Experimental Apparatus Ultra-cold neutrons (UCN) are produced by polarized cold neutron scattering in superfluid He. The polarized neutrons then precess about a small magnetic field as a large applied electric field is reversed. The neutron precession frequency is measured via capture on a small amount of polarized 3He added to the superfluid 4He. This neutron capture yields a time-dependent signal of scintillation light in the 4He that is measured with photomultiplier tubes. The time-dependence results from the difference in neutron capture probability for neutron spins aligned with the 3He spins compared to anti-aligned and the slightly different precession frequencies of the neutrons and 3He due to their different magnetic moments. Precision measurement of the precession frequency for the two orientations of the electric field yields the neutron EDM.