. Because the magnetic-field direction is more
strongly correlated with $\Psi_{2}^{\rm SP}$ than with $\Psi_{2}$, the relative
difference of the chiral magnetic effect observable with respect to
$\Psi_{2}^{\rm SP}$ is expected to be able to reflect much cleaner information
about the chiral magnetic effect with less influences of deformation.
