  We discuss the design and performance of a laser-pumped cesium vapor
magnetometer in the $M_x$-configuration. The device will be implemented for the
control and stabilization of fluctuating magnetic fields and gradients in a new
experiment searching for a permanent electric dipole moment of the neutron. We
have determined the intrinsic sensitivity of the device to be 21 fT with a 1 s
integration time, limited by low frequency noise of the laser power. In the
shot noise limit the magnetometer can reach a sensitivity of 7 fT with a 1 s
integration time. We have used the device to study the magnetic field
fluctuations in a three layer mu-metal shield for integration times in the
range of 2-100 seconds. The lowest values of the fluctuations are traced back
to the stability of the power supply used to generate the field.
