Biot–Savart law
One difference between Gaussian and SI units is in the factors of 4"π" in various formulas. SI electromagnetic units are called "rationalized", because Maxwell's equations have no explicit factors of 4"π" in the formulae. On the other hand, the inverse-square force laws – Coulomb's law and the Biot–Savart law – "do" have a factor of 4"π" attached to the "r". In "unrationalized" Gaussian units (not Lorentz–Heaviside units) the situation is reversed: two of Maxwell's equations have factors of 4"π" in the formulas, while both of the inverse-square force laws, Coulomb's law and the Biot–Savart law, have no factor of 4"π" attached to "r" in the denominator.