Taking into consideration the orbital and spin angular momenta, L and S , respectively, of a single electron in the Hydrogen atom, the perturbation Hamiltonian is given by V ^ ^ = − − ( m ℓ ℓ + m s ) ⋅ ⋅ B where m ℓ ℓ = − − e L / 2 m and m s = − − e S / m . Thus, V ^ ^ = e 2 m ( L + 2 S ) ⋅ ⋅ B Now, in case of the weak-field Zeeman effect, when the applied field is weak compared to the internal field, the spin–orbit coupling dominates and L and S are not separately conserved. The good quantum numbers are n, ℓ, j and m j, and in this basis, the first order energy correction can be shown to be given by E z = − − μ μ B g j B m j , where μ μ B = e ℏ ℏ / 2 m is called the Bohr Magneton. Thus, depending on the value of m j , each degenerate energy level splits into several levels.