What Does EPR Measure?

EPR (electron paramagnetic resonance) measures the tiny shift in the magnetic field that comes from the polarization of $^3$He. More accurately, it measures a small correction to a shift, and this small correction is due to the $^3$He polarization.

$$\Delta\nu \propto B_{tot} = B_0 + B_{^3\vec{He}}$$ (1)

The full equation that we use is

$$\Delta \nu_{EPR} = \frac{8\pi}{3} \frac{d \nu_{EPR}(F,M)}{dB} \kappa_0 \mu_{He} P_{He}$$ (2)