  We report on a method of quantifying spin accumulation in
Co$_{2}$MnSi/$n$-GaAs and Fe/$n$-GaAs heterostructures using a non-magnetic
probe. In the presence of a large non-equilibrium spin polarization, the
combination of a non-constant density of states and energy-dependent
conductivity generates an electromotive force (EMF). We demonstrate that this
signal dephases in the presence of applied and hyperfine fields, scales
quadratically with the polarization, and is comparable in magnitude to the
spin-splitting. Since this spin-generated EMF depends only on experimentally
accessible parameters of the bulk material, its magnitude may be used to
quantify the injected spin polarization in absolute terms.
