Since the divergence of a curl is always zero, this establishes Gauss's law for magnetism. Next, taking the curl of both sides, using the formula for the curl of a curl, and again using the fact that J does not depend on r , we eventually get the result ∇ ∇ × × B = μ μ 0 4 π π ∇ ∇ ∭ ∭ V d 3 ℓ ℓ J ( ℓ ℓ ) ⋅ ⋅ ∇ ∇ ( 1 | r − − ℓ ℓ | ) − − μ μ 0 4 π π ∭ ∭ V d 3 ℓ ℓ J ( ℓ ℓ ) ∇ ∇ 2 ( 1 | r − − ℓ ℓ | )