where μ 0 is the magnetic constant, r̂ is a unit vector parallel to the line joining the centers of the two dipoles, and | r | is the distance between the centers of m 1 and m 2. Last term with δ δ -function vanishes everywhere but the origin, and is necessary to ensure that ∇ ∇ ⋅ ⋅ B vanishes everywhere. Alternatively, suppose γ 1 and γ 2 are gyromagnetic ratios of two particles with spin quanta S 1 and S 2. (Each such quantum is some integral multiple of ⁠ 1 / 2 ⁠.) Then: