  We present a new derivation of the cosmic microwave background anisotropy
spectrum from the Ostriker-Vishniac effect for an open, flat, or closed
Universe, and calculate the anisotropy expected in cold dark-matter (CDM)
models. We provide simple semi-analytic fitting formulas for the Vishniac power
spectrum that can be used to evaluate the expected anisotropy in CDM models for
any arbitrary ionization history. In a flat Universe, CDM models normalized to
cluster abundances produce rms temperature anisotropies of 0.8--2.4 $\mu$K on
arcminute angular scales for a constant ionization fraction of unity, whereas
an ionization fraction of 0.2 yields rms anisotropies of 0.3--0.8 $\mu$K. In an
open and/or high-baryon-density Universe, the level of anisotropy is somewhat
higher. The signal in some of these models may be detectable with planned
interferometry experiments. The damping of the acoustic peaks in the
primary-anisotropy spectrum at degree angular scales depends primarily on the
optical depth and only secondarily on the epoch of reionization. On the other
hand, the amplitude of Ostriker-Vishniac anisotropies depends sensitively on
the epoch of reionization. Therefore, when combined with the estimate of the
reionization optical depth provided by maps of degree-scale anisotropies, the
Ostriker-Vishniac effect can provide a unique probe of the epoch of
reionization.
