  We have developed a technique for precise measurement of small magnetic
fields using nonlinear magneto-optic rotation (NMOR). The technique relies on
the resonant laser beam being chopped. During the on time, the atoms are
optically pumped into an aligned ground state ($\Delta m=2$ coherence). During
the off time, they freely precess around the magnetic field at the Larmor
frequency. If the on-off modulation frequency matches (twice) the Larmor
precession frequency, the rotation is resonantly enhanced in every cycle,
thereby making the process like a repeated Ramsey measurement of the Larmor
frequency. We study chopped-NMOR in a paraffin-coated Cs vapor cell. The
out-of-phase demodulated rotation shows a Lorentzian peak of linewidth 85
$\mu$G, corresponding to a sensitivity of 0.15 nG/$\sqrt{{\rm Hz}}$. We discuss
the potential of this technique for the measurement of an atomic
electric-dipole moment.
