Performance of an atomic mercury magnetometer in the neutron EDM experiment

Previous measurements of the electric dipole moment (EDM) of the neutron have been limited by the systematic effect of changes in the magnetic environment. In this paper we report the performance of a newly-developed "cohabiting" magnetometer based upon measurements of the nuclear precession frequency of free mercury atoms that occupy the same volume as the neutrons. We find that we are able to observe changes as small as 10 ^-9 G in the volume-averaged magnetic field, allowing us to reduce the systematic errors in the EDM experiment by more than an order of magnitude in comparison with previous measurements.