The polarized electron-spin density on a plane that contains Earth's rotation axis and Amherst, Mass. The black arrows indicate only the direction of the electron spins, while the color shading of the plot indicates the magnitude of the polarized electron spin density. The density of the arrows on the plot is not meaningful. The component of the field into or out of the page is not shown. The electron spin density within the core (white central circle) is assumed to be zero. The violet arrow corresponds to the north orientation and location of the Amherst apparatus. The vertical axis is along Earth's rotation axis, and the axis labels have units of kilometers.