  New sources of CP violation beyond the Standard Model of particle physics
could be revealed in the laboratory by measuring a non-zero electric dipole
moment (EDM) of a spin 1/2 particle such as the neutron. Despite the great
sensitivity attained after 60 years of developments, the result of the
experiments is still compatible with zero. Still, new experiments have a high
discovery potential since they probe new physics at the multi-TeV scale, beyond
the reach of direct searches at colliders. Progress in precision on the neutron
EDM is limited by a systematic effect arising from the relativistic motional
field $\vec{E} \times \vec{v} / c^2$ experienced by the particles moving in the
measurement chamber in combination with the residual magnetic gradients. This
effect would normally forbid a significant increase of the size of the chamber,
sadly hindering the increase of neutron statistics. We propose a new
measurement concept to evade this limitation in a room-temperature experiment
employing a mercury co-magnetometer. It consists ajusting the static magnetic
field $B_0$ to a `magic' value which cancels the false EDM of the mercury. The
magic setting is $7.2\,\muT$ for a big cylindrical double-chamber of diameter
$100$~cm.
